Simple POSIX Semaphore for Windows

POSIX semaphores allow processes and threads to synchronize their actions. A semaphore is an integer whose value is never allowed to fall below zero. Two operations can be performed on semaphores: increment the semaphore value by one [sem_post]; and decrement the semaphore value by one [sem_wait]. If the value of a semaphore is currently zero, then a sem_wait operation will block until the value becomes greater than zero.

The semaphore.c & semaphore.h files provide replacement for POSIX semaphores on Windows XP or later. The provided sem_ functions do not behave identically to their POSIX equivalents, but they will work as drop-in replacements for most use cases.

Functions Provided

• int sem_init (sem_t *sem, int pshared, unsigned int value);

• int sem_wait (sem_t *sem);

• int sem_trywait (sem_t *sem);

• int sem_timedwait (sem_t *sem, const struct timespec *abs_timeout);

• int sem_post (sem_t *sem);

• int sem_getvalue (sem_t *sem, int *value);

• int sem_destroy (sem_t *sem);

• sem_t *sem_open (const char *name, int oflag, mode_t mode, unsigned int value);

• int sem_close (sem_t *sem);

• int sem_unlink (const char *name);

Example

#include "semaphore.h"

int main(int argc, char **argv) {
sem_t *sem = sem_open("example_semaphore", O_CREAT | O_EXCL, 0, 1);

/*check if semaphore failed*/
if (sem == SEM_FAILED) {
	printf("Failed to acquired semaphore\n");
	return -1;
	}

printf("About to enter critical code block 1\n");
sem_wait(sem);

/*Critical Code Block 1*/
printf("In critical code block 1\n");
Sleep(2000);
printf("Finished critical code block 1\n\n");
sem_post(sem);

/*Normal Block 1*/
printf("Normal code block 1\n");
printf("About to enter critical code block 1\n");
sem_wait(sem);

/*Critical Code Block 2*/
printf("In critical code block 2\n");
Sleep(2000);
printf("Finished critical code 2\n\n");
sem_post(sem);

/*Normal Block 2*/
printf("Normal code block 2\n\n");

/*clean up*/
sem_unlink("example_semaphore");
printf("exiting\n");
sem_close(sem);
return 0;
}

Requirements

There are no external requirements to use. Just add the two files (semaphore.c & semaphore.h) to your project and include the semaphore.h header in the source file(s) you wish to use semaphores.