How to call a C function from Zig

  • The relative paths of all files are as follows
  |-- main.zig
  |-- c.zig  
  |-- cxmath.h
  |-- cxmath.c


const std = @import("std");
const c = @import("c.zig");

pub fn main() !void {
	_ = c.printf("Hello\n");
	//_ = c.printf("%d\n", cxadd(12, 34));


pub usingnamespace @cImport({



#include <stdio.h>

int cxadd(int a, int b);

void sayHi();



#include "cxmath.h"

void sayHi()

int cxadd(int a, int b)
	return a + b;


const std = @import("std");
const Build =;

// Although this function looks imperative, note that its job is to
// declaratively construct a build graph that will be executed by an external
// runner.
pub fn build(b: *std.Build) void {
    // Standard target options allows the person running `zig build` to choose
    // what target to build for. Here we do not override the defaults, which
    // means any target is allowed, and the default is native. Other options
    // for restricting supported target set are available.
    const target = b.standardTargetOptions(.{});

    // Standard optimization options allow the person running `zig build` to select
    // between Debug, ReleaseSafe, ReleaseFast, and ReleaseSmall. Here we do not
    // set a preferred release mode, allowing the user to decide how to optimize.
    const optimize = b.standardOptimizeOption(.{});

    const exe = b.addExecutable(.{
        .name = "hello-world",
        // In this case the main source file is merely a path, however, in more
        // complicated build scripts, this could be a generated file.
        .root_source_file = .{ .path = "src/main.zig" },
        .target = target,
        .optimize = optimize,
        .file = Build.LazyPath.relative("src/cxmath.c"),
        .flags = &[_][]const u8{"-std=c99"},

    // This declares intent for the executable to be installed into the
    // standard location when the user invokes the "install" step (the default
    // step when running `zig build`).

    // This *creates* a Run step in the build graph, to be executed when another
    // step is evaluated that depends on it. The next line below will establish
    // such a dependency.
    const run_cmd = b.addRunArtifact(exe);

    // By making the run step depend on the install step, it will be run from the
    // installation directory rather than directly from within the cache directory.
    // This is not necessary, however, if the application depends on other installed
    // files, this ensures they will be present and in the expected location.

    // This allows the user to pass arguments to the application in the build
    // command itself, like this: `zig build run -- arg1 arg2 etc`
    if (b.args) |args| {

    // This creates a build step. It will be visible in the `zig build --help` menu,
    // and can be selected like this: `zig build run`
    // This will evaluate the `run` step rather than the default, which is "install".
    const run_step = b.step("run", "Run the app");


I ran 2 commands:

  1. zig init-exe
  2. zig build


src/main.zig:7:2: error: use of undeclared identifier 'sayHi'

How to fix it? I’m new here.

1 Like

Oh silly me

There is no need for c.zig btw. You can achieve the same effect without using usingnamespace with

// main.zig
const std = @import("std");
const c = @cImport({
// ...
1 Like

Thanks. I’m just trying out different features of this language.

1 Like

Thanks for asking, Helped me figure out i wasn’t linking a system library in build.zig