The Linux Programming Interface

990 Chapter 47

Listing 47-10 shows the implementation of the binary semaphore functions. Each

function in this implementation takes two arguments, which identify a semaphore

set and the number of a semaphore within that set. (These functions don’t deal

with the creation and deletion of semaphore sets; nor do they handle the race con-

dition described in Section 47.5.) We employ these functions in the example pro-

grams shown in Section 48.4.

Listing 47-10: Implementing binary semaphores using System V semaphores

–––––––––––––––––––––––––––––––––––––––––––––––––––––– svsem/binary_sems.c

#include <sys/types.h>

#include <sys/sem.h>

#include "semun.h" /* Definition of semun union */

#include "binary_sems.h"

Boolean bsUseSemUndo = FALSE;

Boolean bsRetryOnEintr = TRUE;

int /* Initialize semaphore to 1 (i.e., "available") */

initSemAvailable(int semId, int semNum)

{

union semun arg;

arg.val = 1;

return semctl(semId, semNum, SETVAL, arg);

}

int /* Initialize semaphore to 0 (i.e., "in use") */

initSemInUse(int semId, int semNum)

{

union semun arg;

arg.val = 0;

return semctl(semId, semNum, SETVAL, arg);

}

/* Reserve semaphore (blocking), return 0 on success, or -1 with 'errno'

set to EINTR if operation was interrupted by a signal handler */

int /* Reserve semaphore - decrement it by 1 */

reserveSem(int semId, int semNum)

{

struct sembuf sops;

sops.sem_num = semNum;

sops.sem_op = -1;

sops.sem_flg = bsUseSemUndo? SEM_UNDO : 0;

while (semop(semId, &sops, 1) == -1)

if (errno != EINTR || !bsRetryOnEintr)

return -1;

return 0;

}