The fork() System Call
System call fork() is used to create processes. It takes no arguments and returns a process ID.
The purpose of fork() is to create a new process, which becomes the child process of the caller.
After a new child process is created, both processes will execute the next instruction following
the fork() system call. Therefore, we have to distinguish the parent from the child. This can be
done by testing the returned value of fork():
If fork() returns a negative value, the creation of a child process was unsuccessful.
fork() returns a zero to the newly created child process.
fork() returns a positive value, the process ID of the child process, to the parent. The
returned process ID is of type pid_t defined in sys/types.h. Normally, the process ID is
an integer. Moreover, a process can use function getpid() to retrieve the process ID
assigned to this process.
Therefore, after the system call to fork(), a simple test can tell which process is the child. Please
note that Unix will make an exact copy of the parent's address space and give it to the
child. Therefore, the parent and child processes have separate address spaces.
Let us take an example to make the above points clear. This example does not distinguish parent
and the child processes. Click here to download this file fork-01.c.
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#define MAX_COUNT 200
#define BUF_SIZE 100
void main(void)
{
pid_t pid;
int i;
char buf[BUF_SIZE];
fork();
pid = getpid();
for (i = 1; i <= MAX_COUNT; i++) {
sprintf(buf, "This line is from pid %d, value = %d\n", pid, i);
write(1, buf, strlen(buf));
}
}
Suppose the above program executes up to the point of the call to fork() (marked in red color):
