Zig Language Reference (v0.15)

This skill covers Zig 0.15.x language and standard library. It is retained as a compatibility reference alongside the primary zig-0.16 skill.

Capability Boundaries

✅ Strong Suits

1. Writing and reviewing Zig 0.15.x code
2. Understanding breaking changes from 0.14 to 0.15
3. Using the build.zig / build.zig.zon build system (legacy)
4. Comptime metaprogramming and builtin functions

⚠️ Requirements

1. Target version must be 0.15.x
2. For new projects, prefer zig-0.16 (0.16.0 is the latest stable)

❌ Out of Scope (with alternatives)

1. Do not use this for Zig 0.16.0 development → use zig-0.16 skill instead
2. Do not use this for game/graphics development → use zig-raylib or zig-sdl3-bindings

When to use

Use this skill when the user confirms they are targeting Zig 0.15.x and need to write or review legacy code.

Data Privacy

This skill does not collect, store, or transmit any user data. All code examples are for local development reference only.

Quick Start

Example invocations:

Write a program in Zig 0.15, scaffold the project
Check if this build.zig compiles under 0.15
Review this code for 0.14→0.15 compatibility issues

Workflow

Step 1. Confirm version — Ask the user to run zig version to confirm 0.15.x Step 2. Check breaking changes — Review the Removed Features section for deprecated APIs Step 3. Check I/O patterns — Use the correct std.Io pattern instead of legacy std.io Step 4. Check build system — Use root_module-based build.zig API Step 5. Container init — Use .empty/.init instead of .{ } Step 6. Output code — Provide complete compilable examples

Critical: Removed Features (0.15.x)

usingnamespace - REMOVED

// WRONG - compile error
pub usingnamespace @import("other.zig");

// CORRECT - explicit re-export
const other = @import("other.zig");
pub const foo = other.foo;

async/await - REMOVED

Keywords removed from language. Async I/O support is planned for future releases.

Critical: I/O API Rewrite ("Writergate")

The entire std.io API changed. New std.Io.Writer and std.Io.Reader are non-generic with buffer in the interface.

Writing

// WRONG - old API
const stdout = std.io.getStdOut().writer();
try stdout.print("Hello\n", .{});

// CORRECT - new API: provide buffer, access .interface, flush
var buf: [4096]u8 = undefined;
var stdout_writer = std.fs.File.stdout().writer(&buf);
const stdout = &stdout_writer.interface;
try stdout.print("Hello\n", .{});
try stdout.flush();  // REQUIRED!

Reading

// Reading from file
var buf: [4096]u8 = undefined;
var file_reader = file.reader(&buf);
const r = &file_reader.interface;

// Read line by line (takeDelimiter returns null at EOF)
while (try r.takeDelimiter('\n')) |line| {
    // process line (doesn't include '\n')
}

// Read binary data
const header = try r.takeStruct(Header, .little);
const value = try r.takeInt(u32, .big);

Fixed Buffer Writer (no file)

var buf: [256]u8 = undefined;
var w: std.Io.Writer = .fixed(&buf);
try w.print("Hello {s}", .{"world"});
const result = w.buffered();  // "Hello world"

Fixed Reader (from slice)

var r: std.Io.Reader = .fixed("hello\nworld");
const line = (try r.takeDelimiter('\n')).?;  // "hello" (returns null at EOF)

Deprecated: BufferedWriter, CountingWriter, std.io.bufferedWriter()

Deprecated: GenericWriter, GenericReader, AnyWriter, AnyReader, FixedBufferStream

New: std.Io.Writer, std.Io.Reader - non-generic, buffer in interface

Replacements:

• CountingWriter → std.Io.Writer.Discarding (has .fullCount())
• BufferedWriter → buffer provided to .writer(&buf) call
• Allocating output → std.Io.Writer.Allocating

Critical: Build System (0.15.x)

rootsourcefile is REMOVED from addExecutable/addLibrary/addTest. Use root_module:

// WRONG - removed field
b.addExecutable(.{
    .name = "app",
    .root_source_file = b.path("src/main.zig"),  // ERROR
    .target = target,
});

// CORRECT
b.addExecutable(.{
    .name = "app",
    .root_module = b.createModule(.{
        .root_source_file = b.path("src/main.zig"),
        .target = target,
        .optimize = optimize,
    }),
});

Module imports changed:

// WRONG (old API)
exe.addModule("helper", helper_mod);

// CORRECT
exe.root_module.addImport("helper", helper_mod);

Adding dependency modules:

const dep = b.dependency("lib", .{ .target = target, .optimize = optimize });
exe.root_module.addImport("lib", dep.module("lib"));

Compile-level methods deprecated: exe.linkSystemLibrary(), exe.addCSourceFiles(), exe.addIncludePath(), exe.linkLibC() are deprecated — use exe.root_module.* equivalents instead.

See [std.Build reference](references/std-build.md) for complete build system documentation.

Critical: Container Initialization

Never use .{} for containers. Use .empty or .init:

// WRONG - deprecated
var list: std.ArrayList(u32) = .{};
var gpa: std.heap.DebugAllocator(.{}) = .{};

// CORRECT - use .empty for empty collections
var list: std.ArrayList(u32) = .empty;
var map: std.AutoHashMapUnmanaged(u32, u32) = .empty;

// CORRECT - use .init for stateful types with internal config
var gpa: std.heap.DebugAllocator(.{}) = .init;
var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);

Naming Changes

• std.ArrayListUnmanaged → std.ArrayList (Unmanaged is now default, old name deprecated)
• std.heap.GeneralPurposeAllocator → std.heap.DebugAllocator (GPA alias still works)

std.BoundedArray - REMOVED. Use:

var buffer: [8]i32 = undefined;
var stack = std.ArrayList(i32).initBuffer(&buffer);

Critical: Format Strings (0.15.x)

{f} required to call format methods:

// WRONG - ambiguous error
std.debug.print("{}", .{std.zig.fmtId("x")});

// CORRECT
std.debug.print("{f}", .{std.zig.fmtId("x")});

Format method signature changed:

// OLD - wrong
pub fn format(self: @This(), comptime fmt: []const u8, opts: std.fmt.FormatOptions, writer: anytype) !void

// NEW - correct
pub fn format(self: @This(), writer: *std.Io.Writer) std.Io.Writer.Error!void

Breaking Changes (0.14.0+)

@branchHint replaces @setCold

// WRONG
@setCold(true);

// CORRECT
@branchHint(.cold);  // Must be first statement in block

@export takes pointer

// WRONG
@export(foo, .{ .name = "bar" });

// CORRECT
@export(&foo, .{ .name = "bar" });

Inline asm clobbers are typed

// WRONG
: "rcx", "r11"

// CORRECT
: .{ .rcx = true, .r11 = true }

@fence - REMOVED

Use stronger atomic orderings or RMW operations instead.

Decl Literals (0.14.0+)

.identifier syntax works for declarations:

const S = struct {
    x: u32,
    const default: S = .{ .x = 0 };
    fn init(v: u32) S { return .{ .x = v }; }
};

const a: S = .default;      // S.default
const b: S = .init(42);     // S.init(42)
const c: S = try .init(1);  // works with try

Labeled Switch (0.14.0+)

State machines use continue :label:

state: switch (initial) {
    .idle => continue :state .running,
    .running => if (done) break :state result else continue :state .running,
    .error => return error.Failed,
}

Non-exhaustive Enum Switch (0.15.x)

Can mix explicit tags with _ and else:

switch (value) {
    .a, .b => {},
    else => {},  // other named tags
    _ => {},     // unnamed integer values
}

Quick Fixes

Language References

Load these references when working with core language features:

Code Style

• [Style Guide](references/style-guide.md) - Official Zig naming conventions (TitleCase types, camelCase functions, snake_case variables), whitespace rules, doc comment guidance, redundancy avoidance, zig fmt

Language Basics & Built-ins

• [Language Basics](references/language.md) - Core language: types, control flow (if/while/for/switch), error handling (try/catch/errdefer), optionals, structs, enums, unions, pointers, slices, comptime, functions
• [Built-in Functions](references/builtins.md) - All @ built-ins: type casts (@intCast, @bitCast, @ptrCast), arithmetic (@addWithOverflow, @divExact), bit ops (@clz, @popCount), memory (@memcpy, @sizeOf), atomics (@atomicRmw, @cmpxchgWeak), introspection (@typeInfo, @TypeOf, @hasDecl), SIMD (@Vector, @splat, @reduce), C interop (@cImport, @export)

Standard Library References

Load these references when working with specific modules:

Memory & Slices

• [std.mem](references/std-mem.md) - Slice search/compare, split/tokenize, alignment, endianness, byte conversion

Text & Encoding

• [std.fmt](references/std-fmt.md) - Format strings, integer/float parsing, hex encoding, custom formatters, {f} specifier (0.15.x)
• [std.ascii](references/std-ascii.md) - ASCII character classification (isAlpha, isDigit), case conversion, case-insensitive comparison
• [std.unicode](references/std-unicode.md) - UTF-8/UTF-16 encoding/decoding, codepoint iteration, validation, WTF-8 for Windows
• [std.base64](references/std-base64.md) - Base64 encoding/decoding (standard, URL-safe, with/without padding)

Math & Random

• [std.math](references/std-math.md) - Floating-point ops, trig, overflow-checked arithmetic, constants, complex numbers, big integers
• [std.Random](references/std-random.md) - PRNGs (Xoshiro256, Pcg), CSPRNGs (ChaCha), random integers/floats/booleans, shuffle, distributions
• [std.hash](references/std-hash.md) - Non-cryptographic hash functions (Wyhash, XxHash, FNV, Murmur, CityHash), checksums (CRC32, Adler32), auto-hashing

SIMD & Vectorization

• [std.simd](references/std-simd.md) - SIMD vector utilities: optimal vector length, iota/repeat/join/interlace patterns, element shifting/rotation, parallel searching, prefix scans, branchless selection

Time & Timing

• [std.time](references/std-time.md) - Wall-clock timestamps, monotonic Instant/Timer, epoch conversions, calendar utilities (year/month/day), time unit constants
• [std.Tz](references/std-tz.md) - TZif timezone database parsing (RFC 8536), UTC offsets, DST rules, timezone abbreviations, leap seconds

Sorting & Searching

• [std.sort](references/std-sort.md) - Sorting algorithms (pdq, block, heap, insertion), binary search, min/max

Core Data Structures

• [std.ArrayList](references/std-arraylist.md) - Dynamic arrays, vectors, BoundedArray replacement
• [std.HashMap / AutoHashMap](references/std-hashmap.md) - Hash maps, string maps, ordered maps
• [std.ArrayHashMap](references/std-array-hash-map.md) - Insertion-order preserving hash map, array-style key/value access
• [std.MultiArrayList](references/std-multi-array-list.md) - Struct-of-arrays for cache-efficient struct storage
• [std.SegmentedList](references/std-segmented-list.md) - Stable pointers, arena-friendly, non-copyable types
• [std.DoublyLinkedList / SinglyLinkedList](references/std-linked-list.md) - Intrusive linked lists, O(1) insert/remove
• [std.PriorityQueue](references/std-priority-queue.md) - Binary heap, min/max extraction, task scheduling
• [std.PriorityDequeue](references/std-priority-dequeue.md) - Min-max heap, double-ended priority extraction
• [std.Treap](references/std-treap.md) - Self-balancing BST, ordered keys, min/max/predecessor
• [std.bit_set](references/std-bit-set.md) - Bit sets (Static, Dynamic, Integer, Array), set operations, iteration
• [std.BufMap / BufSet](references/std-buf-map.md) - String-owning maps and sets, automatic key/value memory management
• [std.StaticStringMap](references/std-static-string-map.md) - Compile-time optimized string lookup, perfect hash for keywords
• [std.enums](references/std-enums.md) - EnumSet, EnumMap, EnumArray: bit-backed enum collections

Allocators

• [std.heap](references/std-allocators.md) - Allocator selection guide, ArenaAllocator, DebugAllocator, FixedBufferAllocator, MemoryPool, SmpAllocator, ThreadSafeAllocator, StackFallbackAllocator, custom allocator implementation

I/O & Files

• [std.io](references/std-io.md) - Reader/Writer API (0.15.x): buffered I/O, streaming, binary data, format strings
• [std.fs](references/std-fs.md) - File system: files, directories, iteration, atomic writes, paths
• [std.tar](references/std-tar.md) - Tar archive reading/writing, extraction, POSIX ustar, GNU/pax extensions
• [std.zip](references/std-zip.md) - ZIP archive reading/extraction, ZIP64 support, store/deflate compression
• [std.compress](references/std-compress.md) - Compression: DEFLATE (gzip, zlib), Zstandard, LZMA, LZMA2, XZ decompression/compression

Networking

• [std.http](references/std-http.md) - HTTP client/server, TLS, connection pooling, compression, WebSocket
• [std.net](references/std-net.md) - TCP/UDP sockets, address parsing, DNS resolution
• [std.Uri](references/std-uri.md) - URI parsing/formatting (RFC 3986), percent-encoding/decoding, relative URI resolution

Process Management

• [std.process](references/std-process.md) - Child process spawning, environment variables, argument parsing, exec

OS-Specific APIs

• [std.os](references/std-os.md) - OS-specific APIs: Linux syscalls, io_uring, Windows NT APIs, WASI, direct platform access
• [std.c](references/std-c.md) - C ABI types and libc bindings: platform-specific types (fdt, pidt, timespec), errno values, socket/signal/memory types, fcntl/open flags, FFI with C libraries

Concurrency

• [std.Thread](references/std-thread.md) - Thread spawning, Mutex, RwLock, Condition, Semaphore, WaitGroup, thread pools
• [std.atomic](references/std-atomic.md) - Lock-free atomic operations: Value wrapper, fetch-and-modify (add/sub/and/or/xor), compare-and-swap, atomic ordering semantics, spin loop hints, cache line sizing

Patterns & Best Practices

• [Zig Patterns](references/patterns.md) - Load when writing new code or reviewing code quality. Comprehensive best practices extracted from the Zig standard library: quick patterns (memory/allocators, file I/O, HTTP, JSON, testing, build system) plus idiomatic code patterns covering syntax (closures, context pattern, options structs, destructuring), polymorphism (duck typing, generics, custom formatting, dynamic/static dispatch), safety (diagnostics, error payloads, defer/errdefer, compile-time assertions), and performance (const pointer passing)
• [Code Review](references/code-review.md) - Load when reviewing Zig code. Systematic checklist organized by confidence level: ALWAYS FLAG (removed features, changed syntax, API changes), FLAG WITH CONTEXT (exception safety bugs, missing flush, allocator issues), SUGGEST (style improvements). Includes migration examples for 0.14/0.15 breaking changes

Serialization

• [std.json](references/std-json.md) - JSON parsing, serialization, dynamic values, streaming, custom parse/stringify
• [std.zon](references/std-zon.md) - ZON (Zig Object Notation) parsing and serialization for build.zig.zon, config files, data interchange

Testing & Debug

• [std.testing](references/std-testing.md) - Unit test assertions and utilities
• [std.debug](references/std-debug.md) - Panic, assert, stack traces, hex dump, format specifiers
• [std.log](references/std-log.md) - Scoped logging with configurable levels and output

Metaprogramming

• [Comptime Reference](references/comptime.md) - Comptime fundamentals, type reflection (@typeInfo/@Type/@TypeOf), loop variants (comptime for vs inline for), branch elimination, type generation, comptime limitations
• [std.meta](references/std-meta.md) - Type introspection, field iteration, stringToEnum, generic programming

Compiler Utilities

• [std.zig](references/std-zig.md) - AST parsing, tokenization, source analysis, linters, formatters, ZON parsing

Security & Cryptography

• [std.crypto](references/std-crypto.md) - Hashing (SHA2, SHA3, Blake3), AEAD (AES-GCM, ChaCha20-Poly1305), signatures (Ed25519, ECDSA), key exchange (X25519), password hashing (Argon2, scrypt, bcrypt), secure random, timing-safe operations

Build System

• [std.Build](references/std-build.md) - Build system: build.zig, modules, dependencies, build.zig.zon, steps, options, testing, C/C++ integration

Interoperability

• [C Interop](references/c-interop.md) - Exporting C-compatible APIs: export fn, C calling convention, building static/dynamic libraries, creating headers, macOS universal binaries, XCFramework for Swift/Xcode, module maps

Audience

Customization: specify output format (full code / diff / snippet).

Gotchas

1. Always confirm the version first — User code may come from any older version; run zig version to check
2. build.zig API differences — 0.15.x addExecutable uses rootmodule instead of the old rootsource_file
3. std.Io pattern — 0.15 partially adopts the new std.Io pattern; old std.io is incompatible
4. Container initialization — .{ } for ArrayList etc. will error; use .empty or .init
5. Format strings — Custom formatters require {f} instead of {}
6. Removed features — async/await, usingnamespace, @fence are fully removed

FAQ

Q: Should I use zig-0.15 or zig-0.16? A: For new projects, prefer zig-0.16. zig-0.15 is retained for compatibility with existing 0.15 projects.

Q: What is the biggest difference between 0.15 and 0.14? A: The I/O API rewrite (std.io → std.Io) and the introduction of root_module in build.zig.

Q: How do I create an executable in 0.15? A: Use b.addExecutable(.{ .name = "...", .root_module = b.createModule(...) }).