Forking

#include<sys/types.h>
#include<unistd.h>
pid_t fork(void);
pid_t getpid(void);
pid_t getppid(void);

• A successful call to fork() creates a new process, identical in almost all aspects to the calling process.

• The parent process receives the identifier (pid) of the child process, and the child process receives zero.

• On error and the child process is not created, fork() returns -1 and the error (errno) can be:

Error	Description
EAGAIN	The kernel failed to allocate certain resources, such as a new process identifier (pid).
ENOMEM	There is insufficient kernel memory to execute the request.

Program fragment for starting a child process:

fork.c

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
int main(int argc, char * argv[])
{
   pid_t pid = fork();
   if(pid == -1)
   {
      printf("Error forking a process!\n");
      return EXIT_FAILURE; // -1
   }
   else if(pid == 0)
   {
      printf("Child process!\n");
   }
   else
   {
      printf("Parent process!\n");
   }
   return EXIT_SUCCESS; // 0
}

More information: Linux Programming