Can I delete a named semaphore created with sem_open? - macos

Question

I am experimenting with the named semaphores and there is something that I don't understand about the behavior of sem_close and sem_destroy. In my example, it looks like the semaphore that I create does not get deleted.
#include <iostream>
#include <semaphore.h>
int main() {
char sem_name[] = "/sem-1";
{
sem_t *sptr = sem_open(sem_name, O_CREAT, 0644, 0);
if (sptr != SEM_FAILED) {
printf("sem_open success\n");
// neither of these works
sem_close(sptr);
// sem_destroy(sptr);
} else {
printf("sem_open error #1: %s\n", strerror(errno));
}
}
sem_t *sptr = sem_open(sem_name, O_CREAT | O_EXCL, 0644, 0);
printf("sem_open error #2: %s\n", strerror(errno));
assert(sptr != SEM_FAILED);
return 0;
}
The output:
sem_open success
sem_open error #2: File exists
Assertion failed: (sptr != SEM_FAILED), function main, file /tmp/delete_me/main.cpp, line 22.
I would expect the assertion to not be hit and also the error #2 to not shown.
Additionally, macOS says that the sem_destroy method is deprecated but I am still using both sem_close() and sem_destroy but they both seem to not delete the semaphore.
Background: I am trying to port NASA Core Flight System to macOS. Their POSIX implementation uses sem_ calls and I am trying to understand if I can make them work on macOS with some minor modifications.

Answer 1

Since macOS only recognizes named semaphores, using sem_destroy() won't work. sem_destroy() destroys an unnamed semaphore at the address pointed to by sem. Only a semaphore that has been initialized by sem_init() should be
destroyed using sem_destroy().
You could likely overcome this issue by using sem_unlink:
...
sem_close(sptr);
sem_unlink(sem_name);
...
SEM_UNLINK(2)
NAME
sem_unlink -- remove a named semaphore
SYNOPSIS
#include <semaphore.h>
int sem_unlink(const char *name);
DESCRIPTION
The named semaphore named name is removed. If the semaphore is in use by
other processes, then name is immediately disassociated with the sema-phore, semaphore,
phore, but the semaphore itself will not be removed until all references
to it have been closed. Subsequent calls to sem_open() using name will
refer to or create a new semaphore named name.
If successful, `sem_unlink()` will return 0. Otherwise, -1 is returned and
errno is set, and the state of the semaphore is unchanged.
ERRORS
sem_unlink() succeeds unless:
[EACCES] Permission is denied to be remove the semaphore.
[ENAMETOOLONG] name exceeded SEM_NAME_LEN characters.
[ENOENT] The named semaphore does not exist.
↳ macOS Manual Pages | sem_unlink()