sem_init man page

sem_init — initialize an unnamed semaphore


#include <semaphore.h>

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

Link with -pthread.


sem_init() initializes the unnamed semaphore at the address pointed to by sem. The value argument specifies the initial value for the semaphore.

The pshared argument indicates whether this semaphore is to be shared between the threads of a process, or between processes.

If pshared has the value 0, then the semaphore is shared between the threads of a process, and should be located at some address that is visible to all threads (e.g., a global variable, or a variable allocated dynamically on the heap).

If pshared is nonzero, then the semaphore is shared between processes, and should be located in a region of shared memory (see shm_open(3), mmap(2), and shmget(2)). (Since a child created by fork(2) inherits its parent's memory mappings, it can also access the semaphore.) Any process that can access the shared memory region can operate on the semaphore using sem_post(3), sem_wait(3), and so on.

Initializing a semaphore that has already been initialized results in undefined behavior.

Return Value

sem_init() returns 0 on success; on error, -1 is returned, and errno is set to indicate the error.


value exceeds SEM_VALUE_MAX.
pshared is nonzero, but the system does not support process-shared semaphores (see sem_overview(7)).


For an explanation of the terms used in this section, see attributes(7).

sem_init()Thread safetyMT-Safe

Conforming to



Bizarrely, POSIX.1-2001 does not specify the value that should be returned by a successful call to sem_init(). POSIX.1-2008 rectifies this, specifying the zero return on success.

See Also

sem_destroy(3), sem_post(3), sem_wait(3), sem_overview(7)

Referenced By

sem_destroy(3), sem_overview(7).

Explore man page connections for sem_init(3).