Single app instance with semaphores

// Link with pthread support - gcc -pthread -o A A.c
#include <errno.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <semaphore.h>
#include <stdio.h>

#define SEM_NAME "/sample_app_semaphore"

int main()
{
    sem_t *sem = sem_open(SEM_NAME, O_CREAT | O_EXCL, 0666, 0);
    if (sem == SEM_FAILED)
    {
        if (errno == EEXIST)
        {
            puts("This application is already running.");
            return 1;
        }
        puts("Cannot create semaphore - it may already exist");
        return 2;
    }

    puts("Press any key...");
    getchar();

    sem_close(sem);
    sem_unlink(SEM_NAME);
}