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