IPC Programming in Unix

[PacketThirst]

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.

#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 ?...

[skiddieleet]

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.

[dspeidel]

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

[PacketThirst]

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.

[PacketThirst]

I've modified the code to make

#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 !.