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Embedded Linux Learning

sem_wait / sem_post

sem_wait() / sem_post()¶

Purpose¶

Performs decrement/wait and increment/signal operations on a POSIX semaphore.

#include <semaphore.h>

Prototype¶

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

Parameters¶

sem: pointer to a POSIX semaphore.

Return Value¶

Success: returns 0.
Failure: returns -1 and sets errno.

Minimal Example¶

sem_wait(&shared->items);
/* consume one item */
sem_post(&shared->spaces);

Common Pitfalls¶

sem_wait() can be interrupted by a signal and fail with EINTR.
sem_post() is used to wake waiters.
Keep semaphore ownership and shared memory lifetime clearly defined.

IPC