122 lines
2.3 KiB
C
122 lines
2.3 KiB
C
/*
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** semdemo.c -- demonstrates semaphore use as a file locking mechanism
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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#include <errno.h>
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#include <sys/types.h>
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#include <sys/ipc.h>
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#include <sys/sem.h>
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#define MAX_RETRIES 10
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union semun {
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int val;
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struct semid_ds *buf;
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ushort *array;
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};
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/*
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** initsem() -- more-than-inspired by W. Richard Stevens' UNIX Network
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** Programming 2nd edition, volume 2, lockvsem.c, page 295.
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*/
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int initsem(key_t key, int nsems) /* key from ftok() */
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{
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int i;
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union semun arg;
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struct semid_ds buf;
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struct sembuf sb;
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int semid;
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semid = semget(key, nsems, IPC_CREAT | IPC_EXCL | 0666);
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if (semid >= 0) { /* we got it first */
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sb.sem_op = 1; sb.sem_flg = 0;
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arg.val = 1;
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printf("press return\n"); getchar();
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for(sb.sem_num = 0; sb.sem_num < nsems; sb.sem_num++) {
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/* do a semop() to "free" the semaphores. */
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/* this sets the sem_otime field, as needed below. */
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if (semop(semid, &sb, 1) == -1) {
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int e = errno;
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semctl(semid, 0, IPC_RMID); /* clean up */
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errno = e;
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return -1; /* error, check errno */
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}
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}
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} else if (errno == EEXIST) { /* someone else got it first */
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int ready = 0;
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semid = semget(key, nsems, 0); /* get the id */
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if (semid < 0) return semid; /* error, check errno */
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/* wait for other process to initialize the semaphore: */
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arg.buf = &buf;
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for(i = 0; i < MAX_RETRIES && !ready; i++) {
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semctl(semid, nsems-1, IPC_STAT, arg);
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if (arg.buf->sem_otime != 0) {
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ready = 1;
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} else {
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sleep(1);
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}
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}
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if (!ready) {
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errno = ETIME;
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return -1;
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}
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} else {
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return semid; /* error, check errno */
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}
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return semid;
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}
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int main(void)
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{
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key_t key;
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int semid;
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struct sembuf sb;
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sb.sem_num = 0;
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sb.sem_op = -1; /* set to allocate resource */
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sb.sem_flg = SEM_UNDO;
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if ((key = ftok("semdemo.c", 'J')) == -1) {
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perror("ftok");
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exit(1);
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}
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/* grab the semaphore set created by seminit.c: */
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if ((semid = initsem(key, 1)) == -1) {
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perror("initsem");
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exit(1);
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}
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printf("Press return to lock: ");
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getchar();
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printf("Trying to lock...\n");
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if (semop(semid, &sb, 1) == -1) {
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perror("semop");
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exit(1);
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}
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printf("Locked.\n");
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printf("Press return to unlock: ");
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getchar();
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sb.sem_op = 1; /* free resource */
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if (semop(semid, &sb, 1) == -1) {
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perror("semop");
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exit(1);
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}
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printf("Unlocked\n");
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return 0;
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}
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