Sentinel Termination

Arrays, slices and many pointers may be terminated by a value of their child type. This is known as sentinel termination. These follow the syntax [N:t]T, [:t]T, and [*:t]T, where t is a value of the child type T.

An example of a sentinel terminated array. The built-in @ptrCast is used to perform an unsafe type conversion. This shows us that the last element of the array is followed by a 0 byte.

const expect = @import("std").testing.expect;

test "sentinel termination" {
    const terminated = [3:0]u8{ 3, 2, 1 };
    try expect(terminated.len == 3);
    try expect(@as(*const [4]u8, @ptrCast(&terminated))[3] == 0);
}

The types of string literals is *const [N:0]u8, where N is the length of the string. This allows string literals to coerce to sentinel terminated slices, and sentinel terminated many pointers. Note: string literals are UTF-8 encoded.

const expect = @import("std").testing.expect;

test "string literal" {
    try expect(@TypeOf("hello") == *const [5:0]u8);
}

[*:0]u8 and [*:0]const u8 perfectly model C’s strings.

const expect = @import("std").testing.expect;

test "C string" {
    const c_string: [*:0]const u8 = "hello";
    var array: [5]u8 = undefined;

    var i: usize = 0;
    while (c_string[i] != 0) : (i += 1) {
        array[i] = c_string[i];
    }
}

Sentinel terminated types coerce to their non-sentinel-terminated counterparts.

const expect = @import("std").testing.expect;

test "coercion" {
    const a: [*:0]u8 = undefined;
    const b: [*]u8 = a;

    const c: [5:0]u8 = undefined;
    const d: [5]u8 = c;

    const e: [:0]f32 = undefined;
    const f: []f32 = e;

    _ = .{ b, d, f }; //ignore unused
}

Sentinel terminated slicing is provided which can be used to create a sentinel terminated slice with the syntax x[n..m:t], where t is the terminator value. Doing this is an assertion from the programmer that the memory is terminated where it should be - getting this wrong is detectable illegal behaviour.

const expect = @import("std").testing.expect;

test "sentinel terminated slicing" {
    var x = [_:0]u8{255} ** 3;
    const y = x[0..3 :0];
    _ = y;
}