The real hard limit is 73 bytes [0], to actually run any code in the executable without segfaulting right away. You can cram about 3 simple syscalls into that limit (e.g., my BGGP4 submission [1], which copies the executable to another file), including the final exit() or exit_group() if you want your program to terminate cleanly. Strings are very costly: I couldn't get a Hello World below 77 bytes.