SEM_INIT(3) | Library Functions Manual | SEM_INIT(3)

# NAME

`sem_init` — initialize an unnamed semaphore

# SYNOPSIS

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

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

# DESCRIPTION

The `sem_init`() function initializes the unnamed semaphore pointed to by sem to have the value value. A non-zero value for pshared specifies a shared semaphore that can be used by multiple processes, which this implementation is not capable of.

Following a successful call to `sem_init`(), sem can be used as an argument in subsequent calls to sem_wait(3), sem_trywait(3), sem_post(3), and sem_destroy(3). sem is no longer valid after a successful call to sem_destroy(3).

# RETURN VALUES

The `sem_init`() 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

It is an error to call sem_destroy(3) on a named semaphore created by sem_open(3).

`sem_init`() will fail if:

[`EINVAL`]
  value exceeds SEM_VALUE_MAX.
[`ENOSPC`]
  Memory allocation error.
[`EPERM`]
  Unable to initialize a shared semaphore.

# SEE ALSO

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

# STANDARDS

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

This implementation does not support shared semaphores, and reports this fact by setting errno to `EPERM`. This is perhaps a stretch of the intention of POSIX, but is compliant, with the caveat that `sem_init`() always reports a permissions error when an attempt to create a shared semaphore is made.

June 7, 2025 | Debian