The system call fork is used to create processes. The most important realworld use of fork is in the daemon processes where in which the daemon has to get attached to the pid 1 (the init process).
The process that calls fork and creates another process is called a parent process and the process that is created by the fork system call is called child process.
Fork returns twice unlike any other function in C. The fork call is executed by the kernel and it returns once in the parent process and once in the child process.
Fork returns the pid of the child process in the parent, and returns 0 in the child. If creation of a process was unsuccessful then it returns -1.
The below program gives an example of fork system call.
#include <stdio.h>
#include <unistd.h>
int main(void)
{
pid_t pid;
pid = fork();
if (pid > 0) {
printf("parent process - my pid %d child pid %d\n",
getpid(), pid);
} else {
printf("child process - my pid %d parent pid %d\n",
getpid(), pid);
}
sleep(2);
}Example: demonstration of fork system call
notice that when you run the above program several times, the print statements "parent process" and "child process" never come exactly one after the another for ex: parent first and then the child. This is because of the scheduling job that is done internally in the kernel.
After a child process is created the parent can continue its execution of the remaining code or can exit by calling the exit system call. When the parent exits before the child could run, the child becomes an orphan. This process is then parented by the init process (pid 1). This method is one of the step in creating the system daemon.
In an other case where in the child gets stopped or exited before the parent. In this case, the parent must cleanup the resources that are allocated to the child by calling one of the functions wait, waitpid. In some cases, when the parent process does not perform the task of cleanup, the child process will then go into a state called zombie state. In the zombie state, the child process although is not running, its memory is never get cleaned up by the parent process. Notice the "Z" symbol in the ps -aux command.
fork system call is mostly used in the creation of system daemons.