It appears that there's two clone() things floating around in Linux 2.6
There's a system call:
int clone(int (*fn)(void *), void *child_stack,
int flags, void *arg, ...
/* pid_t *ptid, struct user_desc *tls, pid_t *ctid */ );
This is the "clone()" described by doing man 2 clone.
If you read that man page close enough, you will see this:
It is actually a library function layered on top of the
underlying clone() system call.
Apparently, you're supposed to implement threading using the "library function" layered on the confusingly identically named system call.
I wrote a short program:
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
int
main(int ac, char **av)
{
pid_t cpid;
switch (cpid = fork()) {
case 0: // Child process
break;
case -1: // Error
break;
default: // parent process
break;
}
return 0;
}
Compiled it with: c99 -Wall -Wextra, and ran it under strace -f to see what system calls forking actually do. I got this out of strace on a Linux 2.6.18 machine (x86_64 CPU):
20097 clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x2b4ee9213770) = 20098
20097 exit_group(0) = ?
20098 exit_group(0)
No "fork" call appears in the strace output. The clone() call that shows up in the strace output has very different arguments from the man-page-clone. child_stack=0 as the first argument is different than int (*fn)(void *).
It appears that the fork(2) system call is implemented in terms of the real clone(), just like the "library function" clone() is implemented. The real clone() has a different set of arguments from the man-page-clone.
Simplistically, both of your apparently contradictory statements about fork() and clone() are correct. The "clone" involved is different, though.