Name
sem_init — initialize an unnamed semaphore
Synopsis
#include <semaphore.h>
int
sem_init( |
sem_t *sem, |
| int pshared, | |
unsigned int value); |
![[Note]](../stylesheet/note.png) |
Note |
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Link with |
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
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), 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
-
valueexceedsSEM_VALUE_MAX. - ENOSYS
-
psharedis nonzero, but the system does not support process-shared semaphores (see sem_overview(7)).
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.