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.
sem_init() returns 0 on success; on error, -1 is returned, and
errno is set to indicate the error.
- EINVAL
- value exceeds SEM_VALUE_MAX.
- ENOSYS
- 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).
| Interface |
Attribute |
Value |
| sem_init () |
Thread safety |
MT-Safe |
POSIX.1-2001.
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
shm_open(3) and
sem_wait(3).
sem_destroy(3),
sem_post(3),
sem_wait(3),
sem_overview(7)