[SOLVED] Zig slice of sentinel-terminated array excluding all trailing sentinels

Zig slice of sentinel-terminated array excluding all trailing sentinels

Given a sentinel-terminated array [N:0]T which is only partially filled at the beginning of the array with and unknown amount of entities n < N, and the rest is filled with sentinels (0), what's the most concise way to grab a slice only containing the first entities before any sentinel?

Here's a code example in linux, where utsname.release: [64:0]u8 is populated by the OS with the kernel's semantic version string.

const std = @import("std");

// ...

    const utsname: std.os.linux.utsname = std.os.uname();
    std.debug.print("utsname.release = '{s}'\n", .{utsname.release});
    std.debug.print("utsname.release.len = {}\n", .{utsname.release.len});
    const utsNameReleaseSlice: []const u8 = &utsname.release;
    std.debug.print("utsNameReleaseSlice = '{s}'\n", .{utsNameReleaseSlice});
    std.debug.print("utsNameReleaseSlice.len = {}\n", .{utsNameReleaseSlice.len});

output:

utsname.release = '6.6.8-gnu'
utsname.release.len = 64
utsNameReleaseSlice = '6.6.8-gnu'
utsNameReleaseSlice.len = 64

In the print above, the string looks ok but it prints out the sentinels which are zero-width characters. When I provide the slice to std.SemanticVersion.parse it throws an error when it gets to those sentinel characters.

To fix this I got the index of the first sentinel and took a slice up to there:

    const versionStringEnd: usize = std.mem.indexOf(u8, &utsname.release, &[_]u8{0}) orelse utsname.release.len;
    const utsNameReleaseSlice: []const u8 = utsname.release[0..versionStringEnd];
    std.debug.print("utsNameReleaseSlice = '{s}'\n", .{utsNameReleaseSlice});
    std.debug.print("utsNameReleaseSlice.len = {}\n", .{utsNameReleaseSlice.len});

output:

utsNameReleaseSlice = '6.6.8-gnu'
utsNameReleaseSlice.len = 9

Is there a more concise way of getting this slice which ignores sentinels?

Solution

You're looking for std.mem.span, which does exactly that.

Specifically, with the above example, you would need to cast the sentinel-terminated array as a sentinel-terminated pointer @as([*:0]const u8, &utsname.release) and provide that to std.mem.span:

const utsNameReleaseSlice: []const u8 = std.mem.span(@as([*:0]const u8, &utsname.release));