Semaphore operations on Suns

[Pilli Sujata]

The folowing code is performing some Semaphore operations like wait and
signaling System V features.  But I have problems when I am running it on
The Sun (3/50, 3/60 ) SUN O.S RELEASE 4.0.  The semaphore results in a
negative value sometimes.  The same code was run on a different m/c the
Encore which has the same system calls and it ran fine. Could anyone tell
me why it is not working on the Suns?  Please do run it several times.

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/wait.h>
main()
{
	int i, semid,status, x;
	struct sembuf cw,cs,pw,ps;

	if ((semid=semget(IPC_PRIVATE,2,0666))==-1) perror("semget");
	cw.sem_num = 1; cw.sem_op = -1; cw.sem_flg =~IPC_NOWAIT;
	cs.sem_num = 0; cs.sem_op = 1; cs.sem_flg = IPC_NOWAIT;
	pw.sem_num = 0; pw.sem_op = -1; pw.sem_flg =~IPC_NOWAIT;
	ps.sem_num = 1; ps.sem_op = 1; ps.sem_flg = IPC_NOWAIT;
	if ( fork() == 0) {
	/* child */
	/*Get semaphore value for 1 */
	if ((x = semctl(semid,1,GETVAL,0)) == -1) perror("no-prob:semctl1");
	printf("Semval read by child for sem 1 %d\n",x);

	/*semaphore wait */
	if(semop(semid, &cw, 1) == -1) perror("child-not-waiting:semop1");

	/*Get semaphore value for 1 */
	if ((x = semctl(semid,1,GETVAL,0)) == -1) perror("no-prob:semctl1");
	printf("Semval read by child for sem 1 %d\n",x);

	/*Get semaphore value for 0 */
	if ((x = semctl(semid,0,GETVAL,0)) == -1) perror("no-prob:semctl1");
	printf("Semval read by child for sem 0 %d\n",x);

	/*semaphore signal*/
	if(semop(semid, &cs, 1) == -1) perror("child-signals:semop2");

	/*Get semaphore value for 0 */
	if ((x = semctl(semid,0,GETVAL,0)) == -1) perror("no-prob:semctl3");
	printf("Semval read by child for sem 0 %d\n",x);
	}
	else {
	/*parent*/

	/*Get semaphore value for  1 */
	if ((x = semctl(semid,1,GETVAL,0)) == -1) perror("no-prob:semctl3");
	printf("Semval read by Parent for sem 1 %d\n",x);

	/*semaphore signal*/
	if(semop(semid, &ps, 1) == -1) perror("par-signals:semop2");

	/*Getsemaphore  value for 0 & 1 */
	if ((x = semctl(semid,0,GETVAL,0)) == -1) perror("no-prob:semctl1");
	printf("Semval read by Parent for sem 0 %d\n",x);
	if ((x = semctl(semid,1,GETVAL,0)) == -1) perror("no-prob:semctl1");
	printf("Semval read by Parent for sem 1 %d\n",x);

	/*semaphore wait*/
	printf("Parent shud wait\n");
	if(semop(semid, &pw, 1) == -1) perror("parent-waits:semop2");

	/*Get semaphore value for 0  */
	if ((x = semctl(semid,0,GETVAL,0)) == -1) perror("no-prob:semctl1");
	printf("Semval read by Parent for sem 0 %d\n",x);
	if ((x = semctl(semid,1,GETVAL,0)) == -1) perror("no-prob:semctl1");
	printf("Semval read by Parent for sem 1 %d\n",x);

	/*wait for child*/
	while (wait(&status) == -1) perror("wait");

	}

	semctl(semid,0,IPC_RMID,0);

}

Please e-mail you responses. 
Thanks
Sujata
sujata at zambini.cs.uiowa.edu