CODEWITHSUNDEEP
csemaphore
Mohe TheDreamy
Mohe TheDreamy

Reputation: 387

error creating more than one semaphore

so there's this assignment the teacher gave us, and she already gave us some a procedure to create semaphore along with some other procedures. I could create one a semaphore with that procedure but couldn't do more than one, the it shows an error when trying to create the second one. I really tried to look for a solution for this but I couldn't even find a code with a similar procedure. is there something I should change within the procedure "sem_create" ?

here's the code: 

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/ipc.h>
#include <sys/sem.h>

int sem_create(key_t CLEF,int initval) {
    union semun {
        int val;
        struct semid_ds *buf;
        ushort *array;
    } arg_ctl;
    int semid;
    semid = semget(ftok("Disjktra.h<votre login>",CLEF),
                   1, IPC_CREAT|IPC_EXCL|0666);
    if (semid == -1)
        return -1;
    arg_ctl.val = initval; // sth is missing here
    if (semctl(semid, 0, SETVAL, arg_ctl) == -1)
        return -1;
    return semid;
}


/////////////////////////////////
/////////////////////////////////

void main() {
    int i, CLE=33, S1=0, S2=0;

    if ((S1 = sem_create(CLE,0)) == -1) {
        perror("error in creating semaphore 1");
        exit(-1);
    }

    if ((S2 = sem_create(CLE,0)) == -1) {
        perror("error in creating semaphore 2");
        exit(-1);
    }
}

Output: 

error in creating semaphore 2 :File exists

Upvotes: 0

Views: 3025

Answers (2)

prashad
prashad

Reputation: 107

semaphore exclusivity
first time:
semget() returns the ID of the semaphore. and the check to see if this value is greater than 0 results in a true thereby execting the printf() that prints the ID
second time
the next time round howevever the value returned b semget() is
less than 0 signifying that the semaphore alread exists. as a result the perror() function executed.

reason is :file exists

Upvotes: 0

Tim Pierce
Tim Pierce

Reputation: 5664

According to the Linux manual page for semget(2):

If semflg specifies both IPC_CREAT and IPC_EXCL and a semaphore set already exists for key, then semget() fails with errno set to EEXIST.

This seems pretty clear. Your sem_create function calls semget to create a "Disjktra.h" semaphore, but only if one does not already exist. You can't do that twice in the same program.

Edit: note also, from the ftok(3) man page:

The specified path must specify an existing file that is accessible to the calling process or the call will fail.

I believe that if you check the result of ftok you will find that it is failing to generate a valid key for either semaphore:

key_t semkey = ftok("Disjktra.h<votre login>",CLEF);
if (semkey < 0) {
    perror("error creating key");
    return -1;
}

Based on this, I suggest using a real file for the pathname to ftok, or abandoning ftok entirely and creating a private semaphore each time with semget(IPC_PRIVATE, ...).

On another note, your code was very hard to understand the way it was formatted. Please consider cleaning up your code so it's easier to read when you post a question.

Upvotes: 1

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