openat

pub fn openat(dirfd: i32, path: [*:0]const u8, flags: u32, _: mode_t) usize {
    // we skip perms because only create supports perms
    if (dirfd == AT.FDCWD) { // openat(AT_FDCWD, ...) == open(...)
        return open(path, flags);
    }
    var dir_path_buf: [std.fs.max_path_bytes]u8 = undefined;
    var total_path_buf: [std.fs.max_path_bytes + 1]u8 = undefined;
    const rc = fd2path(dirfd, &dir_path_buf, std.fs.max_path_bytes);
    if (rc != 0) return rc;
    var fba = std.heap.FixedBufferAllocator.init(&total_path_buf);
    var alloc = fba.allocator();
    const dir_path = std.mem.span(@as([*:0]u8, @ptrCast(&dir_path_buf)));
    const total_path = std.fs.path.join(alloc, &.{ dir_path, std.mem.span(path) }) catch unreachable; // the allocation shouldn't fail because it should not exceed max_path_bytes
    fba.reset();
    const total_path_z = alloc.dupeSentinel(u8, total_path, 0) catch unreachable; // should not exceed max_path_bytes + 1
    return open(total_path_z.ptr, flags);
}