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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FORK(2) BSD System Calls Manual FORK(2)
NAME
fork -- create a new process
SYNOPSIS
#include <unistd.h>
pid_t
fork(void);
DESCRIPTION
Fork() causes creation of a new process. The new process (child process) is an exact copy of the calling process (parent process) except for
the following:
o The child process has a unique process ID.
o The child process has a different parent process ID (i.e., the process ID of the parent process).
o The child process has its own copy of the parent's descriptors. These descriptors reference the same underlying objects, so that,
for instance, file pointers in file objects are shared between the child and the parent, so that an lseek(2) on a descriptor in the
child process can affect a subsequent read or write by the parent. This descriptor copying is also used by the shell to establish
standard input and output for newly created processes as well as to set up pipes.
o The child processes resource utilizations are set to 0; see setrlimit(2).
RETURN VALUES
Upon successful completion, fork() returns a value of 0 to the child process and returns the process ID of the child process to the parent
process. Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to
indicate the error.
ERRORS
Fork() will fail and no child process will be created if:
[EAGAIN] The system-imposed limit on the total number of processes under execution would be exceeded. This limit is configuration-
dependent.
[EAGAIN] The system-imposed limit MAXUPRC (<sys/param.h>) on the total number of processes under execution by a single user would
be exceeded.
[ENOMEM] There is insufficient swap space for the new process.
LEGACY SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
The include file <sys/types.h> is necessary.
SEE ALSO
execve(2), sigaction(2), wait(2), compat(5)
HISTORY
A fork() function call appeared in Version 6 AT&T UNIX.
CAVEATS
There are limits to what you can do in the child process. To be totally safe you should restrict yourself to only executing async-signal
safe operations until such time as one of the exec functions is called. All APIs, including global data symbols, in any framework or library
should be assumed to be unsafe after a fork() unless explicitly documented to be safe or async-signal safe. If you need to use these frame-
works in the child process, you must exec. In this situation it is reasonable to exec yourself.
4th Berkeley Distribution June 4, 1993 4th Berkeley Distribution