sem_init — initialise an unnamed semaphore
#include <semaphore.h>
int
sem_init( |
sem_t * | sem, |
int | pshared, | |
unsigned int | value) ; |
sem_init
() initialises 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.
Initialising a semaphore that has already been initialised results in undefined behaviour.
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
non-zero, but the system does not support
process-shared semaphores (see sem_overview(7)).
Bizarrely, POSIX.1-2001 does not specify the value that
should be returned by a successful call to sem_init
(). However, virtually every
implementation returns zero in this case.
sem_destroy(3), sem_post(3), sem_wait(3), sem_overview(7)
|