XS has a reputation for being hard to access and I think it's a shame because I don't think it has to be: it's mostly that the Perl API is hard. What if you offload as much logic as possible to perl, and use XS only to expose functions to perl? That would be much easier for a casual XS writer who doesn't know anything about Perl's internals.

So, in this example I will write a simple XS module for a real-life API, in this case POSIX real-time semaphores. This allows you to synchronize data between different processes or threads. At its core the API is this:

sem_t* sem_open(const char* path, int open_flags, mode_t mode, unsigned int value);
int sem_close(sem_t *sem);
int sem_unlink(const char* path);

int sem_wait(sem_t *sem);
int sem_trywait(sem_t *sem);
int sem_post(sem_t *sem);