SEM_POST(3) | Library Functions Manual | SEM_POST(3)

# NAME

`sem_post` — increment (unlock) a semaphore

# SYNOPSIS

POSIX Threads Library (libpthread, -lpthread)
`#include <semaphore.h>`

int
`sem_post`(sem_t *sem);

# DESCRIPTION

The `sem_post`() function increments (unlocks) the semaphore pointed to by sem. If there are threads blocked on the semaphore when `sem_post`() is called, then the highest priority thread that has been blocked the longest on the semaphore will be allowed to return from sem_wait(3).

`sem_post`() is signal-reentrant and may be called within signal handlers.

# RETURN VALUES

The `sem_post`() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

# ERRORS

`sem_post`() will fail if:

[`EINVAL`]
  sem points to an invalid semaphore.

# SEE ALSO

sem_destroy(3), sem_getvalue(3), sem_init(3), sem_open(3), sem_wait(3)

# STANDARDS

`sem_post`() conforms to ISO/IEC 9945-1:1996 (“POSIX.1”).

June 7, 2025 | Debian