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SubjectRe: need help w/ clone(2)

here is the 'classic' clone() testcode:

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

#include <linux/unistd.h>

#define STACKSIZE 16384

#define CSIGNAL 0x000000ff /* signal mask to be sent at exit */
#define CLONE_VM 0x00000100 /* set if VM shared between processes */
#define CLONE_FS 0x00000200 /* set if fs info shared between processes */
#define CLONE_FILES 0x00000400 /* set if open files shared between processes */
#define CLONE_SIGHAND 0x00000800 /* set if signal handlers shared */


int start_thread(void (*fn)(void *), void *data)
{
long retval;
void **newstack;

/*
* allocate new stack for subthread
*/
newstack = (void **) malloc(STACKSIZE);
if (!newstack)
return -1;

/*
* Set up the stack for child function, put the (void *)
* argument on the stack.
*/
newstack = (void **) (STACKSIZE + (char *) newstack);
*--newstack = data;

/*
* 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 parent, and 0 to the cloned process. If
* an error occurs, the return value will be the negative errno.
*
* In the child process, we will do a "jsr" to the requested function
* and then do a "exit()" system call which 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 parent */
"call *%3\n\t" /* start subthread function */
"movl %2,%0\n\t"
"int $0x80\n" /* exit system call: exit subthread */
"1:\t"
:"=a" (retval)
:"0" (__NR_clone),"i" (__NR_exit),
"r" (fn),
"b" (CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND | SIGCHLD),
"c" (newstack));

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

int show_same_vm;

void cloned_process_starts_here(void * data)
{
printf("child:\t got argument %d as fd\n", (int) data);
show_same_vm = 5;
printf("child:\t vm = %d\n", show_same_vm);
close((int) data);
}

int main()
{
int fd, pid;

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);

pid = start_thread(cloned_process_starts_here, (void *) fd);
if (pid < 0) {
perror("start_thread");
exit(1);
}

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");
}




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