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| /* ipcsemaphore.c
To compile : gcc -o sem ipcsemaphore.c
To run : ./sem */
#define _XOPEN_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
union semun {
int val;
struct semid_ds *buf;
char *array;
};
void die(char *msg) {
perror(msg);
fflush(stdout);
fflush(stderr);
exit(1);
}
int main() {
int i,j;
int pid;
int semid; /* semid of semaphore set */
key_t key ; /* 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 */
/* generate key */
/* if ((key = ftok("Semaphore", 'Q')) == -1)
die("ftok"); */
key=1234567890;
/* set up semaphore */
printf("\nsemget: Setting up semaphore: semget(%d, %d %o)\n", (int)key, nsems, semflg);
fflush(stdout);
fflush(stderr);
if ((semid = semget(key, nsems, semflg)) == -1)
die("semget: semget failed");
if ((pid = fork()) < 0)
die("fork");
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. */
printf("\nsemop:Child Calling semop(%d, &sops, %d) with:", semid, nsops);
for (j = 0; j < nsops; j++)
{
printf("\n\tsops[%d].sem_num = %d, ", j, sops[j].sem_num);
printf("sem_op = %d, ", sops[j].sem_op);
printf("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 {
printf("\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 {
printf("Child Process Giving up Control of Track: %d/3 times\n", i+1);
}
sleep(5); /* halt process to allow parent to catch semaphore change first */
}
++i;
}
} else { /* parent */
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. */
printf("\nsemop:Parent Calling semop(%d, &sops, %d) with:", semid, nsops);
for (j = 0; j < nsops; j++) {
printf("\n\tsops[%d].sem_num = %d, ", j, sops[j].sem_num);
printf("sem_op = %d, ", sops[j].sem_op);
printf("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 {
printf("Parent Process Taking Control of Track: %d/3 times\n", i+1);
sleep(5); /* Sleep 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 {
printf("Parent Process Giving up Control of Track: %d/3 times\n", i+1);
}
sleep(5); /* halt process to allow child to catch semaphore change first */
}
++i;
}
}
return 0;
} |
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