Note: There is no synchronisation here in this simple example
an we have no control over how the OS will schedule the processes.
/* semaphore.c --- simple illustration of dijkstra's semaphore analogy
*
* We fork() a child process so that we have two processes running:
* Each process communicates via a semaphore.
* The respective process can only do its work (not much here)
* When it notices that the semaphore track is free when it returns to 0
* Each process must modify the semaphore accordingly
*/
#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
union semun {
int val;
struct semid_ds *buf;
ushort *array;
};
main(){
int i,j;
int pid;
int semid; /* semid of semaphore set */
key_t key = 1234; /* key to pass to semget() */
int semflg = IPC_CREAT | 0666; /* semflg to pass to semget() */
int nsems = 1; /* nsems to pass to semget() */
int nsops; /* number of operations to do */
struct sembuf *sops = (struct sembuf *) malloc(2*sizeof(struct sembuf));
/* ptr to operations to perform */
/* set up semaphore */
if ((semid = semget(key, nsems, semflg)) == -1) {
perror("semget: semget failed");
exit(1);
};
/* get child process */
if ((pid = fork()) < 0) {
perror("fork");
exit(1);
}
if (pid == 0){ /* child */
i = 0;
while (i < 3) {/* allow for 3 semaphore sets */
nsops = 2;
/* wait for semaphore to reach zero */
sops[0].sem_num = 0; /* We only use one track */
sops[0].sem_op = 0; /* wait for semaphore flag to become zero */
sops[0].sem_flg = SEM_UNDO; /* take off semaphore asynchronous */
sops[1].sem_num = 0;
sops[1].sem_op = 1; /* increment semaphore -- take control of track */
sops[1].sem_flg = SEM_UNDO | IPC_NOWAIT; /* take off semaphore */
/* Recap the call to be made. */
(void) fprintf(stderr,"\nsemop:Child Calling semop(%d, &sops, %d) with:", semid, nsops);
for (j = 0; j < nsops; j++)
{
(void) fprintf(stderr, "\n\tsops[%d].sem_num = %d, ", j, sops[j].sem_num);
(void) fprintf(stderr, "sem_op = %d, ", sops[j].sem_op);
(void) fprintf(stderr, "sem_flg = %#o\n", sops[j].sem_flg);
}
/* Make the semop() call and report the results. */
if ((j = semop(semid, sops, nsops)) == -1) {
perror("semop: semop failed");
}
else
{
(void) fprintf(stderr, "\tsemop: semop returned %d\n", j);
(void) fprintf(stderr, "\n\nChild Process Taking Control of Track: %d/3 times\n", i+1);
sleep(5); /* DO Nothing for 5 seconds */
nsops = 1;
/* wait for semaphore to reach zero */
sops[0].sem_num = 0;
sops[0].sem_op = -1; /* Give UP COntrol of track */
sops[0].sem_flg = SEM_UNDO | IPC_NOWAIT; /* take off semaphore, asynchronous */
if ((j = semop(semid, sops, nsops)) == -1) {
perror("semop: semop failed");
}
else
(void) fprintf(stderr, "Child Process Giving up Control of Track: %d/3 times\n", i+1);
sleep(5); /* halt process to allow parent to catch semaphor change first */
}
++i;
}
}
else /* parent */
{ /* pid hold id of child */
i = 0;
while (i < 3) { /* allow for 3 semaphore sets */
nsops = 2;
/* wait for semaphore to reach zero */
sops[0].sem_num = 0;
sops[0].sem_op = 0; /* wait for semaphore flag to become zero */
sops[0].sem_flg = SEM_UNDO; /* take off semaphore asynchronous */
sops[1].sem_num = 0;
sops[1].sem_op = 1; /* increment semaphore -- take control of track */
sops[1].sem_flg = SEM_UNDO | IPC_NOWAIT; /* take off semaphore */
/* Recap the call to be made. */
(void) fprintf(stderr,"\nsemop:Parent Calling semop(%d, &sops, %d) with:", semid, nsops);
for (j = 0; j < nsops; j++)
{
(void) fprintf(stderr, "\n\tsops[%d].sem_num = %d, ", j, sops[j].sem_num);
(void) fprintf(stderr, "sem_op = %d, ", sops[j].sem_op);
(void) fprintf(stderr, "sem_flg = %#o\n", sops[j].sem_flg);
}
/* Make the semop() call and report the results. */
if ((j = semop(semid, sops, nsops)) == -1) {
perror("semop: semop failed");
}
else
{
(void) fprintf(stderr, "semop: semop returned %d\n", j);
(void) fprintf(stderr, "Parent Process Taking Control of Track: %d/3 times\n", i+1);
sleep(5); /* Do nothing for 5 seconds */
nsops = 1;
/* wait for semaphore to reach zero */
sops[0].sem_num = 0;
sops[0].sem_op = -1; /* Give UP COntrol of track */
sops[0].sem_flg = SEM_UNDO | IPC_NOWAIT; /* take off semaphore, asynchronous */
if ((j = semop(semid, sops, nsops)) == -1) {
perror("semop: semop failed");
}
else
(void) fprintf(stderr, "Parent Process Giving up Control of Track: %d/3 times\n", i+1);
sleep(5); /* halt process to allow child to catch semaphor change first */
}
++i;
}
}
}