CODEWITHSUNDEEP
selfinstructionsquine
Hemispherr
Hemispherr

Reputation: 150

Code that writes itself when executed

Is there any computer program that can display on console or any other medium the instructions that composed it? Does any computer language have capabilities for this? Is it possible?

Upvotes: 0

Views: 603

Answers (3)

Peter Cordes
Peter Cordes

Reputation: 364180

Its own source code?
Write a quine, techniques for which differ by language:

Or as Samuel shows, a program that opens its own source file via the filesystem.

Or output the machine instructions it compiles to?
For that, you can write a program that runs a disassembler on itself, like Linux objdump -drWC (optionally with -Mintel for x86). e.g. on Linux, argv[0] is normally the command name. It might or might not be a full path, but that's enough for a toy program.

#include <spawn.h>

int main(){
        posix_spawn(NULL, "/usr/bin/objdump", NULL, NULL, (char*[]){"objdump", "-drwC", "/proc/self/exe", NULL}, NULL);
     // execution continues here; unlike execve we didn't replace our own process
     // optional: waitpid(child_pid, &status, WUNTRACED | WCONTINUED);
}

I used posix_spawn just to see how it works (and if it works with NULL for envp like Linux execve allows, and most of the other optional things including the child_pid). The system(3) library function or fork+exec* system calls can do the equivalent.

(Linux /proc/self/exe is always the current process's executable. But that would get objdump to disassemble itself, not your program. /dev/fd/0 after your program redirects stdin from /proc/self/exe would work.)

Upvotes: 0

Cem Birler
Cem Birler

Reputation: 21

You don't need to explicitly read the file in your program to achieve this. If you run the following command in Python it would also work:

def write_itself():
    code = 'def write_itself():\n\t code = {!r}\n\t print(code.format(code)) \nwrite_itself()'
    print(code.format(code))
write_itself()

Upvotes: 1

Samuel Littley
Samuel Littley

Reputation: 734

A short python example:

Assuming that the source code is in the file /path/to/file.py

#!/usr/bin/env python3

with open('/path/to/file.py') as file:
    print(file.read())

This is a very contrived example, but it illustrates the point.

EDIT

By the definition of a quine this is technically cheating as it takes input by reading its own source file

Upvotes: 0

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