Merge pull request #8699 from tweag/nix-c-bindings

(Towards) stable C bindings for libutil, libexpr

Makefile

@@ -18,6 +18,9 @@ makefiles = \
   src/libexpr/local.mk \
   src/libcmd/local.mk \
   src/nix/local.mk \
+  src/libutil-c/local.mk \
+  src/libstore-c/local.mk \
+  src/libexpr-c/local.mk \
   src/resolve-system-dependencies/local.mk \
   scripts/local.mk \
   misc/bash/local.mk \
@@ -25,7 +28,10 @@ makefiles = \
   misc/zsh/local.mk \
   misc/systemd/local.mk \
   misc/launchd/local.mk \
-  misc/upstart/local.mk
+  misc/upstart/local.mk \
+  doc/manual/local.mk \
+  doc/internal-api/local.mk \
+  doc/external-api/local.mk
 endif
 
 ifeq ($(ENABLE_UNIT_TESTS), yes)
@@ -60,6 +66,10 @@ ifeq ($(ENABLE_INTERNAL_API_DOCS), yes)
 makefiles-late += doc/internal-api/local.mk
 endif
 
+ifeq ($(ENABLE_EXTERNAL_API_DOCS), yes)
+makefiles-late += doc/external-api/local.mk
+endif
+
 # Miscellaneous global Flags
 
 OPTIMIZE = 1
@@ -124,3 +134,10 @@ internal-api-html:
 	@echo "Internal API docs are disabled. Configure with '--enable-internal-api-docs', or avoid calling 'make internal-api-html'."
 	@exit 1
 endif
+
+ifneq ($(ENABLE_EXTERNAL_API_DOCS), yes)
+.PHONY: external-api-html
+external-api-html:
+	@echo "External API docs are disabled. Configure with '--enable-external-api-docs', or avoid calling 'make external-api-html'."
+	@exit 1
+endif

Makefile.config.in

@@ -12,6 +12,7 @@ ENABLE_BUILD = @ENABLE_BUILD@
 ENABLE_DOC_GEN = @ENABLE_DOC_GEN@
 ENABLE_FUNCTIONAL_TESTS = @ENABLE_FUNCTIONAL_TESTS@
 ENABLE_INTERNAL_API_DOCS = @ENABLE_INTERNAL_API_DOCS@
+ENABLE_EXTERNAL_API_DOCS = @ENABLE_EXTERNAL_API_DOCS@
 ENABLE_S3 = @ENABLE_S3@
 ENABLE_UNIT_TESTS = @ENABLE_UNIT_TESTS@
 GTEST_LIBS = @GTEST_LIBS@

configure.ac

@@ -150,6 +150,11 @@ AC_ARG_ENABLE(unit-tests, AS_HELP_STRING([--disable-unit-tests],[Do not build th
   ENABLE_UNIT_TESTS=$enableval, ENABLE_UNIT_TESTS=$ENABLE_BUILD)
 AC_SUBST(ENABLE_UNIT_TESTS)
 
+# Build external API docs by default
+AC_ARG_ENABLE(external_api_docs, AS_HELP_STRING([--enable-external-api-docs],[Build API docs for Nix's C interface]),
+  external_api_docs=$enableval, external_api_docs=yes)
+AC_SUBST(external_api_docs)
+
 AS_IF(
   [test "$ENABLE_BUILD" == "no" && test "$ENABLE_UNIT_TESTS" == "yes"],
   [AC_MSG_ERROR([Cannot enable unit tests when building overall is disabled. Please do not pass '--enable-unit-tests' or do not pass '--disable-build'.])])
@@ -172,6 +177,10 @@ AC_ARG_ENABLE(internal-api-docs, AS_HELP_STRING([--enable-internal-api-docs],[Bu
   ENABLE_INTERNAL_API_DOCS=$enableval, ENABLE_INTERNAL_API_DOCS=no)
 AC_SUBST(ENABLE_INTERNAL_API_DOCS)
 
+AC_ARG_ENABLE(external-api-docs, AS_HELP_STRING([--enable-external-api-docs],[Build API docs for Nix's external unstable C interfaces]),
+  ENABLE_EXTERNAL_API_DOCS=$enableval, ENABLE_EXTERNAL_API_DOCS=no)
+AC_SUBST(ENABLE_EXTERNAL_API_DOCS)
+
 AS_IF(
   [test "$ENABLE_FUNCTIONAL_TESTS" == "yes" || test "$ENABLE_DOC_GEN" == "yes"],
   [NEED_PROG(jq, jq)])

doc/external-api/.gitignore

@@ -0,0 +1,3 @@
+/doxygen.cfg
+/html
+/latex

doc/external-api/README.md

@@ -0,0 +1,108 @@
+# Getting started
+
+> **Warning** These bindings are **experimental**, which means they can change
+> at any time or be removed outright; nevertheless the plan is to provide a
+> stable external C API to the Nix language and the Nix store.
+
+The language library allows evaluating Nix expressions and interacting with Nix
+language values. The Nix store API is still rudimentary, and only allows
+initialising and connecting to a store for the Nix language evaluator to
+interact with.
+
+Currently there are two ways to interface with the Nix language evaluator
+programmatically:
+
+1. Embedding the evaluator
+2. Writing language plug-ins
+
+Embedding means you link the Nix C libraries in your program and use them from
+there. Adding a plug-in means you make a library that gets loaded by the Nix
+language evaluator, specified through a configuration option.
+
+Many of the components and mechanisms involved are not yet documented, therefore
+please refer to the [Nix source code](https://github.com/NixOS/nix/) for
+details. Additions to in-code documentation and the reference manual are highly
+appreciated.
+
+The following examples, for simplicity, don't include error handling. See the
+[Handling errors](@ref errors) section for more information.
+
+# Embedding the Nix Evaluator
+
+In this example we programmatically start the Nix language evaluator with a
+dummy store (that has no store paths and cannot be written to), and evaluate the
+Nix expression `builtins.nixVersion`.
+
+**main.c:**
+
+```C
+#include <nix_api_util.h>
+#include <nix_api_expr.h>
+#include <nix_api_value.h>
+#include <stdio.h>
+
+// NOTE: This example lacks all error handling. Production code must check for
+// errors, as some return values will be undefined.
+int main() {
+   nix_libexpr_init(NULL);
+
+   Store* store = nix_store_open(NULL, "dummy://", NULL);
+   EvalState* state = nix_state_create(NULL, NULL, store); // empty search path (NIX_PATH)
+   Value *value = nix_alloc_value(NULL, state);
+
+   nix_expr_eval_from_string(NULL, state, "builtins.nixVersion", ".", value);
+   nix_value_force(NULL, state, value);
+   printf("Nix version: %s\n", nix_get_string(NULL, value));
+
+   nix_gc_decref(NULL, value);
+   nix_state_free(state);
+   nix_store_free(store);
+   return 0;
+}
+```
+
+**Usage:**
+
+```ShellSession
+$ gcc main.c $(pkg-config nix-expr-c --libs --cflags) -o main
+$ ./main
+Nix version: 2.17
+```
+
+# Writing a Nix language plug-in
+
+In this example we add a custom primitive operation (_primop_) to `builtins`. It
+will increment the argument if it is an integer and throw an error otherwise.
+
+**plugin.c:**
+
+```C
+#include <nix_api_util.h>
+#include <nix_api_expr.h>
+#include <nix_api_value.h>
+
+void increment(void* user_data, nix_c_context* ctx, EvalState* state, Value** args, Value* v) {
+    nix_value_force(NULL, state, args[0]);
+    if (nix_get_type(NULL, args[0]) == NIX_TYPE_INT) {
+      nix_init_int(NULL, v, nix_get_int(NULL, args[0]) + 1);
+    } else {
+      nix_set_err_msg(ctx, NIX_ERR_UNKNOWN, "First argument should be an integer.");
+    }
+}
+
+void nix_plugin_entry() {
+  const char* args[] = {"n", NULL};
+  PrimOp *p = nix_alloc_primop(NULL, increment, 1, "increment", args, "Example custom built-in function: increments an integer", NULL);
+  nix_register_primop(NULL, p);
+  nix_gc_decref(NULL, p);
+}
+```
+
+**Usage:**
+
+```ShellSession
+$ gcc plugin.c $(pkg-config nix-expr-c --libs --cflags) -shared -o plugin.so
+$ nix --plugin-files ./plugin.so repl
+nix-repl> builtins.increment 1
+2
+```

doc/external-api/doxygen.cfg.in

@@ -0,0 +1,57 @@
+# Doxyfile 1.9.5
+
+# The PROJECT_NAME tag is a single word (or a sequence of words surrounded by
+# double-quotes, unless you are using Doxywizard) that should identify the
+# project for which the documentation is generated. This name is used in the
+# title of most generated pages and in a few other places.
+# The default value is: My Project.
+
+PROJECT_NAME           = "Nix"
+
+# The PROJECT_NUMBER tag can be used to enter a project or revision number. This
+# could be handy for archiving the generated documentation or if some version
+# control system is used.
+
+PROJECT_NUMBER         = @PACKAGE_VERSION@
+
+# Using the PROJECT_BRIEF tag one can provide an optional one line description
+# for a project that appears at the top of each page and should give viewer a
+# quick idea about the purpose of the project. Keep the description short.
+
+PROJECT_BRIEF          = "Nix, the purely functional package manager: C API (experimental)"
+
+# If the GENERATE_LATEX tag is set to YES, doxygen will generate LaTeX output.
+# The default value is: YES.
+
+GENERATE_LATEX         = NO
+
+# The INPUT tag is used to specify the files and/or directories that contain
+# documented source files. You may enter file names like myfile.cpp or
+# directories like /usr/src/myproject. Separate the files or directories with
+# spaces. See also FILE_PATTERNS and EXTENSION_MAPPING
+# Note: If this tag is empty the current directory is searched.
+
+# FIXME Make this list more maintainable somehow. We could maybe generate this
+# in the Makefile, but we would need to change how `.in` files are preprocessed
+# so they can expand variables despite configure variables.
+
+INPUT                  = \
+  src/libutil-c \
+  src/libexpr-c \
+  src/libstore-c \
+  doc/external-api/README.md
+
+FILE_PATTERNS = nix_api_*.h *.md
+
+# The INCLUDE_PATH tag can be used to specify one or more directories that
+# contain include files that are not input files but should be processed by the
+# preprocessor. Note that the INCLUDE_PATH is not recursive, so the setting of
+# RECURSIVE has no effect here.
+# This tag requires that the tag SEARCH_INCLUDES is set to YES.
+
+INCLUDE_PATH           = @RAPIDCHECK_HEADERS@
+EXCLUDE_PATTERNS = *_internal.h
+GENERATE_TREEVIEW = YES
+OPTIMIZE_OUTPUT_FOR_C  = YES
+
+USE_MDFILE_AS_MAINPAGE = doc/external-api/README.md

doc/external-api/local.mk

@@ -0,0 +1,7 @@
+$(docdir)/external-api/html/index.html $(docdir)/external-api/latex: $(d)/doxygen.cfg
+	mkdir -p $(docdir)/external-api
+	{ cat $< ; echo "OUTPUT_DIRECTORY=$(docdir)/external-api" ; } | doxygen -
+
+# Generate the HTML API docs for Nix's unstable C bindings
+.PHONY: external-api-html
+external-api-html: $(docdir)/external-api/html/index.html

flake.nix

@@ -285,6 +285,13 @@
           enableInternalAPIDocs = true;
         };
 
+        # API docs for Nix's C bindings.
+        external-api-docs = nixpkgsFor.x86_64-linux.native.callPackage ./package.nix {
+          inherit fileset;
+          doBuild = false;
+          enableExternalAPIDocs = true;
+        };
+
         # System tests.
         tests = import ./tests/nixos { inherit lib nixpkgs nixpkgsFor; } // {
 

local.mk

@@ -2,7 +2,7 @@ GLOBAL_CXXFLAGS += -Wno-deprecated-declarations -Werror=switch
 # Allow switch-enum to be overridden for files that do not support it, usually because of dependency headers.
 ERROR_SWITCH_ENUM = -Werror=switch-enum
 
-$(foreach i, config.h $(wildcard src/lib*/*.hh), \
+$(foreach i, config.h $(wildcard src/lib*/*.hh) $(wildcard src/lib*/*.h $(filter-out %_internal.h, $(wildcard src/lib*c/*.h))), \
   $(eval $(call install-file-in, $(i), $(includedir)/nix, 0644)))
 
 ifdef HOST_UNIX

package.nix

@@ -5,6 +5,7 @@
 , autoreconfHook
 , aws-sdk-cpp
 , boehmgc
+, buildPackages
 , nlohmann_json
 , bison
 , boost
@@ -91,9 +92,10 @@
 # - readline
 , readlineFlavor ? if stdenv.hostPlatform.isWindows then "readline" else "editline"
 
-# Whether to build the internal API docs, can be done separately from
+# Whether to build the internal/external API docs, can be done separately from
 # everything else.
 , enableInternalAPIDocs ? false
+, enableExternalAPIDocs ? false
 
 # Whether to install unit tests. This is useful when cross compiling
 # since we cannot run them natively during the build, but can do so
@@ -182,6 +184,9 @@ in {
           ./doc/manual
         ] ++ lib.optionals enableInternalAPIDocs [
           ./doc/internal-api
+        ] ++ lib.optionals enableExternalAPIDocs [
+          ./doc/external-api
+        ] ++ lib.optionals (enableInternalAPIDocs || enableExternalAPIDocs) [
           # Source might not be compiled, but still must be available
           # for Doxygen to gather comments.
           ./src
@@ -199,7 +204,7 @@ in {
     ++ lib.optional doBuild "dev"
     # If we are doing just build or just docs, the one thing will use
     # "out". We only need additional outputs if we are doing both.
-    ++ lib.optional (doBuild && (enableManual || enableInternalAPIDocs)) "doc"
+    ++ lib.optional (doBuild && (enableManual || enableInternalAPIDocs || enableExternalAPIDocs)) "doc"
     ++ lib.optional installUnitTests "check";
 
   nativeBuildInputs = [
@@ -221,7 +226,7 @@ in {
   ] ++ lib.optionals (doInstallCheck || enableManual) [
     jq # Also for custom mdBook preprocessor.
   ] ++ lib.optional stdenv.hostPlatform.isLinux util-linux
-    ++ lib.optional enableInternalAPIDocs doxygen
+    ++ lib.optional (enableInternalAPIDocs || enableExternalAPIDocs) doxygen
   ;
 
   buildInputs = lib.optionals doBuild [
@@ -285,6 +290,7 @@ in {
     (lib.enableFeature buildUnitTests "unit-tests")
     (lib.enableFeature doInstallCheck "functional-tests")
     (lib.enableFeature enableInternalAPIDocs "internal-api-docs")
+    (lib.enableFeature enableExternalAPIDocs "external-api-docs")
     (lib.enableFeature enableManual "doc-gen")
     (lib.enableFeature enableGC "gc")
     (lib.enableFeature enableMarkdown "markdown")
@@ -309,7 +315,8 @@ in {
   makeFlags = "profiledir=$(out)/etc/profile.d PRECOMPILE_HEADERS=1";
 
   installTargets = lib.optional doBuild "install"
-    ++ lib.optional enableInternalAPIDocs "internal-api-html";
+    ++ lib.optional enableInternalAPIDocs "internal-api-html"
+    ++ lib.optional enableExternalAPIDocs "external-api-html";
 
   installFlags = "sysconfdir=$(out)/etc";
 
@@ -336,6 +343,10 @@ in {
   '' + lib.optionalString enableInternalAPIDocs ''
     mkdir -p ''${!outputDoc}/nix-support
     echo "doc internal-api-docs $out/share/doc/nix/internal-api/html" >> ''${!outputDoc}/nix-support/hydra-build-products
+  ''
+    + lib.optionalString enableExternalAPIDocs ''
+    mkdir -p ''${!outputDoc}/nix-support
+    echo "doc external-api-docs $out/share/doc/nix/external-api/html" >> ''${!outputDoc}/nix-support/hydra-build-products
   '';
 
   # So the check output gets links for DLLs in the out output.

src/libexpr-c/local.mk

@@ -0,0 +1,25 @@
+libraries += libexprc
+
+libexprc_NAME = libnixexprc
+
+libexprc_DIR := $(d)
+
+libexprc_SOURCES := \
+  $(wildcard $(d)/*.cc) \
+
+# Not just for this library itself, but also for downstream libraries using this library
+
+INCLUDE_libexprc := -I $(d)
+libexprc_CXXFLAGS += $(INCLUDE_libutil) $(INCLUDE_libutilc) \
+                     $(INCLUDE_libfetchers) \
+                     $(INCLUDE_libstore) $(INCLUDE_libstorec) \
+                     $(INCLUDE_libexpr) $(INCLUDE_libexprc)
+
+libexprc_LIBS = libutil libutilc libstore libstorec libexpr
+
+libexprc_LDFLAGS += -pthread
+
+$(eval $(call install-file-in, $(d)/nix-expr-c.pc, $(libdir)/pkgconfig, 0644))
+
+libexprc_FORCE_INSTALL := 1
+

src/libexpr-c/nix-expr-c.pc.in

@@ -0,0 +1,10 @@
+prefix=@prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: Nix
+Description: Nix Language Evaluator - C API
+Version: @PACKAGE_VERSION@
+Requires: nix-store-c
+Libs: -L${libdir} -lnixexprc
+Cflags: -I${includedir}/nix

src/libexpr-c/nix_api_expr.cc

@@ -0,0 +1,178 @@
+#include <cstring>
+#include <iostream>
+#include <stdexcept>
+#include <string>
+
+#include "config.hh"
+#include "eval.hh"
+#include "globals.hh"
+#include "util.hh"
+
+#include "nix_api_expr.h"
+#include "nix_api_expr_internal.h"
+#include "nix_api_store.h"
+#include "nix_api_store_internal.h"
+#include "nix_api_util.h"
+#include "nix_api_util_internal.h"
+
+#ifdef HAVE_BOEHMGC
+#include <mutex>
+#define GC_INCLUDE_NEW 1
+#include "gc_cpp.h"
+#endif
+
+nix_err nix_libexpr_init(nix_c_context * context)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    {
+        auto ret = nix_libutil_init(context);
+        if (ret != NIX_OK)
+            return ret;
+    }
+    {
+        auto ret = nix_libstore_init(context);
+        if (ret != NIX_OK)
+            return ret;
+    }
+    try {
+        nix::initGC();
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_expr_eval_from_string(
+    nix_c_context * context, EvalState * state, const char * expr, const char * path, Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        nix::Expr * parsedExpr = state->state.parseExprFromString(expr, state->state.rootPath(nix::CanonPath(path)));
+        state->state.eval(parsedExpr, *(nix::Value *) value);
+        state->state.forceValue(*(nix::Value *) value, nix::noPos);
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_value_call(nix_c_context * context, EvalState * state, Value * fn, Value * arg, Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        state->state.callFunction(*(nix::Value *) fn, *(nix::Value *) arg, *(nix::Value *) value, nix::noPos);
+        state->state.forceValue(*(nix::Value *) value, nix::noPos);
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_value_force(nix_c_context * context, EvalState * state, Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        state->state.forceValue(*(nix::Value *) value, nix::noPos);
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_value_force_deep(nix_c_context * context, EvalState * state, Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        state->state.forceValueDeep(*(nix::Value *) value);
+    }
+    NIXC_CATCH_ERRS
+}
+
+EvalState * nix_state_create(nix_c_context * context, const char ** searchPath_c, Store * store)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        nix::Strings searchPath;
+        if (searchPath_c != nullptr)
+            for (size_t i = 0; searchPath_c[i] != nullptr; i++)
+                searchPath.push_back(searchPath_c[i]);
+
+        return new EvalState{nix::EvalState(nix::SearchPath::parse(searchPath), store->ptr)};
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+void nix_state_free(EvalState * state)
+{
+    delete state;
+}
+
+#ifdef HAVE_BOEHMGC
+std::unordered_map<
+    const void *,
+    unsigned int,
+    std::hash<const void *>,
+    std::equal_to<const void *>,
+    traceable_allocator<std::pair<const void * const, unsigned int>>>
+    nix_refcounts;
+
+std::mutex nix_refcount_lock;
+
+nix_err nix_gc_incref(nix_c_context * context, const void * p)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        std::scoped_lock lock(nix_refcount_lock);
+        auto f = nix_refcounts.find(p);
+        if (f != nix_refcounts.end()) {
+            f->second++;
+        } else {
+            nix_refcounts[p] = 1;
+        }
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_gc_decref(nix_c_context * context, const void * p)
+{
+
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        std::scoped_lock lock(nix_refcount_lock);
+        auto f = nix_refcounts.find(p);
+        if (f != nix_refcounts.end()) {
+            if (--f->second == 0)
+                nix_refcounts.erase(f);
+        } else
+            throw std::runtime_error("nix_gc_decref: object was not referenced");
+    }
+    NIXC_CATCH_ERRS
+}
+
+void nix_gc_now()
+{
+    GC_gcollect();
+}
+
+#else
+void nix_gc_incref(nix_c_context * context, const void *)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    return NIX_OK;
+}
+void nix_gc_decref(nix_c_context * context, const void *)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    return NIX_OK;
+}
+void nix_gc_now() {}
+#endif
+
+void nix_gc_register_finalizer(void * obj, void * cd, void (*finalizer)(void * obj, void * cd))
+{
+#ifdef HAVE_BOEHMGC
+    GC_REGISTER_FINALIZER(obj, finalizer, cd, 0, 0);
+#endif
+}

src/libexpr-c/nix_api_expr.h

@@ -0,0 +1,213 @@
+#ifndef NIX_API_EXPR_H
+#define NIX_API_EXPR_H
+/** @defgroup libexpr libexpr
+ * @brief Bindings to the Nix language evaluator
+ *
+ * Example (without error handling):
+ * @code{.c}
+ * int main() {
+ *    nix_libexpr_init(NULL);
+ *
+ *    Store* store = nix_store_open(NULL, "dummy", NULL);
+ *    EvalState* state = nix_state_create(NULL, NULL, store); // empty nix path
+ *    Value *value = nix_alloc_value(NULL, state);
+ *
+ *    nix_expr_eval_from_string(NULL, state, "builtins.nixVersion", ".", value);
+ *    nix_value_force(NULL, state, value);
+ *    printf("nix version: %s\n", nix_get_string(NULL, value));
+ *
+ *    nix_gc_decref(NULL, value);
+ *    nix_state_free(state);
+ *    nix_store_free(store);
+ *    return 0;
+ *    }
+ * @endcode
+ * @{
+ */
+/** @file
+ * @brief Main entry for the libexpr C bindings
+ */
+
+#include "nix_api_store.h"
+#include "nix_api_util.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+// cffi start
+
+// Type definitions
+/**
+ * @brief Represents a state of the Nix language evaluator.
+ *
+ * Multiple states can be created for multi-threaded
+ * operation.
+ * @struct EvalState
+ * @see nix_state_create
+ */
+typedef struct EvalState EvalState; // nix::EvalState
+/**
+ * @brief Represents a value in the Nix language.
+ *
+ * Owned by the garbage collector.
+ * @struct Value
+ * @see value_manip
+ */
+typedef void Value; // nix::Value
+
+// Function prototypes
+/**
+ * @brief Initialize the Nix language evaluator.
+ *
+ * This function must be called at least once,
+ * at some point before constructing a EvalState for the first time.
+ * This function can be called multiple times, and is idempotent.
+ *
+ * @param[out] context Optional, stores error information
+ * @return NIX_OK if the initialization was successful, an error code otherwise.
+ */
+nix_err nix_libexpr_init(nix_c_context * context);
+
+/**
+ * @brief Parses and evaluates a Nix expression from a string.
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] state The state of the evaluation.
+ * @param[in] expr The Nix expression to parse.
+ * @param[in] path The file path to associate with the expression.
+ * This is required for expressions that contain relative paths (such as `./.`) that are resolved relative to the given
+ * directory.
+ * @param[out] value The result of the evaluation. You must allocate this
+ * yourself.
+ * @return NIX_OK if the evaluation was successful, an error code otherwise.
+ */
+nix_err nix_expr_eval_from_string(
+    nix_c_context * context, EvalState * state, const char * expr, const char * path, Value * value);
+
+/**
+ * @brief Calls a Nix function with an argument.
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] state The state of the evaluation.
+ * @param[in] fn The Nix function to call.
+ * @param[in] arg The argument to pass to the function.
+ * @param[out] value The result of the function call.
+ * @return NIX_OK if the function call was successful, an error code otherwise.
+ */
+nix_err nix_value_call(nix_c_context * context, EvalState * state, Value * fn, Value * arg, Value * value);
+
+/**
+ * @brief Forces the evaluation of a Nix value.
+ *
+ * The Nix interpreter is lazy, and not-yet-evaluated Values can be
+ * of type NIX_TYPE_THUNK instead of their actual value.
+ *
+ * This function converts these Values into their final type.
+ *
+ * @note You don't need this function for basic API usage, since all functions
+ * that return a value call it for you. The only place you will see a
+ * NIX_TYPE_THUNK is in the arguments that are passed to a PrimOp function
+ * you supplied to nix_alloc_primop.
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] state The state of the evaluation.
+ * @param[in,out] value The Nix value to force.
+ * @post value is not of type NIX_TYPE_THUNK
+ * @return NIX_OK if the force operation was successful, an error code
+ * otherwise.
+ */
+nix_err nix_value_force(nix_c_context * context, EvalState * state, Value * value);
+
+/**
+ * @brief Forces the deep evaluation of a Nix value.
+ *
+ * Recursively calls nix_value_force
+ *
+ * @see nix_value_force
+ * @warning Calling this function on a recursive data structure will cause a
+ * stack overflow.
+ * @param[out] context Optional, stores error information
+ * @param[in] state The state of the evaluation.
+ * @param[in,out] value The Nix value to force.
+ * @return NIX_OK if the deep force operation was successful, an error code
+ * otherwise.
+ */
+nix_err nix_value_force_deep(nix_c_context * context, EvalState * state, Value * value);
+
+/**
+ * @brief Create a new Nix language evaluator state.
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] searchPath Array of strings corresponding to entries in NIX_PATH.
+ * @param[in] store The Nix store to use.
+ * @return A new Nix state or NULL on failure.
+ */
+EvalState * nix_state_create(nix_c_context * context, const char ** searchPath, Store * store);
+
+/**
+ * @brief Frees a Nix state.
+ *
+ * Does not fail.
+ *
+ * @param[in] state The state to free.
+ */
+void nix_state_free(EvalState * state);
+
+/** @addtogroup GC
+ * @brief Reference counting and garbage collector operations
+ *
+ * The Nix language evaluator uses a garbage collector. To ease C interop, we implement
+ * a reference counting scheme, where objects will be deallocated
+ * when there are no references from the Nix side, and the reference count kept
+ * by the C API reaches `0`.
+ *
+ * Functions returning a garbage-collected object will automatically increase
+ * the refcount for you. You should make sure to call `nix_gc_decref` when
+ * you're done with a value returned by the evaluator.
+ * @{
+ */
+/**
+ * @brief Increment the garbage collector reference counter for the given object.
+ *
+ * The Nix language evaluator C API keeps track of alive objects by reference counting.
+ * When you're done with a refcounted pointer, call nix_gc_decref().
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] object The object to keep alive
+ */
+nix_err nix_gc_incref(nix_c_context * context, const void * object);
+/**
+ * @brief Decrement the garbage collector reference counter for the given object
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] object The object to stop referencing
+ */
+nix_err nix_gc_decref(nix_c_context * context, const void * object);
+
+/**
+ * @brief Trigger the garbage collector manually
+ *
+ * You should not need to do this, but it can be useful for debugging.
+ */
+void nix_gc_now();
+
+/**
+ * @brief Register a callback that gets called when the object is garbage
+ * collected.
+ * @note Objects can only have a single finalizer. This function overwrites existing values
+ * silently.
+ * @param[in] obj the object to watch
+ * @param[in] cd the data to pass to the finalizer
+ * @param[in] finalizer the callback function, called with obj and cd
+ */
+void nix_gc_register_finalizer(void * obj, void * cd, void (*finalizer)(void * obj, void * cd));
+
+/** @} */
+// cffi end
+#ifdef __cplusplus
+}
+#endif
+
+/** @} */
+
+#endif // NIX_API_EXPR_H

src/libexpr-c/nix_api_expr_internal.h

@@ -0,0 +1,38 @@
+#ifndef NIX_API_EXPR_INTERNAL_H
+#define NIX_API_EXPR_INTERNAL_H
+
+#include "eval.hh"
+#include "attr-set.hh"
+#include "nix_api_value.h"
+
+struct EvalState
+{
+    nix::EvalState state;
+};
+
+struct BindingsBuilder
+{
+    nix::BindingsBuilder builder;
+};
+
+struct ListBuilder
+{
+    nix::ListBuilder builder;
+};
+
+struct nix_string_return
+{
+    std::string str;
+};
+
+struct nix_printer
+{
+    std::ostream & s;
+};
+
+struct nix_string_context
+{
+    nix::NixStringContext & ctx;
+};
+
+#endif // NIX_API_EXPR_INTERNAL_H

src/libexpr-c/nix_api_external.cc

@@ -0,0 +1,198 @@
+#include "attr-set.hh"
+#include "config.hh"
+#include "eval.hh"
+#include "gc/gc.h"
+#include "globals.hh"
+#include "value.hh"
+
+#include "nix_api_expr.h"
+#include "nix_api_expr_internal.h"
+#include "nix_api_external.h"
+#include "nix_api_util.h"
+#include "nix_api_util_internal.h"
+#include "nix_api_value.h"
+#include "value/context.hh"
+
+#include <nlohmann/json.hpp>
+
+#ifdef HAVE_BOEHMGC
+#define GC_INCLUDE_NEW 1
+#include "gc_cpp.h"
+#endif
+
+void nix_set_string_return(nix_string_return * str, const char * c)
+{
+    str->str = c;
+}
+
+nix_err nix_external_print(nix_c_context * context, nix_printer * printer, const char * c)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        printer->s << c;
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_external_add_string_context(nix_c_context * context, nix_string_context * ctx, const char * c)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto r = nix::NixStringContextElem::parse(c);
+        ctx->ctx.insert(r);
+    }
+    NIXC_CATCH_ERRS
+}
+
+class NixCExternalValue : public nix::ExternalValueBase
+{
+    NixCExternalValueDesc & desc;
+    void * v;
+
+public:
+    NixCExternalValue(NixCExternalValueDesc & desc, void * v)
+        : desc(desc)
+        , v(v){};
+    void * get_ptr()
+    {
+        return v;
+    }
+    /**
+     * Print out the value
+     */
+    virtual std::ostream & print(std::ostream & str) const override
+    {
+        nix_printer p{str};
+        desc.print(v, &p);
+        return str;
+    }
+
+    /**
+     * Return a simple string describing the type
+     */
+    virtual std::string showType() const override
+    {
+        nix_string_return res;
+        desc.showType(v, &res);
+        return std::move(res.str);
+    }
+
+    /**
+     * Return a string to be used in builtins.typeOf
+     */
+    virtual std::string typeOf() const override
+    {
+        nix_string_return res;
+        desc.typeOf(v, &res);
+        return std::move(res.str);
+    }
+
+    /**
+     * Coerce the value to a string.
+     */
+    virtual std::string coerceToString(
+        nix::EvalState & state,
+        const nix::PosIdx & pos,
+        nix::NixStringContext & context,
+        bool copyMore,
+        bool copyToStore) const override
+    {
+        if (!desc.coerceToString) {
+            return nix::ExternalValueBase::coerceToString(state, pos, context, copyMore, copyToStore);
+        }
+        nix_string_context ctx{context};
+        nix_string_return res{""};
+        // todo: pos, errors
+        desc.coerceToString(v, &ctx, copyMore, copyToStore, &res);
+        if (res.str.empty()) {
+            return nix::ExternalValueBase::coerceToString(state, pos, context, copyMore, copyToStore);
+        }
+        return std::move(res.str);
+    }
+
+    /**
+     * Compare to another value of the same type.
+     */
+    virtual bool operator==(const ExternalValueBase & b) const override
+    {
+        if (!desc.equal) {
+            return false;
+        }
+        auto r = dynamic_cast<const NixCExternalValue *>(&b);
+        if (!r)
+            return false;
+        return desc.equal(v, r->v);
+    }
+
+    /**
+     * Print the value as JSON.
+     */
+    virtual nlohmann::json printValueAsJSON(
+        nix::EvalState & state, bool strict, nix::NixStringContext & context, bool copyToStore = true) const override
+    {
+        if (!desc.printValueAsJSON) {
+            return nix::ExternalValueBase::printValueAsJSON(state, strict, context, copyToStore);
+        }
+        nix_string_context ctx{context};
+        nix_string_return res{""};
+        desc.printValueAsJSON(v, (EvalState *) &state, strict, &ctx, copyToStore, &res);
+        if (res.str.empty()) {
+            return nix::ExternalValueBase::printValueAsJSON(state, strict, context, copyToStore);
+        }
+        return nlohmann::json::parse(res.str);
+    }
+
+    /**
+     * Print the value as XML.
+     */
+    virtual void printValueAsXML(
+        nix::EvalState & state,
+        bool strict,
+        bool location,
+        nix::XMLWriter & doc,
+        nix::NixStringContext & context,
+        nix::PathSet & drvsSeen,
+        const nix::PosIdx pos) const override
+    {
+        if (!desc.printValueAsXML) {
+            return nix::ExternalValueBase::printValueAsXML(state, strict, location, doc, context, drvsSeen, pos);
+        }
+        nix_string_context ctx{context};
+        desc.printValueAsXML(
+            v, (EvalState *) &state, strict, location, &doc, &ctx, &drvsSeen,
+            *reinterpret_cast<const uint32_t *>(&pos));
+    }
+
+    virtual ~NixCExternalValue() override{};
+};
+
+ExternalValue * nix_create_external_value(nix_c_context * context, NixCExternalValueDesc * desc, void * v)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto ret = new
+#ifdef HAVE_BOEHMGC
+            (GC)
+#endif
+                NixCExternalValue(*desc, v);
+        nix_gc_incref(nullptr, ret);
+        return (ExternalValue *) ret;
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+void * nix_get_external_value_content(nix_c_context * context, ExternalValue * b)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto r = dynamic_cast<NixCExternalValue *>((nix::ExternalValueBase *) b);
+        if (r)
+            return r->get_ptr();
+        return nullptr;
+    }
+    NIXC_CATCH_ERRS_NULL
+}

src/libexpr-c/nix_api_external.h

@@ -0,0 +1,196 @@
+#ifndef NIX_API_EXTERNAL_H
+#define NIX_API_EXTERNAL_H
+/** @ingroup libexpr
+ * @addtogroup Externals
+ * @brief Deal with external values
+ * @{
+ */
+/** @file
+ * @brief libexpr C bindings dealing with external values
+ */
+
+#include "nix_api_expr.h"
+#include "nix_api_util.h"
+#include "nix_api_value.h"
+#include "stdbool.h"
+#include "stddef.h"
+#include "stdint.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+// cffi start
+
+/**
+ * @brief Represents a string owned by the Nix language evaluator.
+ * @see nix_set_owned_string
+ */
+typedef struct nix_string_return nix_string_return;
+/**
+ * @brief Wraps a stream that can output multiple string pieces.
+ */
+typedef struct nix_printer nix_printer;
+/**
+ * @brief A list of string context items
+ */
+typedef struct nix_string_context nix_string_context;
+
+/**
+ * @brief Sets the contents of a nix_string_return
+ *
+ * Copies the passed string.
+ * @param[out] str the nix_string_return to write to
+ * @param[in]  c   The string to copy
+ */
+void nix_set_string_return(nix_string_return * str, const char * c);
+
+/**
+ * Print to the nix_printer
+ *
+ * @param[out] context Optional, stores error information
+ * @param printer The nix_printer to print to
+ * @param[in] str The string to print
+ * @returns NIX_OK if everything worked
+ */
+nix_err nix_external_print(nix_c_context * context, nix_printer * printer, const char * str);
+
+/**
+ * Add string context to the nix_string_context object
+ * @param[out] context Optional, stores error information
+ * @param[out] string_context The nix_string_context to add to
+ * @param[in] c The context string to add
+ * @returns NIX_OK if everything worked
+ */
+nix_err nix_external_add_string_context(nix_c_context * context, nix_string_context * string_context, const char * c);
+
+/**
+ * @brief Definition for a class of external values
+ *
+ * Create and implement one of these, then pass it to nix_create_external_value
+ * Make sure to keep it alive while the external value lives.
+ *
+ * Optional functions can be set to NULL
+ *
+ * @see nix_create_external_value
+ */
+typedef struct NixCExternalValueDesc
+{
+    /**
+     * @brief Called when printing the external value
+     *
+     * @param[in] self the void* passed to nix_create_external_value
+     * @param[out] printer The printer to print to, pass to nix_external_print
+     */
+    void (*print)(void * self, nix_printer * printer);
+    /**
+     * @brief Called on :t
+     * @param[in] self the void* passed to nix_create_external_value
+     * @param[out] res the return value
+     */
+    void (*showType)(void * self, nix_string_return * res);
+    /**
+     * @brief Called on `builtins.typeOf`
+     * @param self the void* passed to nix_create_external_value
+     * @param[out] res the return value
+     */
+    void (*typeOf)(void * self, nix_string_return * res);
+    /**
+     * @brief Called on "${str}" and builtins.toString.
+     *
+     * The latter with coerceMore=true
+     * Optional, the default is to throw an error.
+     * @param[in] self the void* passed to nix_create_external_value
+     * @param[out] c writable string context for the resulting string
+     * @param[in] coerceMore boolean, try to coerce to strings in more cases
+     * instead of throwing an error
+     * @param[in] copyToStore boolean, whether to copy referenced paths to store
+     * or keep them as-is
+     * @param[out] res the return value. Not touching this, or setting it to the
+     * empty string, will make the conversion throw an error.
+     */
+    void (*coerceToString)(
+        void * self, nix_string_context * c, int coerceMore, int copyToStore, nix_string_return * res);
+    /**
+     * @brief Try to compare two external values
+     *
+     * Optional, the default is always false.
+     * If the other object was not a Nix C external value, this comparison will
+     * also return false
+     * @param[in] self the void* passed to nix_create_external_value
+     * @param[in] other the void* passed to the other object's
+     * nix_create_external_value
+     * @returns true if the objects are deemed to be equal
+     */
+    int (*equal)(void * self, void * other);
+    /**
+     * @brief Convert the external value to json
+     *
+     * Optional, the default is to throw an error
+     * @param[in] self the void* passed to nix_create_external_value
+     * @param[in] state The evaluator state
+     * @param[in] strict boolean Whether to force the value before printing
+     * @param[out] c writable string context for the resulting string
+     * @param[in] copyToStore whether to copy referenced paths to store or keep
+     * them as-is
+     * @param[out] res the return value. Gets parsed as JSON. Not touching this,
+     * or setting it to the empty string, will make the conversion throw an error.
+     */
+    void (*printValueAsJSON)(
+        void * self, EvalState *, bool strict, nix_string_context * c, bool copyToStore, nix_string_return * res);
+    /**
+     * @brief Convert the external value to XML
+     *
+     * Optional, the default is to throw an error
+     * @todo The mechanisms for this call are incomplete. There are no C
+     *       bindings to work with XML, pathsets and positions.
+     * @param[in] self the void* passed to nix_create_external_value
+     * @param[in] state The evaluator state
+     * @param[in] strict boolean Whether to force the value before printing
+     * @param[in] location boolean Whether to include position information in the
+     * xml
+     * @param[out] doc XML document to output to
+     * @param[out] c writable string context for the resulting string
+     * @param[in,out] drvsSeen a path set to avoid duplicating derivations
+     * @param[in] pos The position of the call.
+     */
+    void (*printValueAsXML)(
+        void * self,
+        EvalState *,
+        int strict,
+        int location,
+        void * doc,
+        nix_string_context * c,
+        void * drvsSeen,
+        int pos);
+} NixCExternalValueDesc;
+
+/**
+ * @brief Create an external value, that can be given to nix_init_external
+ *
+ * Owned by the GC. Use nix_gc_decref when you're done with the pointer.
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] desc a NixCExternalValueDesc, you should keep this alive as long
+ * as the ExternalValue lives
+ * @param[in] v the value to store
+ * @returns external value, owned by the garbage collector
+ * @see nix_init_external
+ */
+ExternalValue * nix_create_external_value(nix_c_context * context, NixCExternalValueDesc * desc, void * v);
+
+/**
+ * @brief Extract the pointer from a nix c external value.
+ * @param[out] context Optional, stores error information
+ * @param[in] b The external value
+ * @returns The pointer, or null if the external value was not from nix c.
+ * @see nix_get_external
+ */
+void * nix_get_external_value_content(nix_c_context * context, ExternalValue * b);
+
+// cffi end
+#ifdef __cplusplus
+}
+#endif
+/** @} */
+
+#endif // NIX_API_EXTERNAL_H

src/libexpr-c/nix_api_value.cc

@@ -0,0 +1,530 @@
+#include "attr-set.hh"
+#include "config.hh"
+#include "eval.hh"
+#include "gc/gc.h"
+#include "globals.hh"
+#include "primops.hh"
+#include "value.hh"
+
+#include "nix_api_expr.h"
+#include "nix_api_expr_internal.h"
+#include "nix_api_util.h"
+#include "nix_api_util_internal.h"
+#include "nix_api_value.h"
+
+#ifdef HAVE_BOEHMGC
+#define GC_INCLUDE_NEW 1
+#include "gc_cpp.h"
+#endif
+
+// Helper function to throw an exception if value is null
+static const nix::Value & check_value_not_null(const Value * value)
+{
+    if (!value) {
+        throw std::runtime_error("Value is null");
+    }
+    return *((const nix::Value *) value);
+}
+
+static nix::Value & check_value_not_null(Value * value)
+{
+    if (!value) {
+        throw std::runtime_error("Value is null");
+    }
+    return *((nix::Value *) value);
+}
+
+/**
+ * Helper function to convert calls from nix into C API.
+ *
+ * Deals with errors and converts arguments from C++ into C types.
+ */
+static void nix_c_primop_wrapper(
+    PrimOpFun f, void * userdata, nix::EvalState & state, const nix::PosIdx pos, nix::Value ** args, nix::Value & v)
+{
+    nix_c_context ctx;
+    f(userdata, &ctx, (EvalState *) &state, (Value **) args, (Value *) &v);
+    /* TODO: In the future, this should throw different errors depending on the error code */
+    if (ctx.last_err_code != NIX_OK)
+        state.error<nix::EvalError>("Error from builtin function: %s", *ctx.last_err).atPos(pos).debugThrow();
+}
+
+PrimOp * nix_alloc_primop(
+    nix_c_context * context,
+    PrimOpFun fun,
+    int arity,
+    const char * name,
+    const char ** args,
+    const char * doc,
+    void * user_data)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        using namespace std::placeholders;
+        auto p = new
+#ifdef HAVE_BOEHMGC
+            (GC)
+#endif
+                nix::PrimOp{
+                    .name = name,
+                    .args = {},
+                    .arity = (size_t) arity,
+                    .doc = doc,
+                    .fun = std::bind(nix_c_primop_wrapper, fun, user_data, _1, _2, _3, _4)};
+        if (args)
+            for (size_t i = 0; args[i]; i++)
+                p->args.emplace_back(*args);
+        nix_gc_incref(nullptr, p);
+        return (PrimOp *) p;
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+nix_err nix_register_primop(nix_c_context * context, PrimOp * primOp)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        nix::RegisterPrimOp r(std::move(*((nix::PrimOp *) primOp)));
+    }
+    NIXC_CATCH_ERRS
+}
+
+Value * nix_alloc_value(nix_c_context * context, EvalState * state)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        Value * res = state->state.allocValue();
+        nix_gc_incref(nullptr, res);
+        return res;
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+ValueType nix_get_type(nix_c_context * context, const Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        using namespace nix;
+        switch (v.type()) {
+        case nThunk:
+            return NIX_TYPE_THUNK;
+        case nInt:
+            return NIX_TYPE_INT;
+        case nFloat:
+            return NIX_TYPE_FLOAT;
+        case nBool:
+            return NIX_TYPE_BOOL;
+        case nString:
+            return NIX_TYPE_STRING;
+        case nPath:
+            return NIX_TYPE_PATH;
+        case nNull:
+            return NIX_TYPE_NULL;
+        case nAttrs:
+            return NIX_TYPE_ATTRS;
+        case nList:
+            return NIX_TYPE_LIST;
+        case nFunction:
+            return NIX_TYPE_FUNCTION;
+        case nExternal:
+            return NIX_TYPE_EXTERNAL;
+        }
+        return NIX_TYPE_NULL;
+    }
+    NIXC_CATCH_ERRS_RES(NIX_TYPE_NULL);
+}
+
+const char * nix_get_typename(nix_c_context * context, const Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        auto s = nix::showType(v);
+        return strdup(s.c_str());
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+bool nix_get_bool(nix_c_context * context, const Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        assert(v.type() == nix::nBool);
+        return v.boolean;
+    }
+    NIXC_CATCH_ERRS_RES(false);
+}
+
+const char * nix_get_string(nix_c_context * context, const Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        assert(v.type() == nix::nString);
+        return v.c_str();
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+const char * nix_get_path_string(nix_c_context * context, const Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        assert(v.type() == nix::nPath);
+        // NOTE (from @yorickvP)
+        // v._path.path should work but may not be how Eelco intended it.
+        // Long-term this function should be rewritten to copy some data into a
+        // user-allocated string.
+        // We could use v.path().to_string().c_str(), but I'm concerned this
+        // crashes. Looks like .path() allocates a CanonPath with a copy of the
+        // string, then it gets the underlying data from that.
+        return v._path.path;
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+unsigned int nix_get_list_size(nix_c_context * context, const Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        assert(v.type() == nix::nList);
+        return v.listSize();
+    }
+    NIXC_CATCH_ERRS_RES(0);
+}
+
+unsigned int nix_get_attrs_size(nix_c_context * context, const Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        assert(v.type() == nix::nAttrs);
+        return v.attrs->size();
+    }
+    NIXC_CATCH_ERRS_RES(0);
+}
+
+double nix_get_float(nix_c_context * context, const Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        assert(v.type() == nix::nFloat);
+        return v.fpoint;
+    }
+    NIXC_CATCH_ERRS_RES(0.0);
+}
+
+int64_t nix_get_int(nix_c_context * context, const Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        assert(v.type() == nix::nInt);
+        return v.integer;
+    }
+    NIXC_CATCH_ERRS_RES(0);
+}
+
+ExternalValue * nix_get_external(nix_c_context * context, Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        assert(v.type() == nix::nExternal);
+        return (ExternalValue *) v.external;
+    }
+    NIXC_CATCH_ERRS_NULL;
+}
+
+Value * nix_get_list_byidx(nix_c_context * context, const Value * value, EvalState * state, unsigned int ix)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        assert(v.type() == nix::nList);
+        auto * p = v.listElems()[ix];
+        nix_gc_incref(nullptr, p);
+        if (p != nullptr)
+            state->state.forceValue(*p, nix::noPos);
+        return (Value *) p;
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+Value * nix_get_attr_byname(nix_c_context * context, const Value * value, EvalState * state, const char * name)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        assert(v.type() == nix::nAttrs);
+        nix::Symbol s = state->state.symbols.create(name);
+        auto attr = v.attrs->get(s);
+        if (attr) {
+            nix_gc_incref(nullptr, attr->value);
+            state->state.forceValue(*attr->value, nix::noPos);
+            return attr->value;
+        }
+        nix_set_err_msg(context, NIX_ERR_KEY, "missing attribute");
+        return nullptr;
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+bool nix_has_attr_byname(nix_c_context * context, const Value * value, EvalState * state, const char * name)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        assert(v.type() == nix::nAttrs);
+        nix::Symbol s = state->state.symbols.create(name);
+        auto attr = v.attrs->get(s);
+        if (attr)
+            return true;
+        return false;
+    }
+    NIXC_CATCH_ERRS_RES(false);
+}
+
+Value *
+nix_get_attr_byidx(nix_c_context * context, const Value * value, EvalState * state, unsigned int i, const char ** name)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        const nix::Attr & a = (*v.attrs)[i];
+        *name = ((const std::string &) (state->state.symbols[a.name])).c_str();
+        nix_gc_incref(nullptr, a.value);
+        state->state.forceValue(*a.value, nix::noPos);
+        return a.value;
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+const char * nix_get_attr_name_byidx(nix_c_context * context, const Value * value, EvalState * state, unsigned int i)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        const nix::Attr & a = (*v.attrs)[i];
+        return ((const std::string &) (state->state.symbols[a.name])).c_str();
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+nix_err nix_init_bool(nix_c_context * context, Value * value, bool b)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        v.mkBool(b);
+    }
+    NIXC_CATCH_ERRS
+}
+
+// todo string context
+nix_err nix_init_string(nix_c_context * context, Value * value, const char * str)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        v.mkString(std::string_view(str));
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_init_path_string(nix_c_context * context, EvalState * s, Value * value, const char * str)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        v.mkPath(s->state.rootPath(nix::CanonPath(str)));
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_init_float(nix_c_context * context, Value * value, double d)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        v.mkFloat(d);
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_init_int(nix_c_context * context, Value * value, int64_t i)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        v.mkInt(i);
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_init_null(nix_c_context * context, Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        v.mkNull();
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_init_external(nix_c_context * context, Value * value, ExternalValue * val)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        auto r = (nix::ExternalValueBase *) val;
+        v.mkExternal(r);
+    }
+    NIXC_CATCH_ERRS
+}
+
+ListBuilder * nix_make_list_builder(nix_c_context * context, EvalState * state, size_t capacity)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto builder = state->state.buildList(capacity);
+        return new
+#if HAVE_BOEHMGC
+            (NoGC)
+#endif
+                ListBuilder{std::move(builder)};
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+nix_err nix_list_builder_insert(nix_c_context * context, ListBuilder * list_builder, unsigned int index, Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & e = check_value_not_null(value);
+        list_builder->builder[index] = &e;
+    }
+    NIXC_CATCH_ERRS
+}
+
+void nix_list_builder_free(ListBuilder * list_builder)
+{
+#if HAVE_BOEHMGC
+    GC_FREE(list_builder);
+#else
+    delete bb;
+#endif
+}
+
+nix_err nix_make_list(nix_c_context * context, ListBuilder * list_builder, Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        v.mkList(list_builder->builder);
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_init_primop(nix_c_context * context, Value * value, PrimOp * p)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        v.mkPrimOp((nix::PrimOp *) p);
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_copy_value(nix_c_context * context, Value * value, Value * source)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        auto & s = check_value_not_null(source);
+        v = s;
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_make_attrs(nix_c_context * context, Value * value, BindingsBuilder * b)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        v.mkAttrs(b->builder);
+    }
+    NIXC_CATCH_ERRS
+}
+
+BindingsBuilder * nix_make_bindings_builder(nix_c_context * context, EvalState * state, size_t capacity)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto bb = state->state.buildBindings(capacity);
+        return new
+#if HAVE_BOEHMGC
+            (NoGC)
+#endif
+                BindingsBuilder{std::move(bb)};
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+nix_err nix_bindings_builder_insert(nix_c_context * context, BindingsBuilder * bb, const char * name, Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        nix::Symbol s = bb->builder.state.symbols.create(name);
+        bb->builder.insert(s, &v);
+    }
+    NIXC_CATCH_ERRS
+}
+
+void nix_bindings_builder_free(BindingsBuilder * bb)
+{
+#if HAVE_BOEHMGC
+    GC_FREE((nix::BindingsBuilder *) bb);
+#else
+    delete (nix::BindingsBuilder *) bb;
+#endif
+}

src/libexpr-c/nix_api_value.h

@@ -0,0 +1,434 @@
+#ifndef NIX_API_VALUE_H
+#define NIX_API_VALUE_H
+
+/** @addtogroup libexpr
+ * @{
+ */
+/** @file
+ * @brief libexpr C bindings dealing with values
+ */
+
+#include "nix_api_util.h"
+#include "stdbool.h"
+#include "stddef.h"
+#include "stdint.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+// cffi start
+
+// Type definitions
+typedef enum {
+    NIX_TYPE_THUNK,
+    NIX_TYPE_INT,
+    NIX_TYPE_FLOAT,
+    NIX_TYPE_BOOL,
+    NIX_TYPE_STRING,
+    NIX_TYPE_PATH,
+    NIX_TYPE_NULL,
+    NIX_TYPE_ATTRS,
+    NIX_TYPE_LIST,
+    NIX_TYPE_FUNCTION,
+    NIX_TYPE_EXTERNAL
+} ValueType;
+
+// forward declarations
+typedef void Value;
+typedef struct EvalState EvalState;
+// type defs
+/** @brief Stores an under-construction set of bindings
+ * @ingroup value_manip
+ *
+ * Do not reuse.
+ * @see nix_make_bindings_builder, nix_bindings_builder_free, nix_make_attrs
+ * @see nix_bindings_builder_insert
+ */
+typedef struct BindingsBuilder BindingsBuilder;
+
+/** @brief Stores an under-construction list
+ * @ingroup value_manip
+ *
+ * Do not reuse.
+ * @see nix_make_list_builder, nix_list_builder_free, nix_make_list
+ * @see nix_list_builder_insert
+ */
+typedef struct ListBuilder ListBuilder;
+
+/** @brief PrimOp function
+ * @ingroup primops
+ *
+ * Owned by the GC
+ * @see nix_alloc_primop, nix_init_primop
+ */
+typedef struct PrimOp PrimOp;
+/** @brief External Value
+ * @ingroup Externals
+ *
+ * Owned by the GC
+ */
+typedef struct ExternalValue ExternalValue;
+
+/** @defgroup primops
+ * @brief Create your own primops
+ * @{
+ */
+/** @brief Function pointer for primops
+ * When you want to return an error, call nix_set_err_msg(context, NIX_ERR_UNKNOWN, "your error message here").
+ *
+ * @param[in] user_data Arbitrary data that was initially supplied to nix_alloc_primop
+ * @param[out] context Stores error information.
+ * @param[in] state Evaluator state
+ * @param[in] args list of arguments. Note that these can be thunks and should be forced using nix_value_force before
+ * use.
+ * @param[out] ret return value
+ * @see nix_alloc_primop, nix_init_primop
+ */
+typedef void (*PrimOpFun)(void * user_data, nix_c_context * context, EvalState * state, Value ** args, Value * ret);
+
+/** @brief Allocate a PrimOp
+ *
+ * Owned by the garbage collector.
+ * Use nix_gc_decref() when you're done with the returned PrimOp.
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] fun callback
+ * @param[in] arity expected number of function arguments
+ * @param[in] name function name
+ * @param[in] args array of argument names, NULL-terminated
+ * @param[in] doc optional, documentation for this primop
+ * @param[in] user_data optional, arbitrary data, passed to the callback when it's called
+ * @return primop, or null in case of errors
+ * @see nix_init_primop
+ */
+PrimOp * nix_alloc_primop(
+    nix_c_context * context,
+    PrimOpFun fun,
+    int arity,
+    const char * name,
+    const char ** args,
+    const char * doc,
+    void * user_data);
+
+/** @brief add a primop to the `builtins` attribute set
+ *
+ * Only applies to States created after this call.
+ *
+ * Moves your PrimOp content into the global evaluator
+ * registry, meaning your input PrimOp pointer is no longer usable.
+ * You are free to remove your references to it,
+ * after which it will be garbage collected.
+ *
+ * @param[out] context Optional, stores error information
+ * @return primop, or null in case of errors
+ *
+ */
+nix_err nix_register_primop(nix_c_context * context, PrimOp * primOp);
+/** @} */
+
+// Function prototypes
+
+/** @brief Allocate a Nix value
+ *
+ * Owned by the GC. Use nix_gc_decref() when you're done with the pointer
+ * @param[out] context Optional, stores error information
+ * @param[in] state nix evaluator state
+ * @return value, or null in case of errors
+ *
+ */
+Value * nix_alloc_value(nix_c_context * context, EvalState * state);
+
+/** @addtogroup value_manip Manipulating values
+ * @brief Functions to inspect and change Nix language values, represented by Value.
+ * @{
+ */
+/** @name Getters
+ */
+/**@{*/
+/** @brief Get value type
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @return type of nix value
+ */
+ValueType nix_get_type(nix_c_context * context, const Value * value);
+
+/** @brief Get type name of value as defined in the evaluator
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @return type name, owned string
+ * @todo way to free the result
+ */
+const char * nix_get_typename(nix_c_context * context, const Value * value);
+
+/** @brief Get boolean value
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @return true or false, error info via context
+ */
+bool nix_get_bool(nix_c_context * context, const Value * value);
+
+/** @brief Get string
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @return string
+ * @return NULL in case of error.
+ */
+const char * nix_get_string(nix_c_context * context, const Value * value);
+
+/** @brief Get path as string
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @return string
+ * @return NULL in case of error.
+ */
+const char * nix_get_path_string(nix_c_context * context, const Value * value);
+
+/** @brief Get the length of a list
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @return length of list, error info via context
+ */
+unsigned int nix_get_list_size(nix_c_context * context, const Value * value);
+
+/** @brief Get the element count of an attrset
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @return attrset element count, error info via context
+ */
+unsigned int nix_get_attrs_size(nix_c_context * context, const Value * value);
+
+/** @brief Get float value in 64 bits
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @return float contents, error info via context
+ */
+double nix_get_float(nix_c_context * context, const Value * value);
+
+/** @brief Get int value
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @return int contents, error info via context
+ */
+int64_t nix_get_int(nix_c_context * context, const Value * value);
+
+/** @brief Get external reference
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @return reference to external, NULL in case of error
+ */
+ExternalValue * nix_get_external(nix_c_context * context, Value *);
+
+/** @brief Get the ix'th element of a list
+ *
+ * Owned by the GC. Use nix_gc_decref when you're done with the pointer
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @param[in] state nix evaluator state
+ * @param[in] ix list element to get
+ * @return value, NULL in case of errors
+ */
+Value * nix_get_list_byidx(nix_c_context * context, const Value * value, EvalState * state, unsigned int ix);
+
+/** @brief Get an attr by name
+ *
+ * Owned by the GC. Use nix_gc_decref when you're done with the pointer
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @param[in] state nix evaluator state
+ * @param[in] name attribute name
+ * @return value, NULL in case of errors
+ */
+Value * nix_get_attr_byname(nix_c_context * context, const Value * value, EvalState * state, const char * name);
+
+/** @brief Check if an attribute name exists on a value
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @param[in] state nix evaluator state
+ * @param[in] name attribute name
+ * @return value, error info via context
+ */
+bool nix_has_attr_byname(nix_c_context * context, const Value * value, EvalState * state, const char * name);
+
+/** @brief Get an attribute by index in the sorted bindings
+ *
+ * Also gives you the name.
+ *
+ * Owned by the GC. Use nix_gc_decref when you're done with the pointer
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @param[in] state nix evaluator state
+ * @param[in] i attribute index
+ * @param[out] name will store a pointer to the attribute name
+ * @return value, NULL in case of errors
+ */
+Value *
+nix_get_attr_byidx(nix_c_context * context, const Value * value, EvalState * state, unsigned int i, const char ** name);
+
+/** @brief Get an attribute name by index in the sorted bindings
+ *
+ * Useful when you want the name but want to avoid evaluation.
+ *
+ * Owned by the nix EvalState
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @param[in] state nix evaluator state
+ * @param[in] i attribute index
+ * @return name, NULL in case of errors
+ */
+const char * nix_get_attr_name_byidx(nix_c_context * context, const Value * value, EvalState * state, unsigned int i);
+
+/**@}*/
+/** @name Initializers
+ *
+ * Values are typically "returned" by initializing already allocated memory that serves as the return value.
+ * For this reason, the construction of values is not tied their allocation.
+ * Nix is a language with immutable values. Respect this property by only initializing Values once; and only initialize
+ * Values that are meant to be initialized by you. Failing to adhere to these rules may lead to undefined behavior.
+ */
+/**@{*/
+/** @brief Set boolean value
+ * @param[out] context Optional, stores error information
+ * @param[out] value Nix value to modify
+ * @param[in] b the boolean value
+ * @return error code, NIX_OK on success.
+ */
+nix_err nix_init_bool(nix_c_context * context, Value * value, bool b);
+
+/** @brief Set a string
+ * @param[out] context Optional, stores error information
+ * @param[out] value Nix value to modify
+ * @param[in] str the string, copied
+ * @return error code, NIX_OK on success.
+ */
+nix_err nix_init_string(nix_c_context * context, Value * value, const char * str);
+
+/** @brief Set a path
+ * @param[out] context Optional, stores error information
+ * @param[out] value Nix value to modify
+ * @param[in] str the path string, copied
+ * @return error code, NIX_OK on success.
+ */
+nix_err nix_init_path_string(nix_c_context * context, EvalState * s, Value * value, const char * str);
+
+/** @brief Set a float
+ * @param[out] context Optional, stores error information
+ * @param[out] value Nix value to modify
+ * @param[in] d the float, 64-bits
+ * @return error code, NIX_OK on success.
+ */
+nix_err nix_init_float(nix_c_context * context, Value * value, double d);
+
+/** @brief Set an int
+ * @param[out] context Optional, stores error information
+ * @param[out] value Nix value to modify
+ * @param[in] i the int
+ * @return error code, NIX_OK on success.
+ */
+
+nix_err nix_init_int(nix_c_context * context, Value * value, int64_t i);
+/** @brief Set null
+ * @param[out] context Optional, stores error information
+ * @param[out] value Nix value to modify
+ * @return error code, NIX_OK on success.
+ */
+
+nix_err nix_init_null(nix_c_context * context, Value * value);
+/** @brief Set an external value
+ * @param[out] context Optional, stores error information
+ * @param[out] value Nix value to modify
+ * @param[in] val the external value to set. Will be GC-referenced by the value.
+ * @return error code, NIX_OK on success.
+ */
+nix_err nix_init_external(nix_c_context * context, Value * value, ExternalValue * val);
+
+/** @brief Create a list from a list builder
+ * @param[out] context Optional, stores error information
+ * @param[in] list_builder list builder to use. Make sure to unref this afterwards.
+ * @param[out] value Nix value to modify
+ * @return error code, NIX_OK on success.
+ */
+nix_err nix_make_list(nix_c_context * context, ListBuilder * list_builder, Value * value);
+
+/** @brief Create a list builder
+ * @param[out] context Optional, stores error information
+ * @param[in] state nix evaluator state
+ * @param[in] capacity how many bindings you'll add. Don't exceed.
+ * @return owned reference to a list builder. Make sure to unref when you're done.
+ */
+ListBuilder * nix_make_list_builder(nix_c_context * context, EvalState * state, size_t capacity);
+
+/** @brief Insert bindings into a builder
+ * @param[out] context Optional, stores error information
+ * @param[in] list_builder ListBuilder to insert into
+ * @param[in] index index to manipulate
+ * @param[in] value value to insert
+ * @return error code, NIX_OK on success.
+ */
+nix_err nix_list_builder_insert(nix_c_context * context, ListBuilder * list_builder, unsigned int index, Value * value);
+
+/** @brief Free a list builder
+ *
+ * Does not fail.
+ * @param[in] builder the builder to free
+ */
+void nix_list_builder_free(ListBuilder * list_builder);
+
+/** @brief Create an attribute set from a bindings builder
+ * @param[out] context Optional, stores error information
+ * @param[out] value Nix value to modify
+ * @param[in] b bindings builder to use. Make sure to unref this afterwards.
+ * @return error code, NIX_OK on success.
+ */
+nix_err nix_make_attrs(nix_c_context * context, Value * value, BindingsBuilder * b);
+
+/** @brief Set primop
+ * @param[out] context Optional, stores error information
+ * @param[out] value Nix value to modify
+ * @param[in] op primop, will be gc-referenced by the value
+ * @see nix_alloc_primop
+ * @return error code, NIX_OK on success.
+ */
+nix_err nix_init_primop(nix_c_context * context, Value * value, PrimOp * op);
+/** @brief Copy from another value
+ * @param[out] context Optional, stores error information
+ * @param[out] value Nix value to modify
+ * @param[in] source value to copy from
+ * @return error code, NIX_OK on success.
+ */
+nix_err nix_copy_value(nix_c_context * context, Value * value, Value * source);
+/**@}*/
+
+/** @brief Create a bindings builder
+* @param[out] context Optional, stores error information
+* @param[in] state nix evaluator state
+* @param[in] capacity how many bindings you'll add. Don't exceed.
+* @return owned reference to a bindings builder. Make sure to unref when you're
+done.
+*/
+BindingsBuilder * nix_make_bindings_builder(nix_c_context * context, EvalState * state, size_t capacity);
+
+/** @brief Insert bindings into a builder
+ * @param[out] context Optional, stores error information
+ * @param[in] builder BindingsBuilder to insert into
+ * @param[in] name attribute name, copied into the symbol store
+ * @param[in] value value to give the binding
+ * @return error code, NIX_OK on success.
+ */
+nix_err
+nix_bindings_builder_insert(nix_c_context * context, BindingsBuilder * builder, const char * name, Value * value);
+
+/** @brief Free a bindings builder
+ *
+ * Does not fail.
+ * @param[in] builder the builder to free
+ */
+void nix_bindings_builder_free(BindingsBuilder * builder);
+/**@}*/
+
+// cffi end
+#ifdef __cplusplus
+}
+#endif
+
+/** @} */
+#endif // NIX_API_VALUE_H

src/libexpr/eval.cc

@@ -897,7 +897,6 @@ void Value::mkStringMove(const char * s, const NixStringContext & context)
     copyContextToValue(*this, context);
 }
 
-
 void Value::mkPath(const SourcePath & path)
 {
     mkPath(&*path.accessor, makeImmutableString(path.path.abs()));

src/libexpr/eval.hh

@@ -17,6 +17,7 @@
 #include <optional>
 #include <unordered_map>
 #include <mutex>
+#include <functional>
 
 namespace nix {
 
@@ -79,7 +80,7 @@ struct PrimOp
     /**
      * Implementation of the primop.
      */
-    PrimOpFun fun;
+    std::function<std::remove_pointer<PrimOpFun>::type> fun;
 
     /**
      * Optional experimental for this to be gated on.

src/libexpr/primops.cc

@@ -3404,8 +3404,11 @@ static void prim_sort(EvalState & state, const PosIdx pos, Value * * args, Value
            callFunction. */
         /* TODO: (layus) this is absurd. An optimisation like this
            should be outside the lambda creation */
-        if (args[0]->isPrimOp() && args[0]->primOp->fun == prim_lessThan)
+        if (args[0]->isPrimOp()) {
-            return CompareValues(state, noPos, "while evaluating the ordering function passed to builtins.sort")(a, b);
+            auto ptr = args[0]->primOp->fun.target<decltype(&prim_lessThan)>();
+            if (ptr && *ptr == prim_lessThan)
+                return CompareValues(state, noPos, "while evaluating the ordering function passed to builtins.sort")(a, b);
+        }
 
         Value * vs[] = {a, b};
         Value vBool;

src/libexpr/search-path.cc

@@ -44,7 +44,7 @@ SearchPath::Elem SearchPath::Elem::parse(std::string_view rawElem)
 }
 
 
-SearchPath parseSearchPath(const Strings & rawElems)
+SearchPath SearchPath::parse(const Strings & rawElems)
 {
     SearchPath res;
     for (auto & rawElem : rawElems)

src/libexpr/value.hh

@@ -328,6 +328,7 @@ public:
     }
 
     void mkPath(const SourcePath & path);
+    void mkPath(std::string_view path);
 
     inline void mkPath(InputAccessor * accessor, const char * path)
     {

src/libstore-c/local.mk

@@ -0,0 +1,21 @@
+libraries += libstorec
+
+libstorec_NAME = libnixstorec
+
+libstorec_DIR := $(d)
+
+libstorec_SOURCES := $(wildcard $(d)/*.cc)
+
+libstorec_LIBS = libutil libstore libutilc
+
+libstorec_LDFLAGS += -pthread
+
+# Not just for this library itself, but also for downstream libraries using this library
+
+INCLUDE_libstorec := -I $(d)
+libstorec_CXXFLAGS += $(INCLUDE_libutil) $(INCLUDE_libutilc) \
+		    $(INCLUDE_libstore) $(INCLUDE_libstorec)
+
+$(eval $(call install-file-in, $(d)/nix-store-c.pc, $(libdir)/pkgconfig, 0644))
+
+libstorec_FORCE_INSTALL := 1

src/libstore-c/nix-store-c.pc.in

@@ -0,0 +1,9 @@
+prefix=@prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: Nix
+Description: Nix Store - C API
+Version: @PACKAGE_VERSION@
+Libs: -L${libdir} -lnixstorec -lnixutilc
+Cflags: -I${includedir}/nix

src/libstore-c/nix_api_store.cc

@@ -0,0 +1,134 @@
+#include "nix_api_store.h"
+#include "nix_api_store_internal.h"
+#include "nix_api_util.h"
+#include "nix_api_util_internal.h"
+
+#include "path.hh"
+#include "store-api.hh"
+#include "build-result.hh"
+
+#include "globals.hh"
+
+nix_err nix_libstore_init(nix_c_context * context)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        nix::initLibStore();
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_init_plugins(nix_c_context * context)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        nix::initPlugins();
+    }
+    NIXC_CATCH_ERRS
+}
+
+Store * nix_store_open(nix_c_context * context, const char * uri, const char *** params)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        std::string uri_str = uri ? uri : "";
+
+        if (uri_str.empty())
+            return new Store{nix::openStore()};
+
+        if (!params)
+            return new Store{nix::openStore(uri_str)};
+
+        nix::Store::Params params_map;
+        for (size_t i = 0; params[i] != nullptr; i++) {
+            params_map[params[i][0]] = params[i][1];
+        }
+        return new Store{nix::openStore(uri_str, params_map)};
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+void nix_store_free(Store * store)
+{
+    delete store;
+}
+
+nix_err nix_store_get_uri(nix_c_context * context, Store * store, void * callback, void * user_data)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto res = store->ptr->getUri();
+        return call_nix_get_string_callback(res, callback, user_data);
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_store_get_version(nix_c_context * context, Store * store, void * callback, void * user_data)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto res = store->ptr->getVersion();
+        return call_nix_get_string_callback(res.value_or(""), callback, user_data);
+    }
+    NIXC_CATCH_ERRS
+}
+
+bool nix_store_is_valid_path(nix_c_context * context, Store * store, StorePath * path)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        return store->ptr->isValidPath(path->path);
+    }
+    NIXC_CATCH_ERRS_RES(false);
+}
+
+StorePath * nix_store_parse_path(nix_c_context * context, Store * store, const char * path)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        nix::StorePath s = store->ptr->parseStorePath(path);
+        return new StorePath{std::move(s)};
+    }
+    NIXC_CATCH_ERRS_NULL
+}
+
+nix_err nix_store_realise(
+    nix_c_context * context,
+    Store * store,
+    StorePath * path,
+    void * userdata,
+    void (*callback)(void * userdata, const char *, const char *))
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+
+        const std::vector<nix::DerivedPath> paths{nix::DerivedPath::Built{
+            .drvPath = nix::makeConstantStorePathRef(path->path), .outputs = nix::OutputsSpec::All{}}};
+
+        const auto nixStore = store->ptr;
+        auto results = nixStore->buildPathsWithResults(paths, nix::bmNormal, nixStore);
+
+        if (callback) {
+            for (const auto & result : results) {
+                for (const auto & [outputName, realisation] : result.builtOutputs) {
+                    auto op = store->ptr->printStorePath(realisation.outPath);
+                    callback(userdata, outputName.c_str(), op.c_str());
+                }
+            }
+        }
+    }
+    NIXC_CATCH_ERRS
+}
+
+void nix_store_path_free(StorePath * sp)
+{
+    delete sp;
+}

src/libstore-c/nix_api_store.h

@@ -0,0 +1,148 @@
+#ifndef NIX_API_STORE_H
+#define NIX_API_STORE_H
+/**
+ * @defgroup libstore libstore
+ * @brief C bindings for nix libstore
+ *
+ * libstore is used for talking to a Nix store
+ * @{
+ */
+/** @file
+ * @brief Main entry for the libstore C bindings
+ */
+
+#include "nix_api_util.h"
+#include <stdbool.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+// cffi start
+
+/** @brief Reference to a Nix store */
+typedef struct Store Store;
+/** @brief Nix store path */
+typedef struct StorePath StorePath;
+
+/**
+ * @brief Initializes the Nix store library
+ *
+ * This function should be called before creating a Store
+ * This function can be called multiple times.
+ *
+ * @param[out] context Optional, stores error information
+ * @return NIX_OK if the initialization was successful, an error code otherwise.
+ */
+nix_err nix_libstore_init(nix_c_context * context);
+
+/**
+ * @brief Loads the plugins specified in Nix's plugin-files setting.
+ *
+ * Call this once, after calling your desired init functions and setting
+ * relevant settings.
+ *
+ * @param[out] context Optional, stores error information
+ * @return NIX_OK if the initialization was successful, an error code otherwise.
+ */
+nix_err nix_init_plugins(nix_c_context * context);
+
+/**
+ * @brief Open a nix store
+ * Store instances may share state and resources behind the scenes.
+ * @param[out] context Optional, stores error information
+ * @param[in] uri URI of the nix store, copied
+ * @param[in] params optional, array of key-value pairs, {{"endpoint",
+ * "https://s3.local"}}
+ * @return a Store pointer, NULL in case of errors
+ * @see nix_store_free
+ */
+Store * nix_store_open(nix_c_context *, const char * uri, const char *** params);
+
+/**
+ * @brief Deallocate a nix store and free any resources if not also held by other Store instances.
+ *
+ * Does not fail.
+ *
+ * @param[in] store the store to free
+ */
+void nix_store_free(Store * store);
+
+/**
+ * @brief get the URI of a nix store
+ * @param[out] context Optional, stores error information
+ * @param[in] store nix store reference
+ * @param[in] callback Called with the URI.
+ * @param[in] user_data optional, arbitrary data, passed to the callback when it's called.
+ * @see nix_get_string_callback
+ * @return error code, NIX_OK on success.
+ */
+nix_err nix_store_get_uri(nix_c_context * context, Store * store, void * callback, void * user_data);
+
+// returns: owned StorePath*
+/**
+ * @brief Parse a Nix store path into a StorePath
+ *
+ * @note Don't forget to free this path using nix_store_path_free()!
+ * @param[out] context Optional, stores error information
+ * @param[in] store nix store reference
+ * @param[in] path Path string to parse, copied
+ * @return owned store path, NULL on error
+ */
+StorePath * nix_store_parse_path(nix_c_context * context, Store * store, const char * path);
+
+/** @brief Deallocate a StorePath
+ *
+ * Does not fail.
+ * @param[in] p the path to free
+ */
+void nix_store_path_free(StorePath * p);
+
+/**
+ * @brief Check if a StorePath is valid (i.e. that corresponding store object and its closure of references exists in
+ * the store)
+ * @param[out] context Optional, stores error information
+ * @param[in] store Nix Store reference
+ * @param[in] path Path to check
+ * @return true or false, error info in context
+ */
+bool nix_store_is_valid_path(nix_c_context * context, Store * store, StorePath * path);
+// nix_err nix_store_ensure(Store*, const char*);
+// nix_err nix_store_build_paths(Store*);
+/**
+ * @brief Realise a Nix store path
+ *
+ * Blocking, calls callback once for each realised output
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] store Nix Store reference
+ * @param[in] path Path to build
+ * @param[in] userdata data to pass to every callback invocation
+ * @param[in] callback called for every realised output
+ */
+nix_err nix_store_realise(
+    nix_c_context * context,
+    Store * store,
+    StorePath * path,
+    void * userdata,
+    void (*callback)(void * userdata, const char * outname, const char * out));
+
+/**
+ * @brief get the version of a nix store.
+ * If the store doesn't have a version (like the dummy store), returns an empty string.
+ * @param[out] context Optional, stores error information
+ * @param[in] store nix store reference
+ * @param[in] callback Called with the version.
+ * @param[in] user_data optional, arbitrary data, passed to the callback when it's called.
+ * @see nix_get_string_callback
+ * @return error code, NIX_OK on success.
+ */
+nix_err nix_store_get_version(nix_c_context * context, Store * store, void * callback, void * user_data);
+
+// cffi end
+#ifdef __cplusplus
+}
+#endif
+/**
+ * @}
+ */
+#endif // NIX_API_STORE_H

src/libstore-c/nix_api_store_internal.h

@@ -0,0 +1,15 @@
+#ifndef NIX_API_STORE_INTERNAL_H
+#define NIX_API_STORE_INTERNAL_H
+#include "store-api.hh"
+
+struct Store
+{
+    nix::ref<nix::Store> ptr;
+};
+
+struct StorePath
+{
+    nix::StorePath path;
+};
+
+#endif

src/libstore/globals.cc

@@ -345,6 +345,12 @@ void initPlugins()
                 dlopen(file.c_str(), RTLD_LAZY | RTLD_LOCAL);
             if (!handle)
                 throw Error("could not dynamically open plugin file '%s': %s", file, dlerror());
+
+            /* Older plugins use a statically initialized object to run their code.
+               Newer plugins can also export nix_plugin_entry() */
+            void (*nix_plugin_entry)() = (void (*)())dlsym(handle, "nix_plugin_entry");
+            if (nix_plugin_entry)
+                nix_plugin_entry();
         }
     }
 
@@ -403,6 +409,7 @@ void assertLibStoreInitialized() {
 }
 
 void initLibStore() {
+    if (initLibStoreDone) return;
 
     initLibUtil();
 

src/libstore/globals.hh

@@ -1157,9 +1157,10 @@ public:
         this, {}, "plugin-files",
         R"(
           A list of plugin files to be loaded by Nix. Each of these files will
-          be dlopened by Nix, allowing them to affect execution through static
+          be dlopened by Nix. If they contain the symbol `nix_plugin_entry()`,
-          initialization. In particular, these plugins may construct static
+          this symbol will be called. Alternatively, they can affect execution
-          instances of RegisterPrimOp to add new primops or constants to the
+          through static initialization. In particular, these plugins may construct
+          static instances of RegisterPrimOp to add new primops or constants to the
           expression language, RegisterStoreImplementation to add new store
           implementations, RegisterCommand to add new subcommands to the `nix`
           command, and RegisterSetting to add new nix config settings. See the

src/libutil-c/local.mk

@@ -0,0 +1,18 @@
+libraries += libutilc
+
+libutilc_NAME = libnixutilc
+
+libutilc_DIR := $(d)
+
+libutilc_SOURCES := $(wildcard $(d)/*.cc)
+
+# Not just for this library itself, but also for downstream libraries using this library
+
+INCLUDE_libutilc := -I $(d)
+libutilc_CXXFLAGS += $(INCLUDE_libutil) $(INCLUDE_libutilc)
+
+libutilc_LIBS = libutil
+
+libutilc_LDFLAGS += -pthread
+
+libutilc_FORCE_INSTALL := 1

src/libutil-c/nix_api_util.cc

@@ -0,0 +1,148 @@
+#include "nix_api_util.h"
+#include "config.hh"
+#include "error.hh"
+#include "nix_api_util_internal.h"
+#include "util.hh"
+
+#include <cxxabi.h>
+#include <typeinfo>
+
+nix_c_context * nix_c_context_create()
+{
+    return new nix_c_context();
+}
+
+void nix_c_context_free(nix_c_context * context)
+{
+    delete context;
+}
+
+nix_err nix_context_error(nix_c_context * context)
+{
+    if (context == nullptr) {
+        throw;
+    }
+    try {
+        throw;
+    } catch (nix::Error & e) {
+        /* Storing this exception is annoying, take what we need here */
+        context->last_err = e.what();
+        context->info = e.info();
+        int status;
+        const char * demangled = abi::__cxa_demangle(typeid(e).name(), 0, 0, &status);
+        if (demangled) {
+            context->name = demangled;
+            // todo: free(demangled);
+        } else {
+            context->name = typeid(e).name();
+        }
+        context->last_err_code = NIX_ERR_NIX_ERROR;
+        return context->last_err_code;
+    } catch (const std::exception & e) {
+        context->last_err = e.what();
+        context->last_err_code = NIX_ERR_UNKNOWN;
+        return context->last_err_code;
+    }
+    // unreachable
+}
+
+nix_err nix_set_err_msg(nix_c_context * context, nix_err err, const char * msg)
+{
+    if (context == nullptr) {
+        // todo last_err_code
+        throw nix::Error("Nix C api error: %s", msg);
+    }
+    context->last_err_code = err;
+    context->last_err = msg;
+    return err;
+}
+
+const char * nix_version_get()
+{
+    return PACKAGE_VERSION;
+}
+
+// Implementations
+
+nix_err nix_setting_get(nix_c_context * context, const char * key, void * callback, void * user_data)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        std::map<std::string, nix::AbstractConfig::SettingInfo> settings;
+        nix::globalConfig.getSettings(settings);
+        if (settings.contains(key)) {
+            return call_nix_get_string_callback(settings[key].value, callback, user_data);
+        } else {
+            return nix_set_err_msg(context, NIX_ERR_KEY, "Setting not found");
+        }
+    }
+    NIXC_CATCH_ERRS
+}
+
+nix_err nix_setting_set(nix_c_context * context, const char * key, const char * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    if (nix::globalConfig.set(key, value))
+        return NIX_OK;
+    else {
+        return nix_set_err_msg(context, NIX_ERR_KEY, "Setting not found");
+    }
+}
+
+nix_err nix_libutil_init(nix_c_context * context)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        nix::initLibUtil();
+        return NIX_OK;
+    }
+    NIXC_CATCH_ERRS
+}
+
+const char * nix_err_msg(nix_c_context * context, const nix_c_context * read_context, unsigned int * n)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    if (read_context->last_err) {
+        if (n)
+            *n = read_context->last_err->size();
+        return read_context->last_err->c_str();
+    }
+    nix_set_err_msg(context, NIX_ERR_UNKNOWN, "No error message");
+    return nullptr;
+}
+
+nix_err nix_err_name(nix_c_context * context, const nix_c_context * read_context, void * callback, void * user_data)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    if (read_context->last_err_code != NIX_ERR_NIX_ERROR) {
+        return nix_set_err_msg(context, NIX_ERR_UNKNOWN, "Last error was not a nix error");
+    }
+    return call_nix_get_string_callback(read_context->name, callback, user_data);
+}
+
+nix_err nix_err_info_msg(nix_c_context * context, const nix_c_context * read_context, void * callback, void * user_data)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    if (read_context->last_err_code != NIX_ERR_NIX_ERROR) {
+        return nix_set_err_msg(context, NIX_ERR_UNKNOWN, "Last error was not a nix error");
+    }
+    return call_nix_get_string_callback(read_context->info->msg.str(), callback, user_data);
+}
+
+nix_err nix_err_code(const nix_c_context * read_context)
+{
+    return read_context->last_err_code;
+}
+
+// internal
+nix_err call_nix_get_string_callback(const std::string str, void * callback, void * user_data)
+{
+    ((nix_get_string_callback) callback)(str.c_str(), str.size(), user_data);
+    return NIX_OK;
+}

src/libutil-c/nix_api_util.h

@@ -0,0 +1,301 @@
+#ifndef NIX_API_UTIL_H
+#define NIX_API_UTIL_H
+/**
+ * @defgroup libutil libutil
+ * @brief C bindings for nix libutil
+ *
+ * libutil is used for functionality shared between
+ * different Nix modules.
+ * @{
+ */
+/** @file
+ * @brief Main entry for the libutil C bindings
+ *
+ * Also contains error handling utilities
+ */
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+// cffi start
+
+/** @defgroup errors Handling errors
+ * @brief Dealing with errors from the Nix side
+ *
+ * To handle errors that can be returned from the Nix API,
+ * a nix_c_context can be passed to any function that potentially returns an
+ * error.
+ *
+ * Error information will be stored in this context, and can be retrieved
+ * using nix_err_code and nix_err_msg.
+ *
+ * Passing NULL instead will cause the API to throw C++ errors.
+ *
+ * Example:
+ * @code{.c}
+ * int main() {
+ *     nix_c_context* ctx = nix_c_context_create();
+ *     nix_libutil_init(ctx);
+ *     if (nix_err_code(ctx) != NIX_OK) {
+ *         printf("error: %s\n", nix_err_msg(NULL, ctx, NULL));
+ *         return 1;
+ *     }
+ *     return 0;
+ * }
+ * @endcode
+ *  @{
+ */
+// Error codes
+/**
+ * @brief Type for error codes in the NIX system
+ *
+ * This type can have one of several predefined constants:
+ * - NIX_OK: No error occurred (0)
+ * - NIX_ERR_UNKNOWN: An unknown error occurred (-1)
+ * - NIX_ERR_OVERFLOW: An overflow error occurred (-2)
+ * - NIX_ERR_KEY: A key error occurred (-3)
+ * - NIX_ERR_NIX_ERROR: A generic Nix error occurred (-4)
+ */
+typedef int nix_err;
+
+/**
+ * @brief No error occurred.
+ *
+ * This error code is returned when no error has occurred during the function
+ * execution.
+ */
+#define NIX_OK 0
+
+/**
+ * @brief An unknown error occurred.
+ *
+ * This error code is returned when an unknown error occurred during the
+ * function execution.
+ */
+#define NIX_ERR_UNKNOWN -1
+
+/**
+ * @brief An overflow error occurred.
+ *
+ * This error code is returned when an overflow error occurred during the
+ * function execution.
+ */
+#define NIX_ERR_OVERFLOW -2
+
+/**
+ * @brief A key error occurred.
+ *
+ * This error code is returned when a key error occurred during the function
+ * execution.
+ */
+#define NIX_ERR_KEY -3
+
+/**
+ * @brief A generic Nix error occurred.
+ *
+ * This error code is returned when a generic Nix error occurred during the
+ * function execution.
+ */
+#define NIX_ERR_NIX_ERROR -4
+
+/**
+ * @brief This object stores error state.
+ * @struct nix_c_context
+ *
+ * Passed as a first parameter to functions that can fail, to store error
+ * information.
+ *
+ * Optional wherever it can be used, passing NULL instead will throw a C++
+ * exception.
+ *
+ * The struct is laid out so that it can also be cast to nix_err* to inspect
+ * directly:
+ * @code{.c}
+ * assert(*(nix_err*)ctx == NIX_OK);
+ * @endcode
+ * @note These can be reused between different function calls,
+ *  but make sure not to use them for multiple calls simultaneously (which can
+ * happen in callbacks).
+ */
+typedef struct nix_c_context nix_c_context;
+
+/**
+ * @brief Called to get the value of a string owned by Nix.
+ *
+ * @param[in] start the string to copy.
+ * @param[in] n the string length.
+ * @param[in] user_data optional, arbitrary data, passed to the nix_get_string_callback when it's called.
+ */
+typedef void (*nix_get_string_callback)(const char * start, unsigned int n, void * user_data);
+
+// Function prototypes
+
+/**
+ * @brief Allocate a new nix_c_context.
+ * @throws std::bad_alloc
+ * @return allocated nix_c_context, owned by the caller. Free using
+ * `nix_c_context_free`.
+ */
+nix_c_context * nix_c_context_create();
+/**
+ * @brief Free a nix_c_context. Does not fail.
+ * @param[out] context The context to free, mandatory.
+ */
+void nix_c_context_free(nix_c_context * context);
+/**
+ *  @}
+ */
+
+/**
+ * @brief Initializes nix_libutil and its dependencies.
+ *
+ * This function can be called multiple times, but should be called at least
+ * once prior to any other nix function.
+ *
+ * @param[out] context Optional, stores error information
+ * @return NIX_OK if the initialization is successful, or an error code
+ * otherwise.
+ */
+nix_err nix_libutil_init(nix_c_context * context);
+
+/** @defgroup settings
+ *  @{
+ */
+/**
+ * @brief Retrieves a setting from the nix global configuration.
+ *
+ * This function requires nix_libutil_init() to be called at least once prior to
+ * its use.
+ *
+ * @param[out] context optional, Stores error information
+ * @param[in] key The key of the setting to retrieve.
+ * @param[in] callback Called with the setting value.
+ * @param[in] user_data optional, arbitrary data, passed to the callback when it's called.
+ * @see nix_get_string_callback
+ * @return NIX_ERR_KEY if the setting is unknown, or NIX_OK if the setting was retrieved
+ * successfully.
+ */
+nix_err nix_setting_get(nix_c_context * context, const char * key, void * callback, void * user_data);
+
+/**
+ * @brief Sets a setting in the nix global configuration.
+ *
+ * Use "extra-<setting name>" to append to the setting's value.
+ *
+ * Settings only apply for new State%s. Call nix_plugins_init() when you are
+ * done with the settings to load any plugins.
+ *
+ * @param[out] context optional, Stores error information
+ * @param[in] key The key of the setting to set.
+ * @param[in] value The value to set for the setting.
+ * @return NIX_ERR_KEY if the setting is unknown, or NIX_OK if the setting was
+ * set successfully.
+ */
+nix_err nix_setting_set(nix_c_context * context, const char * key, const char * value);
+
+/**
+ *  @}
+ */
+// todo: nix_plugins_init()
+
+/**
+ * @brief Retrieves the nix library version.
+ *
+ * Does not fail.
+ * @return A static string representing the version of the nix library.
+ */
+const char * nix_version_get();
+
+/** @addtogroup errors
+ *  @{
+ */
+/**
+ * @brief Retrieves the most recent error message from a context.
+ *
+ * @pre This function should only be called after a previous nix function has
+ * returned an error.
+ *
+ * @param[out] context optional, the context to store errors in if this function
+ * fails
+ * @param[in] ctx the context to retrieve the error message from
+ * @param[out] n optional: a pointer to an unsigned int that is set to the
+ * length of the error.
+ * @return nullptr if no error message was ever set,
+ *         a borrowed pointer to the error message otherwise.
+ */
+const char * nix_err_msg(nix_c_context * context, const nix_c_context * ctx, unsigned int * n);
+
+/**
+ * @brief Retrieves the error message from errorInfo in a context.
+ *
+ * Used to inspect nix Error messages.
+ *
+ * @pre This function should only be called after a previous nix function has
+ * returned a NIX_ERR_NIX_ERROR
+ *
+ * @param[out] context optional, the context to store errors in if this function
+ * fails
+ * @param[in] read_context the context to retrieve the error message from.
+ * @param[in] callback Called with the error message.
+ * @param[in] user_data optional, arbitrary data, passed to the callback when it's called.
+ * @see nix_get_string_callback
+ * @return NIX_OK if there were no errors, an error code otherwise.
+ */
+nix_err
+nix_err_info_msg(nix_c_context * context, const nix_c_context * read_context, void * callback, void * user_data);
+
+/**
+ * @brief Retrieves the error name from a context.
+ *
+ * Used to inspect nix Error messages.
+ *
+ * @pre This function should only be called after a previous nix function has
+ * returned a NIX_ERR_NIX_ERROR
+ *
+ * @param context optional, the context to store errors in if this function
+ * fails
+ * @param[in] read_context the context to retrieve the error message from
+ * @param[in] callback Called with the error name.
+ * @param[in] user_data optional, arbitrary data, passed to the callback when it's called.
+ * @see nix_get_string_callback
+ * @return NIX_OK if there were no errors, an error code otherwise.
+ */
+nix_err nix_err_name(nix_c_context * context, const nix_c_context * read_context, void * callback, void * user_data);
+
+/**
+ * @brief Retrieves the most recent error code from a nix_c_context
+ *
+ * Equivalent to reading the first field of the context.
+ *
+ * Does not fail
+ *
+ * @param[in] read_context the context to retrieve the error message from
+ * @return most recent error code stored in the context.
+ */
+nix_err nix_err_code(const nix_c_context * read_context);
+
+/**
+ * @brief Set an error message on a nix context.
+ *
+ * This should be used when you want to throw an error from a PrimOp callback.
+ *
+ * All other use is internal to the API.
+ *
+ * @param context context to write the error message to, or NULL
+ * @param err The error code to set and return
+ * @param msg The error message to set.
+ * @returns the error code set
+ */
+nix_err nix_set_err_msg(nix_c_context * context, nix_err err, const char * msg);
+
+/**
+ *  @}
+ */
+
+// cffi end
+#ifdef __cplusplus
+}
+#endif
+
+/** @} */
+#endif // NIX_API_UTIL_H

src/libutil-c/nix_api_util_internal.h

@@ -0,0 +1,49 @@
+#ifndef NIX_API_UTIL_INTERNAL_H
+#define NIX_API_UTIL_INTERNAL_H
+
+#include <string>
+#include <optional>
+
+#include "error.hh"
+#include "nix_api_util.h"
+
+struct nix_c_context
+{
+    nix_err last_err_code = NIX_OK;
+    std::optional<std::string> last_err = {};
+    std::optional<nix::ErrorInfo> info = {};
+    std::string name = "";
+};
+
+nix_err nix_context_error(nix_c_context * context);
+
+/**
+ * Internal use only.
+ *
+ * Helper to invoke nix_get_string_callback
+ * @param context optional, the context to store errors in if this function
+ * fails
+ * @param str The string to observe
+ * @param callback Called with the observed string.
+ * @param user_data optional, arbitrary data, passed to the callback when it's called.
+ * @return NIX_OK if there were no errors.
+ * @see nix_get_string_callback
+ */
+nix_err call_nix_get_string_callback(const std::string str, void * callback, void * user_data);
+
+#define NIXC_CATCH_ERRS \
+    catch (...) \
+    { \
+        return nix_context_error(context); \
+    } \
+    return NIX_OK;
+
+#define NIXC_CATCH_ERRS_RES(def) \
+    catch (...) \
+    { \
+        nix_context_error(context); \
+        return def; \
+    }
+#define NIXC_CATCH_ERRS_NULL NIXC_CATCH_ERRS_RES(nullptr)
+
+#endif // NIX_API_UTIL_INTERNAL_H

src/libutil/error.hh

@@ -150,6 +150,10 @@ public:
         : err(e)
     { }
 
+    std::string message() {
+        return err.msg.str();
+    }
+
     const char * what() const noexcept override { return calcWhat().c_str(); }
     const std::string & msg() const { return calcWhat(); }
     const ErrorInfo & info() const { calcWhat(); return err; }

tests/unit/libexpr-support/tests/nix_api_expr.hh

@@ -0,0 +1,31 @@
+#pragma once
+///@file
+#include "nix_api_expr.h"
+#include "nix_api_value.h"
+#include "tests/nix_api_store.hh"
+
+#include <gtest/gtest.h>
+
+namespace nixC {
+
+class nix_api_expr_test : public nix_api_store_test
+{
+protected:
+
+    nix_api_expr_test()
+    {
+        nix_libexpr_init(ctx);
+        state = nix_state_create(nullptr, nullptr, store);
+        value = nix_alloc_value(nullptr, state);
+    }
+    ~nix_api_expr_test()
+    {
+        nix_gc_decref(nullptr, value);
+        nix_state_free(state);
+    }
+
+    EvalState * state;
+    Value * value;
+};
+
+}

tests/unit/libexpr/local.mk

@@ -24,14 +24,17 @@ libexpr-tests_EXTRA_INCLUDES = \
     -I tests/unit/libstore-support \
     -I tests/unit/libutil-support \
     $(INCLUDE_libexpr) \
+    $(INCLUDE_libexprc) \
     $(INCLUDE_libfetchers) \
     $(INCLUDE_libstore) \
-    $(INCLUDE_libutil)
+    $(INCLUDE_libstorec) \
+    $(INCLUDE_libutil) \
+    $(INCLUDE_libutilc)
 
 libexpr-tests_CXXFLAGS += $(libexpr-tests_EXTRA_INCLUDES)
 
 libexpr-tests_LIBS = \
     libexpr-test-support libstore-test-support libutils-test-support \
-    libexpr libfetchers libstore libutil
+    libexpr libexprc libfetchers libstore libstorec libutil libutilc
 
 libexpr-tests_LDFLAGS := -lrapidcheck $(GTEST_LIBS) -lgmock

tests/unit/libexpr/nix_api_expr.cc

@@ -0,0 +1,100 @@
+#include "nix_api_store.h"
+#include "nix_api_store_internal.h"
+#include "nix_api_util.h"
+#include "nix_api_util_internal.h"
+#include "nix_api_expr.h"
+#include "nix_api_value.h"
+
+#include "tests/nix_api_expr.hh"
+
+#include <gtest/gtest.h>
+
+namespace nixC {
+
+TEST_F(nix_api_expr_test, nix_expr_eval_from_string)
+{
+    nix_expr_eval_from_string(nullptr, state, "builtins.nixVersion", ".", value);
+    nix_value_force(nullptr, state, value);
+    auto result = nix_get_string(nullptr, value);
+
+    ASSERT_STREQ(PACKAGE_VERSION, result);
+}
+
+TEST_F(nix_api_expr_test, nix_expr_eval_add_numbers)
+{
+    nix_expr_eval_from_string(nullptr, state, "1 + 1", ".", value);
+    nix_value_force(nullptr, state, value);
+    auto result = nix_get_int(nullptr, value);
+
+    ASSERT_EQ(2, result);
+}
+
+TEST_F(nix_api_expr_test, nix_expr_eval_drv)
+{
+    auto expr = R"(derivation { name = "myname"; builder = "mybuilder"; system = "mysystem"; })";
+    nix_expr_eval_from_string(nullptr, state, expr, ".", value);
+    ASSERT_EQ(NIX_TYPE_ATTRS, nix_get_type(nullptr, value));
+
+    EvalState * stateFn = nix_state_create(nullptr, nullptr, store);
+    Value * valueFn = nix_alloc_value(nullptr, state);
+    nix_expr_eval_from_string(nullptr, stateFn, "builtins.toString", ".", valueFn);
+    ASSERT_EQ(NIX_TYPE_FUNCTION, nix_get_type(nullptr, valueFn));
+
+    EvalState * stateResult = nix_state_create(nullptr, nullptr, store);
+    Value * valueResult = nix_alloc_value(nullptr, stateResult);
+    nix_value_call(ctx, stateResult, valueFn, value, valueResult);
+    ASSERT_EQ(NIX_TYPE_STRING, nix_get_type(nullptr, valueResult));
+
+    std::string p = nix_get_string(nullptr, valueResult);
+    std::string pEnd = "-myname";
+    ASSERT_EQ(pEnd, p.substr(p.size() - pEnd.size()));
+
+    // Clean up
+    nix_gc_decref(nullptr, valueFn);
+    nix_state_free(stateFn);
+
+    nix_gc_decref(nullptr, valueResult);
+    nix_state_free(stateResult);
+}
+
+TEST_F(nix_api_expr_test, nix_build_drv)
+{
+    auto expr = R"(derivation { name = "myname";
+                                system = builtins.currentSystem;
+                                builder = "/bin/sh";
+                                args = [ "-c" "echo foo > $out" ];
+                              })";
+    nix_expr_eval_from_string(nullptr, state, expr, ".", value);
+
+    Value * drvPathValue = nix_get_attr_byname(nullptr, value, state, "drvPath");
+    const char * drvPath = nix_get_string(nullptr, drvPathValue);
+
+    std::string p = drvPath;
+    std::string pEnd = "-myname.drv";
+    ASSERT_EQ(pEnd, p.substr(p.size() - pEnd.size()));
+
+    StorePath * drvStorePath = nix_store_parse_path(ctx, store, drvPath);
+    ASSERT_EQ(true, nix_store_is_valid_path(ctx, store, drvStorePath));
+
+    Value * outPathValue = nix_get_attr_byname(ctx, value, state, "outPath");
+    const char * outPath = nix_get_string(ctx, outPathValue);
+
+    p = outPath;
+    pEnd = "-myname";
+    ASSERT_EQ(pEnd, p.substr(p.size() - pEnd.size()));
+    ASSERT_EQ(true, drvStorePath->path.isDerivation());
+
+    StorePath * outStorePath = nix_store_parse_path(ctx, store, outPath);
+    ASSERT_EQ(false, nix_store_is_valid_path(ctx, store, outStorePath));
+
+    // TODO figure out why fails.
+    // `make libexpr-tests_RUN` works, but `nix build .` enters an infinite loop
+    /* nix_store_realise(ctx, store, drvStorePath, nullptr, nullptr); */
+    /* auto is_valid_path = nix_store_is_valid_path(ctx, store, outStorePath); */
+    /* ASSERT_EQ(true, is_valid_path); */
+
+    // Clean up
+    nix_store_path_free(drvStorePath);
+    nix_store_path_free(outStorePath);
+}
+}

tests/unit/libexpr/nix_api_external.cc

@@ -0,0 +1,63 @@
+#include "nix_api_store.h"
+#include "nix_api_store_internal.h"
+#include "nix_api_util.h"
+#include "nix_api_util_internal.h"
+#include "nix_api_expr.h"
+#include "nix_api_expr_internal.h"
+#include "nix_api_value.h"
+#include "nix_api_external.h"
+#include "tests/nix_api_expr.hh"
+
+#include <gtest/gtest.h>
+
+namespace nixC {
+
+class MyExternalValueDesc : public NixCExternalValueDesc
+{
+public:
+    MyExternalValueDesc(int x)
+        : _x(x)
+    {
+        print = print_function;
+        showType = show_type_function;
+        typeOf = type_of_function;
+    }
+
+private:
+    int _x;
+    static void print_function(void * self, nix_printer * printer) {}
+
+    static void show_type_function(void * self, nix_string_return * res) {}
+
+    static void type_of_function(void * self, nix_string_return * res)
+    {
+        MyExternalValueDesc * obj = static_cast<MyExternalValueDesc *>(self);
+
+        std::string type_string = "nix-external<MyExternalValueDesc( ";
+        type_string += std::to_string(obj->_x);
+        type_string += " )>";
+        res->str = &*type_string.begin();
+    }
+};
+
+TEST_F(nix_api_expr_test, nix_expr_eval_external)
+{
+    MyExternalValueDesc * external = new MyExternalValueDesc(42);
+    ExternalValue * val = nix_create_external_value(ctx, external, external);
+    nix_init_external(ctx, value, val);
+
+    EvalState * stateResult = nix_state_create(nullptr, nullptr, store);
+    Value * valueResult = nix_alloc_value(nullptr, stateResult);
+
+    EvalState * stateFn = nix_state_create(nullptr, nullptr, store);
+    Value * valueFn = nix_alloc_value(nullptr, stateFn);
+
+    nix_expr_eval_from_string(nullptr, state, "builtins.typeOf", ".", valueFn);
+
+    ASSERT_EQ(NIX_TYPE_EXTERNAL, nix_get_type(nullptr, value));
+
+    nix_value_call(ctx, state, valueFn, value, valueResult);
+
+    ASSERT_STREQ("nix-external<MyExternalValueDesc( 42 )>", nix_get_string(nullptr, valueResult));
+}
+}

tests/unit/libexpr/nix_api_value.cc

@@ -0,0 +1,184 @@
+#include "nix_api_store.h"
+#include "nix_api_store_internal.h"
+#include "nix_api_util.h"
+#include "nix_api_util_internal.h"
+#include "nix_api_expr.h"
+#include "nix_api_value.h"
+
+#include "tests/nix_api_expr.hh"
+
+#include <cstdlib>
+#include <gtest/gtest.h>
+
+namespace nixC {
+
+TEST_F(nix_api_expr_test, nix_value_set_get_int)
+{
+    ASSERT_EQ(0, nix_get_int(ctx, nullptr));
+    ASSERT_DEATH(nix_get_int(ctx, value), "");
+
+    int myInt = 1;
+    nix_init_int(ctx, value, myInt);
+
+    ASSERT_EQ(myInt, nix_get_int(ctx, value));
+    ASSERT_STREQ("an integer", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_INT, nix_get_type(ctx, value));
+}
+
+TEST_F(nix_api_expr_test, nix_value_set_get_float)
+{
+    ASSERT_FLOAT_EQ(0.0, nix_get_float(ctx, nullptr));
+    ASSERT_DEATH(nix_get_float(ctx, value), "");
+
+    float myDouble = 1.0;
+    nix_init_float(ctx, value, myDouble);
+
+    ASSERT_FLOAT_EQ(myDouble, nix_get_float(ctx, value));
+    ASSERT_STREQ("a float", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_FLOAT, nix_get_type(ctx, value));
+}
+
+TEST_F(nix_api_expr_test, nix_value_set_get_bool)
+{
+    ASSERT_EQ(false, nix_get_bool(ctx, nullptr));
+    ASSERT_DEATH(nix_get_bool(ctx, value), "");
+
+    bool myBool = true;
+    nix_init_bool(ctx, value, myBool);
+
+    ASSERT_EQ(myBool, nix_get_bool(ctx, value));
+    ASSERT_STREQ("a Boolean", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_BOOL, nix_get_type(ctx, value));
+}
+
+TEST_F(nix_api_expr_test, nix_value_set_get_string)
+{
+    ASSERT_EQ(nullptr, nix_get_string(ctx, nullptr));
+    ASSERT_DEATH(nix_get_string(ctx, value), "");
+
+    const char * myString = "some string";
+    nix_init_string(ctx, value, myString);
+
+    ASSERT_STREQ(myString, nix_get_string(ctx, value));
+    ASSERT_STREQ("a string", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_STRING, nix_get_type(ctx, value));
+}
+
+TEST_F(nix_api_expr_test, nix_value_set_get_null)
+{
+    ASSERT_DEATH(nix_get_typename(ctx, value), "");
+
+    nix_init_null(ctx, value);
+
+    ASSERT_STREQ("null", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_NULL, nix_get_type(ctx, value));
+}
+
+TEST_F(nix_api_expr_test, nix_value_set_get_path)
+{
+    ASSERT_EQ(nullptr, nix_get_path_string(ctx, nullptr));
+    ASSERT_DEATH(nix_get_path_string(ctx, value), "");
+
+    const char * p = "/nix/store/40s0qmrfb45vlh6610rk29ym318dswdr-myname";
+    nix_init_path_string(ctx, state, value, p);
+
+    ASSERT_STREQ(p, nix_get_path_string(ctx, value));
+    ASSERT_STREQ("a path", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_PATH, nix_get_type(ctx, value));
+}
+
+TEST_F(nix_api_expr_test, nix_build_and_init_list)
+{
+    ASSERT_EQ(nullptr, nix_get_list_byidx(ctx, nullptr, state, 0));
+    ASSERT_EQ(0, nix_get_list_size(ctx, nullptr));
+
+    ASSERT_DEATH(nix_get_list_byidx(ctx, value, state, 0), "");
+    ASSERT_DEATH(nix_get_list_size(ctx, value), "");
+
+    int size = 10;
+    ListBuilder * builder = nix_make_list_builder(ctx, state, size);
+
+    Value * intValue = nix_alloc_value(ctx, state);
+    nix_init_int(ctx, intValue, 42);
+    nix_list_builder_insert(ctx, builder, 0, intValue);
+    nix_make_list(ctx, builder, value);
+    nix_list_builder_free(builder);
+
+    ASSERT_EQ(42, nix_get_int(ctx, nix_get_list_byidx(ctx, value, state, 0)));
+    ASSERT_EQ(nullptr, nix_get_list_byidx(ctx, value, state, 1));
+    ASSERT_EQ(10, nix_get_list_size(ctx, value));
+
+    ASSERT_STREQ("a list", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_LIST, nix_get_type(ctx, value));
+
+    // Clean up
+    nix_gc_decref(ctx, intValue);
+}
+
+TEST_F(nix_api_expr_test, nix_build_and_init_attr)
+{
+    ASSERT_EQ(nullptr, nix_get_attr_byname(ctx, nullptr, state, 0));
+    ASSERT_EQ(nullptr, nix_get_attr_byidx(ctx, nullptr, state, 0, nullptr));
+    ASSERT_EQ(nullptr, nix_get_attr_name_byidx(ctx, nullptr, state, 0));
+    ASSERT_EQ(0, nix_get_attrs_size(ctx, nullptr));
+    ASSERT_EQ(false, nix_has_attr_byname(ctx, nullptr, state, "no-value"));
+
+    ASSERT_DEATH(nix_get_attr_byname(ctx, value, state, 0), "");
+    ASSERT_DEATH(nix_get_attr_byidx(ctx, value, state, 0, nullptr), "");
+    ASSERT_DEATH(nix_get_attr_name_byidx(ctx, value, state, 0), "");
+    ASSERT_DEATH(nix_get_attrs_size(ctx, value), "");
+    ASSERT_DEATH(nix_has_attr_byname(ctx, value, state, "no-value"), "");
+
+    int size = 10;
+    const char ** out_name = (const char **) malloc(sizeof(char *));
+
+    BindingsBuilder * builder = nix_make_bindings_builder(ctx, state, size);
+
+    Value * intValue = nix_alloc_value(ctx, state);
+    nix_init_int(ctx, intValue, 42);
+
+    Value * stringValue = nix_alloc_value(ctx, state);
+    nix_init_string(ctx, stringValue, "foo");
+
+    nix_bindings_builder_insert(ctx, builder, "a", intValue);
+    nix_bindings_builder_insert(ctx, builder, "b", stringValue);
+    nix_make_attrs(ctx, value, builder);
+    nix_bindings_builder_free(builder);
+
+    ASSERT_EQ(2, nix_get_attrs_size(ctx, value));
+
+    Value * out_value = nix_get_attr_byname(ctx, value, state, "a");
+    ASSERT_EQ(42, nix_get_int(ctx, out_value));
+    nix_gc_decref(ctx, out_value);
+
+    out_value = nix_get_attr_byidx(ctx, value, state, 0, out_name);
+    ASSERT_EQ(42, nix_get_int(ctx, out_value));
+    ASSERT_STREQ("a", *out_name);
+    nix_gc_decref(ctx, out_value);
+
+    ASSERT_STREQ("a", nix_get_attr_name_byidx(ctx, value, state, 0));
+
+    ASSERT_EQ(true, nix_has_attr_byname(ctx, value, state, "b"));
+    ASSERT_EQ(false, nix_has_attr_byname(ctx, value, state, "no-value"));
+
+    out_value = nix_get_attr_byname(ctx, value, state, "b");
+    ASSERT_STREQ("foo", nix_get_string(ctx, out_value));
+    nix_gc_decref(nullptr, out_value);
+
+    out_value = nix_get_attr_byidx(ctx, value, state, 1, out_name);
+    ASSERT_STREQ("foo", nix_get_string(ctx, out_value));
+    ASSERT_STREQ("b", *out_name);
+    nix_gc_decref(nullptr, out_value);
+
+    ASSERT_STREQ("b", nix_get_attr_name_byidx(ctx, value, state, 1));
+
+    ASSERT_STREQ("a set", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_ATTRS, nix_get_type(ctx, value));
+
+    // Clean up
+    nix_gc_decref(ctx, intValue);
+    nix_gc_decref(ctx, stringValue);
+    free(out_name);
+}
+
+}

tests/unit/libstore-support/tests/nix_api_store.hh

@@ -0,0 +1,54 @@
+#pragma once
+///@file
+#include "tests/nix_api_util.hh"
+
+#include "nix_api_store.h"
+#include "nix_api_store_internal.h"
+
+#include <filesystem>
+#include <gtest/gtest.h>
+
+namespace fs = std::filesystem;
+
+namespace nixC {
+class nix_api_store_test : public nix_api_util_context
+{
+public:
+    nix_api_store_test()
+    {
+        nix_libstore_init(ctx);
+        init_local_store();
+    };
+
+    ~nix_api_store_test() override
+    {
+        nix_store_free(store);
+
+        for (auto & path : fs::recursive_directory_iterator(nixDir)) {
+            fs::permissions(path, fs::perms::owner_all);
+        }
+        fs::remove_all(nixDir);
+    }
+
+    Store * store;
+    std::string nixDir;
+    std::string nixStoreDir;
+
+protected:
+    void init_local_store()
+    {
+        auto tmpl = nix::getEnv("TMPDIR").value_or("/tmp") + "/tests_nix-store.XXXXXX";
+        nixDir = mkdtemp((char *) tmpl.c_str());
+        nixStoreDir = nixDir + "/my_nix_store";
+
+        // Options documented in `nix help-stores`
+        const char * p1[] = {"store", nixStoreDir.c_str()};
+        const char * p2[] = {"state", (new std::string(nixDir + "/my_state"))->c_str()};
+        const char * p3[] = {"log", (new std::string(nixDir + "/my_log"))->c_str()};
+
+        const char ** params[] = {p1, p2, p3, nullptr};
+
+        store = nix_store_open(ctx, "local", params);
+    }
+};
+}

tests/unit/libstore/local.mk

@@ -20,12 +20,14 @@ libstore-tests_EXTRA_INCLUDES = \
     -I tests/unit/libstore-support \
     -I tests/unit/libutil-support \
     $(INCLUDE_libstore) \
-    $(INCLUDE_libutil)
+    $(INCLUDE_libstorec) \
+    $(INCLUDE_libutil) \
+    $(INCLUDE_libutilc)
 
 libstore-tests_CXXFLAGS += $(libstore-tests_EXTRA_INCLUDES)
 
 libstore-tests_LIBS = \
     libstore-test-support libutil-test-support \
-    libstore libutil
+    libstore libstorec libutil libutilc
 
 libstore-tests_LDFLAGS := -lrapidcheck $(GTEST_LIBS)

tests/unit/libstore/nix_api_store.cc

@@ -0,0 +1,93 @@
+#include "nix_api_util.h"
+#include "nix_api_util_internal.h"
+#include "nix_api_store.h"
+#include "nix_api_store_internal.h"
+
+#include "tests/nix_api_store.hh"
+
+namespace nixC {
+
+void observe_string_cb(const char * start, unsigned int n, std::string * user_data)
+{
+    *user_data = std::string(start);
+}
+
+std::string PATH_SUFFIX = "/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-name";
+
+TEST_F(nix_api_util_context, nix_libstore_init)
+{
+    auto ret = nix_libstore_init(ctx);
+    ASSERT_EQ(NIX_OK, ret);
+}
+
+TEST_F(nix_api_store_test, nix_store_get_uri)
+{
+    std::string str;
+    auto ret = nix_store_get_uri(ctx, store, (void *) observe_string_cb, &str);
+    ASSERT_EQ(NIX_OK, ret);
+    ASSERT_STREQ("local", str.c_str());
+}
+
+TEST_F(nix_api_store_test, InvalidPathFails)
+{
+    nix_store_parse_path(ctx, store, "invalid-path");
+    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
+}
+
+TEST_F(nix_api_store_test, ReturnsValidStorePath)
+{
+    StorePath * result = nix_store_parse_path(ctx, store, (nixStoreDir + PATH_SUFFIX).c_str());
+    ASSERT_NE(result, nullptr);
+    ASSERT_STREQ("name", result->path.name().data());
+    ASSERT_STREQ(PATH_SUFFIX.substr(1).c_str(), result->path.to_string().data());
+}
+
+TEST_F(nix_api_store_test, SetsLastErrCodeToNixOk)
+{
+    nix_store_parse_path(ctx, store, (nixStoreDir + PATH_SUFFIX).c_str());
+    ASSERT_EQ(ctx->last_err_code, NIX_OK);
+}
+
+TEST_F(nix_api_store_test, DoesNotCrashWhenContextIsNull)
+{
+    ASSERT_NO_THROW(nix_store_parse_path(ctx, store, (nixStoreDir + PATH_SUFFIX).c_str()));
+}
+
+TEST_F(nix_api_store_test, get_version)
+{
+    std::string str;
+    auto ret = nix_store_get_version(ctx, store, (void *) observe_string_cb, &str);
+    ASSERT_EQ(NIX_OK, ret);
+    ASSERT_STREQ(PACKAGE_VERSION, str.c_str());
+}
+
+TEST_F(nix_api_util_context, nix_store_open_dummy)
+{
+    nix_libstore_init(ctx);
+    Store * store = nix_store_open(ctx, "dummy://", nullptr);
+    ASSERT_EQ(NIX_OK, ctx->last_err_code);
+    ASSERT_STREQ("dummy", store->ptr->getUri().c_str());
+
+    std::string str;
+    nix_store_get_version(ctx, store, (void *) observe_string_cb, &str);
+    ASSERT_STREQ("", str.c_str());
+
+    nix_store_free(store);
+}
+
+TEST_F(nix_api_util_context, nix_store_open_invalid)
+{
+    nix_libstore_init(ctx);
+    Store * store = nix_store_open(ctx, "invalid://", nullptr);
+    ASSERT_EQ(NIX_ERR_NIX_ERROR, ctx->last_err_code);
+    ASSERT_EQ(nullptr, store);
+    nix_store_free(store);
+}
+
+TEST_F(nix_api_store_test, nix_store_is_valid_path_not_in_store)
+{
+    StorePath * path = nix_store_parse_path(ctx, store, (nixStoreDir + PATH_SUFFIX).c_str());
+    ASSERT_EQ(false, nix_store_is_valid_path(ctx, store, path));
+}
+
+}

tests/unit/libutil-support/tests/nix_api_util.hh

@@ -0,0 +1,27 @@
+#pragma once
+///@file
+#include "nix_api_util.h"
+
+#include <gtest/gtest.h>
+
+namespace nixC {
+
+class nix_api_util_context : public ::testing::Test
+{
+protected:
+
+    nix_api_util_context()
+    {
+        ctx = nix_c_context_create();
+        nix_libutil_init(ctx);
+    };
+
+    ~nix_api_util_context() override
+    {
+        nix_c_context_free(ctx);
+        ctx = nullptr;
+    }
+
+    nix_c_context * ctx;
+};
+}

tests/unit/libutil/local.mk

@@ -18,11 +18,12 @@ libutil-tests_SOURCES := $(wildcard $(d)/*.cc)
 
 libutil-tests_EXTRA_INCLUDES = \
     -I tests/unit/libutil-support \
-    $(INCLUDE_libutil)
+    $(INCLUDE_libutil) \
+    $(INCLUDE_libutilc)
 
 libutil-tests_CXXFLAGS += $(libutil-tests_EXTRA_INCLUDES)
 
-libutil-tests_LIBS = libutil-test-support libutil
+libutil-tests_LIBS = libutil-test-support libutil libutilc
 
 libutil-tests_LDFLAGS := -lrapidcheck $(GTEST_LIBS)
 

tests/unit/libutil/nix_api_util.cc

@@ -0,0 +1,145 @@
+#include "config.hh"
+#include "args.hh"
+#include "nix_api_util.h"
+#include "nix_api_util_internal.h"
+#include "tests/nix_api_util.hh"
+
+#include <gtest/gtest.h>
+
+namespace nixC {
+
+void observe_string_cb(const char * start, unsigned int n, std::string * user_data)
+{
+    *user_data = std::string(start);
+}
+
+TEST_F(nix_api_util_context, nix_context_error)
+{
+    std::string err_msg_ref;
+    try {
+        throw nix::Error("testing error");
+    } catch (nix::Error & e) {
+        err_msg_ref = e.what();
+        nix_context_error(ctx);
+    }
+    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
+    ASSERT_EQ(ctx->name, "nix::Error");
+    ASSERT_EQ(*ctx->last_err, err_msg_ref);
+    ASSERT_EQ(ctx->info->msg.str(), "testing error");
+
+    try {
+        throw std::runtime_error("testing exception");
+    } catch (std::exception & e) {
+        err_msg_ref = e.what();
+        nix_context_error(ctx);
+    }
+    ASSERT_EQ(ctx->last_err_code, NIX_ERR_UNKNOWN);
+    ASSERT_EQ(*ctx->last_err, err_msg_ref);
+}
+
+TEST_F(nix_api_util_context, nix_set_err_msg)
+{
+    ASSERT_EQ(ctx->last_err_code, NIX_OK);
+    nix_set_err_msg(ctx, NIX_ERR_UNKNOWN, "unknown test error");
+    ASSERT_EQ(ctx->last_err_code, NIX_ERR_UNKNOWN);
+    ASSERT_EQ(*ctx->last_err, "unknown test error");
+}
+
+TEST(nix_api_util, nix_version_get)
+{
+    ASSERT_EQ(std::string(nix_version_get()), PACKAGE_VERSION);
+}
+
+struct MySettings : nix::Config
+{
+    nix::Setting<std::string> settingSet{this, "empty", "setting-name", "Description"};
+};
+
+MySettings mySettings;
+static nix::GlobalConfig::Register rs(&mySettings);
+
+TEST_F(nix_api_util_context, nix_setting_get)
+{
+    ASSERT_EQ(ctx->last_err_code, NIX_OK);
+    std::string setting_value;
+    nix_err result = nix_setting_get(ctx, "invalid-key", (void *) observe_string_cb, &setting_value);
+    ASSERT_EQ(result, NIX_ERR_KEY);
+
+    result = nix_setting_get(ctx, "setting-name", (void *) observe_string_cb, &setting_value);
+    ASSERT_EQ(result, NIX_OK);
+    ASSERT_STREQ("empty", setting_value.c_str());
+}
+
+TEST_F(nix_api_util_context, nix_setting_set)
+{
+    nix_err result = nix_setting_set(ctx, "invalid-key", "new-value");
+    ASSERT_EQ(result, NIX_ERR_KEY);
+
+    result = nix_setting_set(ctx, "setting-name", "new-value");
+    ASSERT_EQ(result, NIX_OK);
+
+    std::string setting_value;
+    result = nix_setting_get(ctx, "setting-name", (void *) observe_string_cb, &setting_value);
+    ASSERT_EQ(result, NIX_OK);
+    ASSERT_STREQ("new-value", setting_value.c_str());
+}
+
+TEST_F(nix_api_util_context, nix_err_msg)
+{
+    // no error
+    EXPECT_THROW(nix_err_msg(nullptr, ctx, NULL), nix::Error);
+
+    // set error
+    nix_set_err_msg(ctx, NIX_ERR_UNKNOWN, "unknown test error");
+
+    // basic usage
+    std::string err_msg = nix_err_msg(NULL, ctx, NULL);
+    ASSERT_EQ(err_msg, "unknown test error");
+
+    // advanced usage
+    unsigned int sz;
+    err_msg = nix_err_msg(nix_c_context_create(), ctx, &sz);
+    ASSERT_EQ(sz, err_msg.size());
+}
+
+TEST_F(nix_api_util_context, nix_err_info_msg)
+{
+    std::string err_info;
+
+    // no error
+    EXPECT_THROW(nix_err_info_msg(NULL, ctx, (void *) observe_string_cb, &err_info), nix::Error);
+
+    try {
+        throw nix::Error("testing error");
+    } catch (...) {
+        nix_context_error(ctx);
+    }
+    nix_err_info_msg(nix_c_context_create(), ctx, (void *) observe_string_cb, &err_info);
+    ASSERT_STREQ("testing error", err_info.c_str());
+}
+
+TEST_F(nix_api_util_context, nix_err_name)
+{
+    std::string err_name;
+
+    // no error
+    EXPECT_THROW(nix_err_name(NULL, ctx, (void *) observe_string_cb, &err_name), nix::Error);
+
+    std::string err_msg_ref;
+    try {
+        throw nix::Error("testing error");
+    } catch (...) {
+        nix_context_error(ctx);
+    }
+    nix_err_name(nix_c_context_create(), ctx, (void *) observe_string_cb, &err_name);
+    ASSERT_EQ(std::string(err_name), "nix::Error");
+}
+
+TEST_F(nix_api_util_context, nix_err_code)
+{
+    ASSERT_EQ(nix_err_code(ctx), NIX_OK);
+    nix_set_err_msg(ctx, NIX_ERR_UNKNOWN, "unknown test error");
+    ASSERT_EQ(nix_err_code(ctx), NIX_ERR_UNKNOWN);
+}
+
+}