why are some posix shared memory segments and posix semaphores not visible to ipcs

Question

I built a client server application using posix shared memory and posix unnamed semaphores with pshared=1. The semaphores are placed inside the shared memory. The program runs fine, but when I type ipcs -m or ipcs -s, I do not see any shared memory segments or semaphores that I created. Why is it so?

/* Server main function for implementing client server program using Posix Shared Memory and Posix Unnamed Semaphores*/  
#include "shm_sem.h"
int main(int argc,char ** argv)  
{  
    int fd;  
    struct shmstruct *ptr;  
    shm_unlink(MYSHM); // delete shared memory segment, if it already exists     
    /* create shared memory, set its size, map it and close descriptor */
    fd=shm_open(MYSHM,O_RDWR|O_CREAT|O_EXCL,0777);  
    ptr=mmap(NULL,sizeof(struct shmstruct),PROT_READ|PROT_WRITE,MAP_SHARED,fd,0);  
    // truncate the size of shared memory to the size of shmstruct  
    ftruncate(fd,sizeof(struct shmstruct)); 
    close(fd);  
    // initialize the semaphores in shared memory  
    sem_init(&ptr->client_mutex,1,1); // set client semaphore to 1  
    sem_init(&ptr->server_mutex,1,0); // set server semaphore to 0  
    for(;;)
        {
        serverPosixShmSem(ptr); // calling server
        }  
}

Answer 1

A couple of questions:

• Are you running ipcs as the same user that created the shared memory/semaphores (or as the superuser)?
• Are you running ipcs while the program is running? (Are you sure it's not removing them when it exits?)

Update:

Actually, after reading this thread I'm not sure ipcs is supposed to be able to show the POSIX semaphores. I tried your sample code (with some edits to fix compile errors) and you can see the shared memory segment in the /dev/shm directory.

Answer 2

/* Server main function for implementing client server program using Posix Shared Memory and Posix Unnamed Semaphores*/

include "shm_sem.h"

int main(int argc,char ** argv)
{
int fd;
struct shmstruct *ptr;
shm_unlink(MYSHM); // delete shared memory segment, if it already exists
/* create shared memory, set its size, map it and close descriptor */ fd=shm_open(MYSHM,O_RDWR|O_CREAT|O_EXCL,0777);
ptr=mmap(NULL,sizeof(struct shmstruct),PROT_READ|PROT_WRITE,MAP_SHARED,fd,0);
// truncate the size of shared memory to the size of shmstruct
ftruncate(fd,sizeof(struct shmstruct)); close(fd);

// initialize the semaphores in shared memory
sem_init(&ptr->client_mutex,1,1); // set client semaphore to 1
sem_init(&ptr->server_mutex,1,0); // set server semaphore to 0
for(;;) {

    serverPosixShmSem(ptr); // calling server

    }

}

Answer 3

ipcs displays information on the System V IPC system. POSIX semaphores and shared memory are an independent (and better) system which is not monitored by 'ipcs'.