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std.debug (Zig 0.16.0)

Debugging utilities: panic handling, assertions, stack traces, hex dumps, and value tracing.

Primary Zig 0.16 release-note source: https://ziglang.org/download/0.16.0/release-notes.html

Zig 0.16 reworked debug information and expanded target support for segfault handling/unwinding. For application-owned output, create a writer from the caller's std.Io (for example std.Io.File.stderr().writer(io, &buf)). Low-level std.debug.print and stack-dump helpers intentionally use the configured debug I/O path instead.

Quick Reference

Function	Purpose
`print(fmt, args)`	Printf-style debug output to stderr
`panic(fmt, args)`	Format message and abort
`assert(bool)`	Crash if false (optimized out in ReleaseFast)
`dumpCurrentStackTrace(addr)`	Print stack trace to stderr
`dumpHex(bytes)`	Print hexdump to stderr

Debug Printing

const std = @import("std");

// Quick debug output (64-byte buffer, auto-flush)
std.debug.print("value: {}\n", .{x});
std.debug.print("name: {s}, count: {d}\n", .{name, count});

// Print without newline
std.debug.print("loading...", .{});

Note: std.debug.print silently ignores errors. For production logging, use std.log.

Format Specifiers

Format string syntax: {[argument][specifier]:[fill][alignment][width].[precision]}. Named arguments use square brackets, for example {[name]s}.

Type Specifiers

Specifier	Types	Output
`{}`	any	Default formatting
`{s}`	`[]const u8`, `[*:0]const u8`	String
`{d}`	int, float, enum	Decimal
`{b}`	int, enum	Binary
`{o}`	int, enum	Octal
`{x}`	int, float, `[]u8`, enum	Lowercase hex
`{X}`	int, float, `[]u8`, enum	Uppercase hex
`{c}`	u8, u21	ASCII character
`{u}`	u21	Unicode codepoint
`{e}`	float	Scientific notation
`{*}`	pointer	Address (`Type@0x...`)
`{f}`	has `format` method	Custom formatter
`{any}`	any	Debug representation with depth limit

Examples

std.debug.print("{d}\n", .{42});           // "42"
std.debug.print("{x}\n", .{255});          // "ff"
std.debug.print("{X}\n", .{255});          // "FF"
std.debug.print("{b}\n", .{5});            // "101"
std.debug.print("{o}\n", .{64});           // "100"
std.debug.print("{s}\n", .{"hello"});      // "hello"
std.debug.print("{c}\n", .{'A'});          // "A"
std.debug.print("{*}\n", .{&value});       // "i32@7fff5fbff8a0"

// Floats
std.debug.print("{d}\n", .{3.14159});      // "3.14159"
std.debug.print("{e}\n", .{1234.5});       // "1.2345e+03"
std.debug.print("{x}\n", .{@as(f32, 1.0)}); // "0x1.0p0"

// Hex dump of bytes
std.debug.print("{x}\n", .{"hello"});      // "68656c6c6f"

Width and Alignment

std.debug.print("{d:5}\n", .{42});         // "   42" (right-aligned, width 5)
std.debug.print("{d:<5}\n", .{42});        // "42   " (left-aligned)
std.debug.print("{d:^5}\n", .{42});        // " 42  " (center-aligned)
std.debug.print("{d:0>5}\n", .{42});       // "00042" (zero-padded)
std.debug.print("{s:_<10}\n", .{"hi"});    // "hi________" (custom fill)

Precision

std.debug.print("{d:.2}\n", .{3.14159});   // "3.14"
std.debug.print("{e:.3}\n", .{1234.5});    // "1.234e+03"
std.debug.print("{x:.4}\n", .{@as(f32, 1.0)}); // "0x1.0000p0"

Named and Positional Arguments

// Positional
std.debug.print("{0} {1} {0}\n", .{"a", "b"});  // "a b a"

// Named (with struct)
std.debug.print("{[name]s}: {[value]d}\n", .{ .name = "x", .value = 42 });

// Runtime width/precision
std.debug.print("{[value]d:[width]}\n", .{ .value = 42, .width = 5 });
std.debug.print("{[value]d:.[precision]}\n", .{ .value = 3.14159, .precision = 2 });

Escape Braces

std.debug.print("{{literal braces}}\n", .{});  // "{literal braces}"

Custom Format Method

Types can implement a format method for {f}:

const Point = struct {
    x: f32,
    y: f32,

    pub fn format(self: @This(), writer: *std.Io.Writer) std.Io.Writer.Error!void {
        try writer.print("({d:.2}, {d:.2})", .{ self.x, self.y });
    }
};

const p = Point{ .x = 1.5, .y = 2.5 };
std.debug.print("{f}\n", .{p});  // "(1.50, 2.50)"

Any Format (Debug Representation)

const data = .{ .x = 1, .list = &[_]u8{ 1, 2, 3 } };
std.debug.print("{any}\n", .{data});
// Prints struct with depth-limited recursion

Assertions

// Runtime assertion (triggers illegal instruction on failure)
std.debug.assert(x > 0);
std.debug.assert(ptr != null);

// Debug/ReleaseSafe: generates check
// ReleaseFast/ReleaseSmall: optimized away (undefined behavior if false)

Specialized Assertions

// Assert slice is readable (checks memory mapping)
std.debug.assertReadable(slice);

// Assert pointer alignment
std.debug.assertAligned(ptr, .@"16");  // 16-byte alignment

Panic

// Formatted panic message
std.debug.panic("invalid state: {}", .{state});

// With explicit return address
std.debug.panicExtra(@returnAddress(), "error: {s}", .{msg});

Panic prints message + stack trace to stderr, then aborts.

Stack Traces

Dump Current Stack

// Print current stack trace to stderr
std.debug.dumpCurrentStackTrace(.{});

// Skip frames until this address
std.debug.dumpCurrentStackTrace(.{ .first_address = @returnAddress() });

Dump to Writer

var buf: [4096]u8 = undefined;
var locked = try io.lockStderr(&buf, null);
defer io.unlockStderr();
try std.debug.writeCurrentStackTrace(.{}, locked.terminal());

Capture Stack Trace

var addrs: [32]usize = undefined;
const trace = std.debug.captureCurrentStackTrace(.{}, &addrs);

// Later: print captured trace
std.debug.dumpStackTrace(&trace);

StackUnwindOptions can also carry a target-specific cpu_context.Native pointer and opt into unsafe fallback unwinding. The stack iterator itself is an implementation detail; use captureCurrentStackTrace, writeCurrentStackTrace, and dumpCurrentStackTrace as the public interface.

Hex Dump

const data = "Hello, World!\x00\x01\x02";

// Quick dump to stderr
std.debug.dumpHex(data);
// Output:
// 7fff5fbff8a0  48 65 6C 6C 6F 2C 20 57  6F 72 6C 64 21 00 01 02  Hello, World!...

// Fallible dump to an existing terminal abstraction
try std.debug.dumpHexFallible(terminal, data);

dumpHexFallible accepts std.Io.Terminal, not a bare writer, because it uses terminal color/mode operations. Use .no_color when constructing a terminal for a non-terminal sink.

Output format:

Address (lowercase hex)
16 bytes per line (uppercase hex)
ASCII representation (. for non-printable, special chars for \n, \r, \t)

Value Tracing

Track where values originate and mutate during debugging:

const Trace = std.debug.Trace;  // Pre-configured: 2 traces, 4 stack frames

const MyStruct = struct {
    value: u32,
    trace: Trace = .init,

    fn setValue(self: *@This(), v: u32) void {
        self.value = v;
        self.trace.add("setValue called");
    }
};

var s = MyStruct{ .value = 0 };
s.setValue(42);
s.trace.dump();  // Prints stack traces with notes

Configurable Trace

// Custom configuration: 4 trace slots, 8 stack frames per trace
const MyTrace = std.debug.ConfigurableTrace(4, 8, true);

var trace: MyTrace = .init;
trace.add("first mutation");
trace.addAddr(@returnAddress(), "with explicit address");

// Check if tracing is enabled
if (MyTrace.enabled) {
    trace.dump();
}

// Use in format strings
std.debug.print("trace: {}", .{trace});

The predefined std.debug.Trace is enabled only in Debug mode. A custom ConfigurableTrace follows its explicit is_enabled argument; when disabled, its operations are no-ops and its storage is zero-sized.

SafetyLock

Debug helper to detect concurrent access violations:

const SafetyLock = std.debug.SafetyLock;

var lock: SafetyLock = .{};

fn criticalSection() void {
    lock.lock();
    defer lock.unlock();
    // ... protected code
}

fn checkNotLocked() void {
    lock.assertUnlocked();  // Panics if locked
}

SafetyLock follows runtime-safety mode: active in Debug and ReleaseSafe, and a no-op in ReleaseFast and ReleaseSmall.
Trace has the separate Debug-only default described above.

Source Location

const SourceLocation = std.debug.SourceLocation;

const loc: SourceLocation = .{
    .line = 42,
    .column = 10,
    .file_name = "src/main.zig",
};

// Invalid/unknown location
const unknown = SourceLocation.invalid;

Symbol Information

const Symbol = std.debug.Symbol;

// Resolved fields are optional because debug information may be incomplete.
const sym: Symbol = .{
    .name = "myFunction",
    .compile_unit_name = "main.zig",
    .source_location = .{ .line = 100, .column = 1, .file_name = "src/main.zig" },
};

// Unknown symbol
const unknown: Symbol = .unknown; // all three fields are null

Segfault Handling

// Check if platform supports segfault handling
if (std.debug.have_segfault_handling_support) {
    // Attach handler (prints stack trace on SIGSEGV/SIGBUS/etc)
    std.debug.attachSegfaultHandler();

    // Later: reset to default handler
    std.debug.resetSegfaultHandler();
}

// Check if handler is enabled by default
const enabled = std.debug.default_enable_segfault_handler;

Note: maybeEnableSegfaultHandler() is called automatically by the runtime if std.options.enable_segfault_handler is true.

CPU Context Unwinding

Signal handlers that already receive a native CPU context can pass it through std.debug.StackUnwindOptions.context to captureCurrentStackTrace, writeCurrentStackTrace, or dumpCurrentStackTrace. There are no public std.debug.ThreadContext, getContext, copyContext, or dumpStackTraceFromBase helpers in Zig 0.16; native context capture is target- and signal-handler-specific under std.debug.cpu_context.

Valgrind Detection

if (std.debug.inValgrind()) {
    // Running under Valgrind - may want different behavior
    std.debug.print("Valgrind detected\n", .{});
}

Debug Info Access

// Get debug info for current executable
const info = try std.debug.getSelfDebugInfo();

// Public methods are target-specific through SelfInfo. A broadly available
// operation is resolving the owning module name; returned storage is owned by
// SelfInfo rather than by the caller.
const module_name = try info.getModuleName(io, address);
std.debug.print("module: {s}\n", .{module_name});

Constants

// Whether runtime safety checks are enabled
std.debug.runtime_safety  // deprecated; reflects stdlib mode, not necessarily the caller's module

// Whether platform can produce stack traces
std.debug.sys_can_stack_trace  // false on WASM, MIPS, etc.

Submodules

Module	Purpose
`std.debug.Dwarf`	DWARF debug info parser
`std.debug.Pdb`	Windows PDB debug info parser
`std.debug.SelfInfo`	Debug info for current executable
`std.debug.Coverage`	Code coverage support
`std.debug.cpu_context`	Target-specific native CPU context definitions

FullPanic

Create custom panic handler with formatted safety messages:

pub const panic = std.debug.FullPanic(myPanicFn);

fn myPanicFn(msg: []const u8, ret_addr: ?usize) noreturn {
    // Custom panic handling (log to file, send telemetry, etc.)
    std.posix.abort();
}

// Now safety checks use myPanicFn with descriptive messages:
// - "sentinel mismatch: expected X, found Y"
// - "index out of bounds: index N, len M"
// - "attempt to unwrap error: ErrorName"
// etc.

Locking stderr

For multi-line debug output without interleaving:

// Lock stderr and clear any progress indicators. The returned object exposes
// a file writer and terminal; unlock performs the final flush automatically.
var lock_buf: [256]u8 = undefined;
const locked = std.debug.lockStderr(&lock_buf);
defer std.debug.unlockStderr();

// Safe to write multiple lines
try locked.file_writer.interface.writeAll("Line 1\n");
try locked.file_writer.interface.writeAll("Line 2\n");

The matching function is spelled unlockStderr:

var buf: [256]u8 = undefined;
const locked = std.debug.lockStderr(&buf);
defer std.debug.unlockStderr();
try locked.file_writer.interface.print("Complex output: {}\n", .{value});

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