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Creating new process-fork()

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.
#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));
 }
}
if the call to fork() is executed successfully, Unix will
  • make two identical copies of address spaces, one for the parent and the other for the child.
  • Both processes will start their execution at the next statement following the fork() call. In this case, both processes will start their execution at the assignment statement  pid=getpid()
Consider one more simple example, which distinguishes the parent from the child.
#include  <sys/types.h>
#define   MAX_COUNT  200
void  ChildProcess(void);   /* child process prototype  */
void  ParentProcess(void);  /* parent process prototype */
void  main(void)
{
     pid_t  pid;
     pid = fork();
     if (pid == 0)
          ChildProcess();
     else
          ParentProcess();
}
void  ChildProcess(void)
{
 int   i;
 for (i = 1; i <= MAX_COUNT; i++)
   printf("   This line is from child, value = %d\n", i);
 printf("   *** Child process is done ***\n");
}
void  ParentProcess(void)
{
 int   i;
 for (i = 1; i <= MAX_COUNT; i++)
   printf("This line is from parent, value = %d\n", i);
 printf("*** Parent is done ***\n");
}
In this program, both processes print lines that indicate whether the line is printed by the child or by the parent process, and the value of variable i. For simplicity, printf() is used.
When the main program executes fork(), an identical copy of its address space, including the program and all data, is created. System call fork() returns the child process ID to the parent and returns 0 to the child process. The following figure shows that in both address spaces there is a variable pid. The one in the parent receives the child's process ID 3456 and the one in the child receives 0.
Due to the fact that the CPU scheduler will assign a time quantum to each process, the parent or the child process will run for some time before the control is switched to the other and the running process will print some lines before you can see any line printed by the other process. Therefore, the value of MAX_COUNT should be large enough so that both processes will run for at least two or more time quanta. If the value of MAX_COUNT is so small that a process can finish in one time quantum, you will see two groups of lines, each of which contains all lines printed by

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