zig-debugging

name	zig-debugging
description	Zig debugging skill. Use when debugging Zig programs with GDB or LLDB, interpreting Zig runtime panics, using std.debug.print for tracing, configuring debug builds, or debugging Zig programs in VS Code. Activates on queries about debugging Zig, Zig panics, zig gdb, zig lldb, std.debug.print, Zig stack traces, or Zig error return traces.

Zig Debugging

Purpose

Guide agents through debugging Zig programs: GDB/LLDB sessions, interpreting Zig panics and error return traces, std.debug.print logging, debug build configuration, and IDE integration.

Triggers

• "How do I debug a Zig program with GDB?"
• "How do I interpret a Zig panic message?"
• "How do I use std.debug.print for debugging?"
• "Zig is showing an error return trace — what does it mean?"
• "How do I set up Zig debugging in VS Code?"
• "How do I get a stack trace from a Zig crash?"

Workflow

1. Build for debugging

# Debug build (default) — full debug info, safety checks
zig build-exe src/main.zig -O Debug

# With build system
zig build            # uses Debug by default
zig build -Doptimize=Debug

# Run directly with debug output
zig run src/main.zig

2. GDB with Zig

Zig emits standard DWARF debug information compatible with GDB:

# Build with debug info
zig build-exe src/main.zig -O Debug -femit-bin=myapp

# Launch GDB
gdb ./myapp

# GDB session
(gdb) break main
(gdb) run arg1 arg2
(gdb) next          # step over
(gdb) step          # step into
(gdb) continue
(gdb) print my_var
(gdb) info locals
(gdb) bt            # backtrace

Break on Zig panics:

(gdb) break __zig_panic_start
(gdb) break std.builtin.default_panic

3. LLDB with Zig

lldb ./myapp

(lldb) b main
(lldb) r arg1 arg2
(lldb) n            # next
(lldb) s            # step into
(lldb) p my_var     # print
(lldb) frame variable
(lldb) bt           # backtrace
(lldb) c            # continue

# Break on panic
(lldb) b __zig_panic

4. Interpreting Zig panics

Zig panics include the source location and reason:

thread 'main' panic: index out of bounds: index 5, len 3
/home/user/src/main.zig:15:14
/home/user/src/main.zig:42:9
???:?:?: (name not available)

Common panic messages:

Panic	Cause
index out of bounds: index N, len M	Slice/array OOB access
integer overflow	Arithmetic overflow in Debug/ReleaseSafe
attempt to unwrap null	Optional access .? on null
reached unreachable code	unreachable executed
casting...	Invalid enum tag or union access
integer cast truncated bits	@intCast with value out of range
out of memory	Allocator failed

5. Error return traces

Zig tracks where errors propagate with error return traces:

error: FileNotFound
/home/user/src/main.zig:30:20: 0x10a3b in openConfig (main)
    const f = try std.fs.openFileAbsolute(path, .{});
                   ^
/home/user/src/main.zig:15:25: 0x10b12 in run (main)
    const cfg = try openConfig("/etc/myapp.conf");
                    ^
/home/user/src/main.zig:8:20: 0x10c44 in main (main)
    try run();
               ^

The trace shows the exact try chain where the error propagated. Read bottom-up: main → run → openConfig.

Enable in release builds:

// build.zig
const exe = b.addExecutable(.{
    .name = "myapp",
    .root_source_file = b.path("src/main.zig"),
    .target = target,
    .optimize = optimize,
    .error_tracing = true,  // enable even in ReleaseFast
});

6. std.debug.print for tracing

const std = @import("std");

pub fn main() !void {
    const x: u32 = 42;
    const name = "world";

    // Basic print (always to stderr)
    std.debug.print("x = {d}, name = {s}\n", .{ x, name });

    // Print any value (useful for structs)
    const point = Point{ .x = 1, .y = 2 };
    std.debug.print("point = {any}\n", .{point});

    // Formatted output
    std.debug.print("hex: {x}, binary: {b}\n", .{ x, x });

    // Log levels (respects compile-time log level)
    const log = std.log.scoped(.my_module);
    log.debug("debug info: {d}", .{x});
    log.info("started processing", .{});
    log.warn("unusual condition", .{});
    log.err("failed: {s}", .{"reason"});
}

7. std.log configuration

// Override default log level at root
pub const std_options = std.Options{
    .log_level = .debug,  // .debug | .info | .warn | .err
};

// Custom log handler
pub fn logFn(
    comptime level: std.log.Level,
    comptime scope: @TypeOf(.enum_literal),
    comptime format: []const u8,
    args: anytype,
) void {
    const prefix = "[" ++ @tagName(level) ++ "] (" ++ @tagName(scope) ++ "): ";
    std.debug.print(prefix ++ format ++ "\n", args);
}

pub const std_options = std.Options{
    .logFn = logFn,
};

8. VS Code / IDE integration

Install the zig.vscode-zig extension and CodeLLDB.

.vscode/launch.json:

{
    "version": "0.2.0",
    "configurations": [
        {
            "type": "lldb",
            "request": "launch",
            "name": "Debug Zig",
            "program": "${workspaceFolder}/zig-out/bin/myapp",
            "args": [],
            "cwd": "${workspaceFolder}",
            "preLaunchTask": "zig build"
        }
    ]
}

.vscode/tasks.json:

{
    "version": "2.0.0",
    "tasks": [
        {
            "label": "zig build",
            "type": "shell",
            "command": "zig build",
            "group": { "kind": "build", "isDefault": true },
            "problemMatcher": ["$zig"]
        }
    ]
}

Related skills

• Use skills/zig/zig-compiler for build modes and debug info flags
• Use skills/debuggers/gdb for GDB fundamentals
• Use skills/debuggers/lldb for LLDB fundamentals
• Use skills/zig/zig-cinterop when debugging mixed Zig/C code