sem_open returns 0 under linux

Question

This sounds like a trivial question, but I couldn't easily find a solution around.

Here is my code:

#include <iostream>
#include <fcntl.h>
#include <semaphore.h>

using namespace std;

int main()
{
    sem_t * my_semaphore = sem_open("./my_semaphore", O_CREAT, 0755, 1);
    cout<<my_semaphore<<endl;
}

This prints out 0: the semaphore wasn't created. What am I doing wrong? I tested this code on an OSX environment and it worked perfectly.

Answer 1

The Linux manpage for sem_open says (emphasis added):

ENOENT The O_CREAT flag was not specified in oflag and no semaphore with this name exists; or, O_CREAT was specified, but name wasn't well formed .

The sem_overview says this about semaphore names:

A named semaphore is identified by a name of the form /somename ; that is, a null-terminated string of up to NAME_MAX-4 (ie, 251) characters consisting of an initial slash, followed by one or more characters, none of which are slashes.

Answer 2

I had similar problem starting from GLIBC version 2.34.

sem_open returned always NULL and error code was set to ENOENT (2).

The problem was, that on my Linux system there was no /dev/shm directory. Starting from GLIBC version 2.34, this folder is a requirement to use shared semaphores.

Answer 3

From the documentation at http://pubs.opengroup.org/onlinepubs/009695399/functions/sem_open.html

If name begins with the slash character, then processes calling sem_open() with the same value of name shall refer to the same semaphore object, as long as that name has not been removed. If name does not begin with the slash character, the effect is implementation-defined. The interpretation of slash characters other than the leading slash character in name is implementation-defined.

Implementors of the sem_open() function can define what happens if the name doesn't start with a /. It appears that linux doesn't allow such names.