Shared memory - shm_open causing permission denied

Question

I am writing a program that uses POSIX shared memory and have an error that I am unsure how to fix. I looked for similar questions but could not find any relevant to this specific problem.

Two files are involved - server.c, which contains the code run by the program, and shm.c, which contains functions that provide abstraction for handling the shared memory. This is an assignment, so I cannot deviate very far from the current structure.

Below is the relevant code from each file:

server.c

int shmFd;

shmFd = createSHM(SHNAME);

shm.c

int createSHM(char * shname)
{
    int fileDesc;

    fileDesc = shm_open(shname, O_CREAT | O_RDWR, 0);

    if(fileDesc == -1)
    {
        perror("Error: Could not create shared memory space");
    }

  return fileDesc;
}

shm.h

#define SHNAME "/shmserver" 

When I attempt to run the built program in the terminal, the following error appears:

Error: Could not create shared memory space: Permission denied 

Any help would be much appreciated.

Answer 1

The line

fileDesc = shm_open(shname, O_CREAT | O_RDWR, 0);

gives no-one any access rights to the shared memory object. Once you create a shared memory object with no access rights, only the root user will be able to open it.

Instead, use (for example)

fileDesc = shm_open(shname, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR | S_IXUSR);

(You could allow other users to access the shared memory, obviously. But you need, at a minimum, to allow yourself to access it; otherwise, you won't be able to open it once you've created it.)

Perhaps worth noting that your error message is incorrect, so you might be misleading yourself (and others). The call to shm_open does not fail when it is creating the shared memory object. What fails is opening an already created shared memory object without all permissions for the user.