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SEM_POST(3) Linux Programmer's Manual SEM_POST(3)

sem_post - unlock a semaphore

#include <semaphore.h>
int sem_post(sem_t *sem);

Link with -pthread.

sem_post() increments (unlocks) the semaphore pointed to by sem. If the semaphore's value consequently becomes greater than zero, then another process or thread blocked in a sem_wait(3) call will be woken up and proceed to lock the semaphore.

sem_post() returns 0 on success; on error, the value of the semaphore is left unchanged, -1 is returned, and errno is set to indicate the error.

sem is not a valid semaphore.
The maximum allowable value for a semaphore would be exceeded.

For an explanation of the terms used in this section, see attributes(7).
Interface Attribute Value
sem_post () Thread safety MT-Safe


sem_post() is async-signal-safe: it may be safely called within a signal handler.

See sem_wait(3).

sem_getvalue(3), sem_wait(3), sem_overview(7), signal-safety(7)

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2017-03-13 Linux