Structured Logging for Zig

slog is a configurable, structured logging package for Zig with support for hierarchical loggers.

Usage

Add slog to your build.zig.zon

zig fetch --save git+https://github.com/graveland/slog

Example code:

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

pub fn main() !void {
    var log = try slog.initRootLogger(std.heap.page_allocator, .{});
    defer log.deinit();

    var log2 = try log.initChildLogger("mod1");
    var log3 = try log2.initChildLogger("mod2");
    var log4 = try log3.initChildLogger("mod3");

    log.info("Hello slog!", .{ .field1 = "value1", .field2 = "value1", .rate = 30 });
    log2.trace("Hello slog!", .{ .field1 = "value1", .field2 = "value2", .rate = 30 });
    log2.debug("Hello slog!", .{ .field1 = "value1", .field2 = "value3", .rate = 30 });
    log2.info("Hello slog!", .{ .field1 = "value1", .field2 = "value4", .rate = 30e2 });
    log3.warn("Hello slog!", .{ .field1 = "value1", .field2 = "value5", .rate = 30.34534 });
    log3.err("Hello slog!", .{ .field1 = "value1", .field2 = "value6", .rate = 30, .active = true, .metadata = null });
    log4.err("Hello slog!", .{ .field1 = "value1", .field2 = "value6", .rate = 30, .active = true, .metadata = null });
}

Compile-Time Log Level Filtering

slog supports compile-time log level filtering to completely eliminate log calls from release builds. This provides zero runtime overhead for filtered log levels.

Build Options

Option	Description	Default
-Dmin_log_level=<level>	Minimum log level to compile	trace (Debug), info (Release)
-Dlog_compile_verbose=true	Show compile-time messages when logs are filtered	false

Valid levels: trace, debug, info, warn, error

Usage

# Default: trace in debug builds, info in release builds
zig build

# Force only warn+ logs in any build
zig build -Dmin_log_level=warn

# Release build with debug logs enabled
zig build -Doptimize=ReleaseFast -Dmin_log_level=debug

Runtime Introspection

You can check the compiled minimum level at runtime:

const slog = @import("slog");
std.debug.print("min_log_level = {s}\n", .{@tagName(slog.min_log_level)});

Performance

In release builds, LLVM completely eliminates filtered log calls including argument evaluation:

// When min_log_level > debug, this entire line compiles to nothing
logger.debug("msg", .{ .value = expensive_computation() });

Benchmark results (10,000 iterations calling expensiveComputation()):

Build Mode	min_log_level	Time	Dead Code Eliminated
Debug	trace	10,798ms	No
Debug	info	10,080ms	No (args still evaluated)
ReleaseSafe	info	0ms	Yes
ReleaseFast	info	0ms	Yes
ReleaseSmall	info	0ms	Yes

Interaction with Runtime Filtering

Compile-time filtering sets a floor. Runtime filtering via ZIG_LOG can further restrict but cannot enable levels below the compile-time minimum:

• Compile with -Dmin_log_level=info → trace/debug calls don't exist in binary
• Setting ZIG_LOG=debug at runtime has no effect on debug logs (they're not in the binary)
• Setting ZIG_LOG=warn at runtime restricts to warn/error only

Environment Variables

• ZIG_LOG - configures runtime log level, e.g. info,mod1=debug,mod2.mod3=warn.
• ZIG_LOG_COLORS - configures color scheme for text logger, e.g. timestamp=31;1,logger=33.

Dependencies

• zeit - a great date and time library for Zig