Semaphore 3

In previous lesson, I introduced some functions of semaphore, like semget( ), semctl( ). There is another function semop( ).
This function is used to communicate between parents and child process.

Prototype:

int semop(int semid, struct sembuf *sops, size_t nsops)

The semid argument is the semaphore ID returned by a previous semget() call.
The sops  argument is a pointer of a structure, sembuf.
The nsops argument specifies the length of the array.


The sembuf structure specifies a semaphore operation, as defined in <sys/sem.h>.

struct sembuf {
        ushort_t      sem_num;      /* semaphore number */
        short           sem_op;         /* semaphore operation */
        short           sem_flg;         /* operation flags */
};


Now, there is a problem to understand that what operation will done ?
There are 3 types of operations.

-    A positive integer increments the semaphore value by that amount.
-    A negative integer decrements the semaphore value by that amount.
-    A value of zero means to wait for the semaphore value to reach zero.

OH ! Too much description. So, there is an example that help you to understand.



// **********************************************************************
// **********************************************************************
//   Write " ipcs -s "  in Terminal to see semaphore list
//   Here we set 1 in semaphore at parent. So, child will wait until it
//   get 0 at semaphore
//   when we again set 0 at parent, then child will execute
// **********************************************************************
// **********************************************************************

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <error.h>

int main()
{
    int semid, pid;
    struct sembuf sop;
   
    semid = semget( 0x20 , 1, IPC_CREAT | 0666 ) ;
   
    pid = fork();

    if( pid ==0 ){
            sleep(2);
            printf("child \n");
            sop.sem_num =0;
            sop.sem_op  =0;                /// set value  0
            sop.sem_flg =0;
            semop ( semid, &sop , 1 );   
           
            printf(" child over \n");   
    }
    else{
        printf("parent \n");
        semctl ( semid, 0, SETVAL, 1 ); /// as set value 1,  child will wait
        printf("parent  sleep\n");
        sleep(5);
        printf("parent  second\n");
        semctl ( semid, 0, SETVAL, 0 ); ///  now set value 0,  so  child will not wait now
        printf("parent  end\n");
    }

return 0;
}

// **********************************************************************
// **********************************************************************