Fixes errno to work on single-threaded targets, adds CI

.github/workflows/build.yml

@@ -0,0 +1,28 @@
+name: Continuous Integration
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+        single_thread: [true, false]
+
+    runs-on: ${{ matrix.os }}
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Zig
+        uses: goto-bus-stop/setup-zig@v2
+        with:
+          version: 0.11.0
+
+      - name: Generate and validate packages
+        run: |
+          zig build -Dsingle-threaded=${{ matrix.single_thread }}

build.zig

@@ -22,11 +22,14 @@ pub fn build(b: *std.Build) void {
     const target = b.standardTargetOptions(.{});
     const optimize = b.standardOptimizeOption(.{});
 
+    const single_threaded = b.option(bool, "single-threaded", "Create a single-threaded libc implementation (default: false)") orelse false;
+
     // check if the host has a gcc or clang available:
     const maybe_gcc = b.findProgram(&.{"gcc"}, &.{}) catch null;
     const maybe_clang = b.findProgram(&.{"clang"}, &.{}) catch null;
 
     const libc = createLibrary(b, target, optimize);
+    libc.single_threaded = single_threaded;
     b.installArtifact(libc);
 
     for (header_files) |header_name| {

src/modules/errno.zig

@@ -1,9 +1,17 @@
 //! implementation of `errno.h`
 
 const std = @import("std");
+const builtin = @import("builtin");
 
-threadlocal var errno: c_int = 0;
+const storage = if (builtin.single_threaded)
+    struct {
+        var errno: c_int = 0; // regular variable on single-threaded systems
+    }
+else
+    struct {
+        threadlocal var errno: c_int = 0; // thread-local variable on multi-threaded systems
+    };
 
 export fn __get_errno() *c_int {
-    return &errno;
+    return &storage.errno;
 }