clone() and the CLONE_CHILD_CLEARTID flag

[misterade]

First of all: hello to everybody ):P, I'm new to this forum but not to ubuntu. I used to post on the italian ubuntu forum and now I'm moving here :-).

I have a quite frustrating problem with the clone() system call in the C language.
I'm trying to use it with the flag CLONE_CHILD_CLEARTID but I cant see any change in the value of the field I specify as argument. Here it is a simple code:


int the_child(){
exit(0);
}
int main(int argc, char * argv[])
{
pid_t child_id = 99;
printf("child %p\n",child_id);

clone((int (*)(void *))the_child,
NULL,
CLONE_VM | CLONE_CHILD_CLEARTID | SIGCHLD,
NULL, NULL,NULL, child_id);

sleep(1);
printf("child %p\n",child_id);
}

However when the two printf display always 99, What am I doing wrong?

[r-senior]

Welcome :)

I don't know, but you ought to check the return value:


RETURN VALUE
On success, the thread ID of the child process is returned in the call‐
er's thread of execution. On failure, -1 is returned in the caller's
context, no child process will be created, and errno will be set appro‐
priately.

[misterade]

Thank you for your reply.
I know that I can use different way but I'm interested in that flag because I really would like to understand what's wrong.

[misterade]

I get it. I necessary have to specify a value for the child stack. I realized that
the_child() wasn't called at all -.-".

[r-senior]

I know, I thought clone might be throwing up an error, in which case testing the return value and using perror might give you a clue?

EDIT: That was a reply to #3 but great that you figured it out.