06-07-2001
fork() is used to create a copy of a process. When you call fork(), the calling process is copied and you now have a parent and child process. THe only difference is the return value of fork(). It will return 0 to the child process, and a process ID number for the parent. That's how you can tell which process you're currently in.
Well, I won't write your assignment for you, but here's how a fork goes:
<pre>
int main(int argc, char* argv[])
{
pid_t pid;
printf("This will be seen once.");
pid = fork();
printf("This will get seen twice. Once for the parent, once for the child process");
if (pid == 0) /* This is the child process */
{
/* Do child process stuff here */
exit(0);
}
else if (pid < 0) /* Some sort of error in fork() */
{
/* Process error here */
}
else /* This means we are in the parent process */
{
/* Do parent process stuff */
wait(NULL); /* wait for the child to finish */
}
return(0);
}
</pre>
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VFORK(2) BSD System Calls Manual VFORK(2)
NAME
vfork -- spawn new process in a virtual memory efficient way
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
pid_t
vfork(void);
DESCRIPTION
The vfork system call creates a new process that does not have a new virtual address space, but rather shares address space with the parent,
thus avoiding potentially expensive copy-on-write operations normally associated with creating a new process. It is useful when the purpose
of fork(2) would have been to create a new system context for an execve(2). The vfork system call differs from fork(2) in that the child
borrows the parent's memory and thread of control until a call to execve(2) or an exit (either by a call to _exit(2) or abnormally). The
parent process is suspended while the child is using its resources.
The vfork system call returns 0 in the child's context and (later) the pid of the child in the parent's context.
The vfork system call can normally be used just like fork(2). It does not work, however, to return while running in the childs context from
the procedure that called vfork() since the eventual return from vfork() would then return to a no longer existent stack frame. Be careful,
also, to call _exit(2) rather than exit(3) if you can't execve(2), since exit(3) will flush and close standard I/O channels, and thereby mess
up the standard I/O data structures in the parent process. (Even with fork(2) it is wrong to call exit(3) since buffered data would then be
flushed twice.)
RETURN VALUES
Same as for fork(2).
ERRORS
Same as for fork(2).
SEE ALSO
execve(2), fork(2), sigaction(2), wait(2)
HISTORY
The vfork() function call appeared in 3.0BSD. In 4.4BSD, the semantics were changed to only suspend the parent. The original semantics were
reintroduced in NetBSD 1.4.
BUGS
Users should not depend on the memory sharing semantics of vfork() as other ways of speeding up the fork process may be developed in the
future.
To avoid a possible deadlock situation, processes that are children in the middle of a vfork() are never sent SIGTTOU or SIGTTIN signals;
rather, output or ioctl(2) calls are allowed and input attempts result in an end-of-file indication.
BSD
January 3, 1998 BSD