books/apitue/sample-code/08/semdemo.c

/* This file is part of the sample code and exercises
 * used by the class "Advanced Programming in the UNIX
 * Environment" taught by Jan Schaumann
 * <jschauma@netmeister.org> at Stevens Institute of
 * Technology.
 *
 * This file is in the public domain.
 *
 * You don't have to, but if you feel like
 * acknowledging where you got this code, you may
 * reference me by name, email address, or point
 * people to the course website:
 * https://stevens.netmeister.org/631/
 */

/* A simple program to illustrate the use of
 * semaphores.  Derived from:
 * https://www.beej.us/guide/bgipc/html/multi/semaphores.html
 *
 * Run in parallel in multiple processes to illustrate
 * the blocking as another process holds a lock.
 *
 * Use ipcs(1) to show that semaphores remain in the
 * kernel until explicitly removed.
 */
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/types.h>

#include <err.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define MAX_RETRIES 10

 /* Code examples often use a char as the ftok(3) id,
  * but really any int will do. */
#define SEMID       42
#define SEMLOCK     -1
#define SEMUNLOCK   1

#ifndef __DARWIN_UNIX03
union semun {
	int val;
	struct semid_ds *buf;
	ushort *array;
};
#endif

/*
* initsem() -- simplified version of W. Richard
* Stevens' UNIX Network Programming 2nd edition,
* volume 2, lockvsem.c, page 295;
*/
int
initsem(key_t key) {
	int i;
	union semun arg;
	struct semid_ds buf;
	struct sembuf sb;
	int semid;

	arg.buf = &buf;

	semid = semget(key, 1, IPC_CREAT | IPC_EXCL | 0666);
	if (semid >= 0) {       /* we got it first */
		sb.sem_num = 0; /* operate on the first semaphore in the set */
		sb.sem_op  = 1; /* "free" the semaphore */
		sb.sem_flg = 0; /* no flags are set */

		/* "Free" the semaphore.  Note: since
		 * this semget, semop sequence is
		 * non-atomic, it poses a possible
		 * race condition whereby this process
		 * that created the semaphore might be
		 * suspended prior to freeing it.  In
		 * that case, the second process in
		 * the 'EEXIST' block below...
		 */
		if (semop(semid, &sb, 1) == -1) {
			int e = errno;
			if (semctl(semid, 0, IPC_RMID) < 0) {
				perror("semctl rm");
			}
			errno = e;
			return -1; /* error, check errno */
		}

	} else if (errno == EEXIST) { /* someone else got it first */
		int ready = 0;
		int e;

		semid = semget(key, 1, 0); /* get the id */
		if (semid < 0)
			return semid; /* error, check errno */

		/* ...would see sem_otime as '0',
		 * sleep for a second, and attempt
		 * again up to MAX_TRIES to give the
		 * semaphore-creating process above a
		 * chance to free it.  Without this
		 * check, the race condition above
		 * could otherwise lead to a deadlock.
		 */
		for (i = 0; i < MAX_RETRIES && !ready; i++) {
			if ((e = semctl(semid, 0, IPC_STAT, arg)) < 0) {
				perror("semctl stat");
				return -1;
			}

			if (arg.buf->sem_otime != 0) {
				ready = 1;
			} else {
				sleep(1);
			}
		}

		/* If the other process didn't free
		 * the semaphore after MAX_TRIES
		 * seconds, give up.  Note: we set
		 * errno explicitly ourselves here. */
		if (!ready) {
			errno = ETIME;
			return -1;
		}
	} else {
		return semid; /* error, check errno */
	}

	return semid;
}

int
main(void) {
	key_t key;
	int semid;
	struct sembuf sb = {
		.sem_num = 0,
		.sem_op  = SEMLOCK,
		.sem_flg = SEM_UNDO,
	};

	/* What happens if you use e.g., getuid() as
	 * the 'id' argument to ftok(3)?
	 *
	 * What if the user running the command
	 * doesn't have read permissions to the file
	 * here?
	 *
	 * Give it a try. */
	if ((key = ftok("./semdemo.c", SEMID)) == -1) {
		err(EXIT_FAILURE, "ftok");
		/* NOTREACHED */
	}

	/* What happens if you don't use ftok(3)?
	 * Give it a try. */
	//key = IPC_PRIVATE;

	if ((semid = initsem(key)) == -1) {
		err(EXIT_FAILURE, "initsem");
		/* NOTREACHED */
	}

	(void)printf("Press return to lock: ");
	(void)getchar();
	(void)printf("Trying to lock...\n");

	/* This call will block if another process
	 * already has a lock. */
	if (semop(semid, &sb, 1) == -1) {
		err(EXIT_FAILURE, "semop");
		/* NOTREACHED */
	}

	(void)printf("Locked.\n");
	(void)printf("Press return to unlock: ");
	(void)getchar();

	/* Since we specified SEM_UNDO above, we don't
	 * stricly need to free the resource here; our
	 * process exiting would increment the
	 * semaphore value.  (Try it out: comment out
	 * this block and set sem_flg = 0 above.)
	 * However, just as with cleaning up file
	 * descriptors and other resources, it's a
	 * good idea to be explicit and free the
	 * resource on exit. */
	sb.sem_op = SEMUNLOCK;
	if (semop(semid, &sb, 1) == -1) {
		err(EXIT_FAILURE, "semop");
		/* NOTREACHED */
	}

	(void)printf("Unlocked.\n");

	return EXIT_SUCCESS;
}