svsem/event_flags.c

This is svsem/event_flags.c, an example to accompany the book, The Linux Programming Interface.

This file is not printed in the book; it is the solution to Exercise 47-5:a (page 995).

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

Download svsem/event_flags.c

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Function list (Bold in this list means a function is not static)

waitForEventFlag()
clearEventFlag()
setEventFlag()
getFlagState()

/* event_flags.c

   Implement an event flags protocol using System V semaphores.

   See event_flags.h for a summary of the interface.
*/
#include <sys/types.h>
#include <sys/sem.h>
#include "semun.h"              /* Definition of semun union */
#include "event_flags.h"
#include "tlpi_hdr.h"

/* Wait for the specified flag to become "set" (0) */

int
waitForEventFlag(int semId, int semNum)
{
    struct sembuf sops;

    sops.sem_num = semNum;
    sops.sem_op = 0;                    /* Wait for semaphore to equal 0 */
    sops.sem_flg = 0;

    /* Waiting for a semaphore to become zero may block, so we
       program to retry if interrupted by a signal handler */

    while (semop(semId, &sops, 1) == -1)
        if (errno != EINTR)
            return -1;
    return 0;
}

/* "Clear" the event flag (give it the value 1) */

int
clearEventFlag(int semId, int semNum)
{
    union semun arg;

    arg.val = 1;
    return semctl(semId, semNum, SETVAL, arg);
}

/* "Set" the event flag (give it the value 0) */

int
setEventFlag(int semId, int semNum)
{
    union semun arg;

    arg.val = 0;
    return semctl(semId, semNum, SETVAL, arg);
}

/* Get current state of event flag */

int
getFlagState(int semId, int semNum, Boolean *isSet)
{
    union semun dummy;

    int sem_val = semctl(semId, semNum, GETVAL, dummy);
    if (sem_val == -1)
        return -1;

    *isSet = (sem_val == 0) ? TRUE : FALSE;
    return 0;
}

Download svsem/event_flags.c

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