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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LEARN ABOUT V7

fork

FORK(2) 							System Calls Manual							   FORK(2)

NAME

       fork  -	spawn new process

SYNOPSIS

       fork( )

DESCRIPTION

       Fork  is  the only way new processes are created.  The new process's core image is a copy of that of the caller of fork.  The only distinc-
       tion is the fact that the value returned in the old (parent) process contains the process ID of the new (child) process,  while	the  value
       returned in the child is 0.  Process ID's range from 1 to 30,000.  This process ID is used by wait(2).

       Files open before the fork are shared, and have a common read-write pointer.  In particular, this is the way that standard input and output
       files are passed and also how pipes are set up.

SEE ALSO

       wait(2), exec(2)

DIAGNOSTICS

       Returns -1 and fails to create a process if: there is inadequate swap space, the user is not super-user and has too many processes, or  the
       system's process table is full.	Only the super-user can take the last process-table slot.

ASSEMBLER

       (fork = 2.)
       sys fork
       (new process return)
       (old process return, new process ID in r0)

       The return locations in the old and new process differ by one word.  The C-bit is set in the old process if a new process could not be cre-
       ated.

																	   FORK(2)