Build Modes

Zig has four build modes that make different trade-offs between safety and speed.

Let’s take another look at our example for checked illegal behavior:

const std = @import("std");
const print = std.debug.print;

fn f() u64 {
    return 0;
}

pub fn main() void {
    print("{}\n", .{1 / f()});
}

Running the compiler with -O Debug selects Debug build mode. That’s the default, so the output here is what we’ve already seen:

$ zig build-exe -O Debug build-modes.zig
$ ./build-modes
thread 216374 panic: division by zero
[...]/build-modes.zig:9:23: 0x11d7ed0 in main (build-modes.zig)
    print("{}\n", .{1 / f()});
                      ^
[...]
Aborted

ReleaseFast disables safety checks so all illegal behavior becomes unchecked. Instead of an error message we get unpredictable behavior:

$ zig build-exe -O ReleaseFast build-modes.zig
$ ./build-modes
00

There’s four build modes in total:

Debug: for testing and debugging.
ReleaseSafe: enables optimizations, keeps safety checks.
ReleaseFast: disables safety checks, optimizes for speed.
ReleaseSmall: disables safety checks, optimizes for small binaries.

Next example: Integers.