mirror of
https://github.com/privatevoid-net/nix-super.git
synced 2024-11-24 14:56:15 +02:00
121 lines
3.6 KiB
Markdown
121 lines
3.6 KiB
Markdown
# 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>
|
|
#include <stdlib.h>
|
|
#include <string.h>
|
|
|
|
// NOTE: This example lacks all error handling. Production code must check for
|
|
// errors, as some return values will be undefined.
|
|
|
|
void my_get_string_cb(const char * start, unsigned int n, char ** user_data)
|
|
{
|
|
*user_data = strdup(start);
|
|
}
|
|
|
|
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);
|
|
|
|
char * version;
|
|
nix_get_string(NULL, value, my_get_string_cb, version);
|
|
printf("Nix version: %s\n", version);
|
|
|
|
free(version);
|
|
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
|
|
```
|