POSIX named semaphore name without slash

The man page sem_overview(7) explains that the name of named semaphores ought to start with /. However, I couldn't find any specification of what should or would happen if such a name does not start by a slash.

Trying it out on Linux seems to reveal no difference at all and xxd /dev/shm/sem.mysem also shows no difference when using sem_open("mysem", 0) vs. sem_open("/mysem", 0) in a (C) program.

Is the naming scheme actually of any relevance?

Solution

TLDR:

The GLIBC implementation ignores leading / characters despite what the documentation says.

Full answer:

Per POSIX sem_open():

SYNOPSIS
#include <semaphore.h>

sem_t *sem_open(const char *name, int oflag, ...);
DESCRIPTION

...

If name does not begin with the <slash> character, the effect is implementation-defined.

...

So, in this case the name will be treated per the Linux sem_open() man page:

DESCRIPTION

sem_open() creates a new POSIX semaphore or opens an existing semaphore. The semaphore is identified by name. For details of the construction of name, see sem_overview(7).

Which brings us back to the sem_overview(7) man page as you noted:

Named semaphores

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

As you noted, that doesn't specify what happens if the leading / is omitted.

So, how is it implemented?

The actual name of the semaphore file (that's all a "POSIX named semaphore" is - a file in a file system that is memory mapped and contains a semaphore object) is created in GLIBC by the __shm_get_name() function in the posix/shm-directory.c file:

int
__shm_get_name (struct shmdir_name *result, const char *name, bool sem_prefix)
{
  struct alloc_buffer buffer;
  size_t namelen;

  buffer = alloc_buffer_create (result->name, sizeof (result->name));
  alloc_buffer_copy_bytes (&buffer, SHMDIR, strlen (SHMDIR));

#if defined (SHM_ANON) && defined (O_TMPFILE)
  if (name == SHM_ANON)
    {
      /* For SHM_ANON, we want shm_open () to pass O_TMPFILE to open (),
         with SHMDIR itself as the path.  So, leave it at that.  */
      alloc_buffer_add_byte (&buffer, 0);
      if (alloc_buffer_has_failed (&buffer))
        return -1;
      return 0;
    }
#endif

  while (name[0] == '/')
    ++name;
  namelen = strlen (name);

  if (sem_prefix)
    alloc_buffer_copy_bytes (&buffer, "sem.", strlen ("sem."));
  alloc_buffer_copy_bytes (&buffer, name, namelen + 1);
  if (namelen == 0 || memchr (name, '/', namelen) != NULL)
    return EINVAL;
  if (alloc_buffer_has_failed (&buffer))
    {
      if (namelen > NAME_MAX)
        return ENAMETOOLONG;
      return EINVAL;
    }
  return 0;
}

Note this code:

while (name[0] == '/')
    ++name;

The code skips over all leading / characters if they exist. If there are no leading / characters, GLIBC proceeds without error.

GLIBC's implementation makes leading / characters use irrelevant. The result is the same no matter how many leading / characters are used, even if that number is zero, despite what the sem_overview man page states.