a fork-like C-wrapper for clone(), DONE!

[Francesc Oller <francesc@mat.upc.es>]

Hi all,

Some days before I asked for a fork-like C-wrapper for clone() which
could be used like fork() thinking that somebody could have done it
before but I only received two e-mails saying that probably it
wasn't worth it or even it was complete non-sense.

Therefore, I've done it myself. Code follows.

Please, before beginning to flame me for doing "such kind of non-
standard threading model", I've to say that IMHO it has some merit.
After all it was E.W.Dijkstra who invented it in the late sixties.

Greetings from Barcelona

Francesc Oller


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/*
* Implementation of Dijkstra's parbegin/parend using clone()
* Modified from original Linus' clone.c example
* A proof of concept for academic purposes
* (c) Francesc Oller 2001, Linus Torvalds
* Under GPL license
*/

#include <unistd.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sched.h>

#include <linux/unistd.h>

#include "tinyx.h"

#define STACKSIZE 4096 /* 4 KB */

pid_t myclone(void)
{
	long retval;
	long *childsp;
	register long * motherbp __asm__ ("%ebp");

	/*
	 * allocate new stack for child
	 */
	childsp = malloc(STACKSIZE);
	if (!childsp)
		return -1;
	childsp = (long *)(((char *)childsp) + STACKSIZE);
	*--childsp = *(motherbp + 1);	/* push return address */
	*--childsp = *motherbp;		/* push mother's bp */

	/*
	 * Do clone() system call. We need to do the low-level stuff
	 * entirely in assembly as we're returning with a different
	 * stack in the child process and we couldn't otherwise guarantee
	 * that the program doesn't use the old stack incorrectly.
	 *
	 * Parameters to clone() system call:
	 *	%eax - __NR_clone, clone system call number
	 *	%ebx - clone_flags, bitmap of cloned data
	 *	%ecx - new stack pointer for cloned child
	 *
	 * In this example %ebx is CLONE_VM | CLONE_FS | CLONE_FILES |
	 * CLONE_SIGHAND which shares as much as possible between parent
	 * and child. (We or in the signal to be sent on child termination
	 * into clone_flags: SIGCHLD makes the cloned process work like
	 * a "normal" unix child process)
	 *
	 * The clone() system call returns (in %eax) the pid of the newly
	 * cloned process to the mother, and 0 to the cloned process. If
	 * an error occurs, the return value will be the negative errno.
	 *
	 * Prior to the creation of the child process, we have stored
	 * return adress and caller's bp in child's stack. Child will 
	 * restore caller's bp and jmp to the post-clone adress. The
	 * "_exit()" system call at the child's body end will terminate
	 * the child.
	 */
	__asm__ __volatile__(
		"int $0x80\n\t"		/* Linux/i386 system call */
		"testl %0,%0\n\t"	/* check return value */
		"jne 1f\n\t"		/* jump if mother */
		"popl %%ebp\n\t"	/* restore caller's bp */
		"ret\n"			/* jmp to return address */
		"1:\t"
		:"=a" (retval)
		:"0" (__NR_clone),
		 "b" (CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND | SIGCHLD),
		 "c" (childsp));

	if (retval < 0) {
		errno = -retval;
		retval = -1;
	}
	return retval;
}

int show_same_vm;

int main()
{
	int fd;

	fd = open("/dev/null", O_RDWR);
	if (fd < 0) {
		perror("/dev/null");
		exit(1);
	}
	printf("mother:\t fd = %d\n", fd);
	show_same_vm = 10;
	printf("mother:\t vm = %d\n", show_same_vm);

	switch(myclone())
	{
		case -1: perror("clone"); exit(1);
		case  0: 
			printf("child:\t fd = %d\n", fd);
			show_same_vm = 5;
			printf("child:\t vm = %d\n", show_same_vm);
			close(fd);
			_exit(0);
		default:
			sleep(1);
			printf("mother:\t vm = %d\n", show_same_vm);
			if (write(fd, "c", 1) < 0)
				printf("mother:\t child closed our file descriptor\n");
	}

	/*
	 * The implementation of this construct is left as an exercise
	 * to the reader ;-)
	 */
	/*parbegin
		altpar
			printf("I'm thread 1\n");
			parbegin
				altpar
					printf("I'm thread 4, child of thread 1\n");
				altpar
					printf("I'm thread 5, child of thread 1\n");
				altpar
					printf("I'm thread 6, child of thread 1\n");
			parend
		altpar
			printf("I'm thread 2\n");
		altpar
			printf("I'm thread 3\n");
	parend*/
	return 0;
}

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