#include <semaphore.h>int sem_init(sem_t *sem, int pshared, unsigned int value);
Link with -lrt or -pthread.
DESCRIPTION
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 non-zero, 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),
etc.
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.
ERRORS
EINVAL
value
exceeds
SEM_VALUE_MAX.
ENOSYS
pshared
is non-zero,
but the system does not support process-shared semaphores (see
sem_overview(7)).
CONFORMING TO
POSIX.1-2001.
NOTES
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.
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
