IPC Programming in Unix

Hey ,

I'm trying to implement a semaphore which provides mutual exclusion among two processes to a given critical section on Linux. Below is the code which does that. I had written the code initially without the lines commented as "Check1" and "Check2". The code could not provide any mutual exclusion. I added the two lines explained previously for debugging purposes. After adding the lines, the code started giving the required output. I can't figure out what happened. Commenting the lines and running the code leads to the previous undesired output.

Code:

---

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



int s;

signal()
{
        printf("\ns:%d here \n",getpid()); // Check 1
        struct sembuf sa={0,1,0};
        semop(s,&sa,1);
 }

 wait()
{
 
        printf("\nw:%d here \n",getpid()); //Check 2
        struct sembuf sa={0,-1,0};
        semop(s,&sa,1);
}


void cs() // Critical Section
{
         
        printf("\nProcess%d entering ",getpid());
        sleep(1);
        printf("\nProcess%d exiting ",getpid());
        sleep(1);
}




union semun  // The Union Structure for semget
{
        int val;
        struct semid_ds *buff;
        unsigned short int *array;
}

main()
{      int i;
        union semun x;
        int pid;
        s=semget(IPC_PRIVATE,1,0666|IPC_CREAT); // Creating a single semaphore
        x.val=1;
        semctl(s,0,SETVAL,x);  // Initializing semaphore's value to be 1
        pid=fork();
        for(i=0;i<2;i++)
        {     
                      wait(); // Decrement Semaphore
                            cs();
                      signal(); // Increment Semaphore
                      sleep(1);
                     
        }


}

---

Does it ring a bell ?...

I'm not getting the right output with those lines in there. I would try and help but using that stuff is completely foreign to me. I just wanted to let you know I don't think the output is right even with those lines, and maybe it was a weird coincidence for you.

I'm not sure (its been awhile since I've done this level of coding) maybe you need to flush the buffer. If you replace printf with fflush(); are the results the same?

Cheers,
-D

Skiddieleet: I do realize that the lines are of no significance. But the output is correct as far as mutual exclusion is concerned. I've tried many times. The process scheduler is indeed a wild beast

dspeidel: I'm away from my pc now. Shall do it ASAP :-). Thanks anyway.

I've modified the code to make

Code:

---

#include<unistd.h>
#include<sys/types.h>
#include<sys/ipc.h>
#include<sys/sem.h>
#include<stdio.h>
int s;
void wait()
{      struct sembuf w={0,-2,0};
        printf("\nSemval of %d before wait is %d",getpid(),semctl(s,0,GETVAL));
                semop(s,&w,1);
        printf("\nnSemval of %d after wait is %d",getpid(),semctl(s,0,GETVAL));
}
void sigz()
{
        struct sembuf w={0,+2,0};
        printf("\nSemval of %d before wait is %d",getpid(),semctl(s,0,GETVAL));
        semop(s,&w,1);
/*mark1*/printf("\nSemval of %d after signal is %d" , getpid(),semctl (s,0,GETVAL)); 

}
void cs()
{
        printf("\nProcess %d entering cs",getpid());
        sleep(2);
        printf("\nProcesss %d quiting cs",getpid());
        sleep(2);
}

union semnu
{
        int val;
        struct semid_ds *buf;
        unsigned short *array;
}
main()
{
        int i;

        union semnu ss;
        s=semget(IPC_PRIVATE,1,0666|IPC_CREAT);
        ss.val=2;
        semctl(s,0,SETVAL,ss);
        fork();
        for(i=0;i<3;i++)
        {
                wait();
                cs();
                sigz();
                sleep(2);
        }
}

---

The Output if fine. But the debugging results are strange. Even after doing a signal on the semaphore, the value is still the same ( the line is commented as mark1 ) . And strangely processes is able to enter the critical section inspite of the 0 value of the semaphore !.