shm_open() leads to a No such file or directory

Question

Description :

I have a project directory named "Projet" which contain two directory named "Serveur" and "Client".

(1) Serveur contains serveur.c (2) Client contains client.c

Referenting to the man, I choose as a name : "/shm_request_stack".

Source files description :

serveur.c :

#define SHM_REQUEST "/shm_request_stack"

int main(void) {
    sem_t shm = open_shm(SHM_REQUEST,
                         O_RDWR | O_CREAT | O_EXCL,
                         S_IRUSR | S_IWUSR);

    unlink_shm(SHM_REQUEST);

    size_t memsize = sizeof(int);
    setsize_shm(shm, memsize);

    int * ptr = project_shm(shm, memsize);
    *ptr = 0;

    while(*ptr == 0);
    printf("Client modify the value\n");
}

client.c :

#define SHM_REQUEST "/shm_request_stack"

int main(void) {
    sem_t shm = open_shm(SHM_REQUEST,
                         O_RDWR,
                         S_IRUSR | S_IWUSR);

    unlink_shm(SHM_REQUEST);

    size_t memsize = sizeof(int);
    int * ptr = project_shm(shm, memsize);

    *ptr = 1;
}

Envelope functions

int open_shm(char *name, int oflag, mode_t mode) {
    int shm = shm_open(name, oflag, mode);
    if (shm == -1) {
        fprintf(stderr, "Error while opening %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    }
    return shm;
}

void unlink_shm(char *name) {
    if (shm_unlink(name) == -1) {
        perror("sem_unlink");
        exit(EXIT_FAILURE);
    }
}

void setsize_shm(int shm, size_t size) {
    if (ftruncate(shm, size) == -1) {
        perror("ftruncate");
        exit(EXIT_FAILURE);
    }
}

void * project_shm(int shm, size_t size) {
    int *ptr = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, shm, 0);
    if (ptr == MAP_FAILED) {
        perror("mmap");
        exit(EXIT_FAILURE);
    }
    return ptr;
}

Problems :

The client can't find the named shm create by the server. I tried to found the shared memory with ipcs -m but I've not.

I try to modify the value from the server and it's works so the memory exist.

How can I successfully open the shm from the client ?

Answer 1

You appear to be deleting the shared object immediately after creating it (unlink).

It is a bit like a file. If you have an open reference to the object then it is retained, but the unlink removes the name. That is why the server can still write to the object after the unlink (she is still in scope), but the client cannot open the object by name.

The critical words in the doc you quote are : "all open and map references" - what you can't do is create a new reference after the unlink.