SEM_WAIT(3) | Library Functions Manual | SEM_WAIT(3)

# NAME

`sem_wait`, `sem_timedwait`, `sem_trywait` — decrement (lock) a semaphore

# SYNOPSIS

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

int
`sem_wait`(sem_t *sem);

int
`sem_timedwait`(sem_t *sem, const struct timespec *abstime);

int
`sem_trywait`(sem_t *sem);

# DESCRIPTION

The `sem_wait`() function decrements (locks) the semaphore pointed to by sem, but blocks if the value of sem is zero, until the value is non-zero and the value can be decremented.

The `sem_timedwait`() function decrements (locks) the semaphore pointed to by sem, but blocks if the value of sem is zero, until either the value is non-zero and can be decremented or the system time specified by abstime is reached.

The `sem_trywait`() function decrements (locks) the semaphore pointed to by sem only if the value is non-zero. Otherwise, the semaphore is not decremented and an error is returned.

# RETURN VALUES

The `sem_wait`(), `sem_timedwait`(), and `sem_trywait`() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

# ERRORS

`sem_wait`(), `sem_timedwait`(), and `sem_trywait`() will fail if:

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

Additionally, `sem_wait`() and `sem_timedwait`() will fail if:

[`EINTR`]
  The call was interrupted by a signal.

Additionally, `sem_timedwait`() will fail if:

[`EINVAL`]
  abstime was `NULL` or specified a nanosecond value less than zero or greater than 1000 million.
[`ETIMEDOUT`]
  The semaphore value was zero and could not be decremented before abstime was reached.

Additionally, `sem_trywait`() will fail if:

[`EAGAIN`]
  The semaphore value was zero, and thus could not be decremented.

# SEE ALSO

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

# STANDARDS

`sem_wait`(), `sem_timedwait`(), and `sem_trywait`() conform to IEEE Std 1003.1-2008 (“POSIX.1”).

June 7, 2025 | Debian