Forking in Unix using C++ Post: 2856

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Top Forums Programming Forking in Unix using C++ Post 2856 by TelePlayer on Thursday 7th of June 2001 04:25:58 PM

06-07-2001

Registered User

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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fork

FORK(2) 						     Linux Programmer's Manual							   FORK(2)

NAME

       fork - create a child process

SYNOPSIS

       #include <sys/types.h>
       #include <unistd.h>

       pid_t fork(void);

DESCRIPTION

       fork  creates a child process that differs from the parent process only in its PID and PPID, and in the fact that resource utilizations are
       set to 0.  File locks and pending signals are not inherited.

       Under Linux, fork is implemented using copy-on-write pages, so the only penalty incurred by fork is the time and memory required to  dupli-
       cate the parent's page tables, and to create a unique task structure for the child.

RETURN VALUE

       On success, the PID of the child process is returned in the parent's thread of execution, and a 0 is returned in the child's thread of exe-
       cution.	On failure, a -1 will be returned in the parent's context, no child process will be created, and errno will be set appropriately.

ERRORS

       EAGAIN fork cannot allocate sufficient memory to copy the parent's page tables and allocate a task structure for the child.

       ENOMEM fork failed to allocate the necessary kernel structures because memory is tight.

CONFORMING TO

       The fork call conforms to SVr4, SVID, POSIX, X/OPEN, BSD 4.3.

SEE ALSO

       clone(2), execve(2), vfork(2), wait(2)

Linux 1.2.9							    1995-06-10								   FORK(2)