preallocate.zig

const std = @import("std");

pub fn preallocate_file(fd: std.os.fd_t, offset: std.os.off_t, len: std.os.off_t) void {
    _ = std.os.linux.fallocate(fd, 0, @intCast(i64, offset), len);
}

pub fn main() anyerror!void {
    const argv = std.process.argsAlloc(std.heap.c_allocator) catch unreachable;
    if (argv.len < 3) {
        std.log.warn("Usage: {s} <length> <file> [--preallocate]\n", .{argv[0]});
        return;
    }
    const length = try std.fmt.parseInt(usize, argv[1], 10);
    const file = try std.fs.cwd().createFileZ(argv[2], .{});
    const should_preallocate = std.mem.eql(u8, argv[argv.len - 1], "--preallocate");
    const bytes = try std.heap.c_allocator.alloc(u8, length);

    if (should_preallocate) {
        preallocate_file(file.handle, 0, @intCast(std.os.off_t, length));
    }

    try file.writeAll(bytes);
}