nix-super/doc/external-api/README.md

# 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>

int main() {
   nix_libexpr_init(NULL);

   Store* store = nix_store_open(NULL, "dummy://", NULL);
   State* 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_unref(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, State* state, Value** args, Value* v) {
    nix_value_force(NULL, state, args[0]);
    if (nix_get_type(NULL, args[0]) == NIX_TYPE_INT) {
      nix_set_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
```