Initializes a semaphore in a mapped file or shared memory region.
Standard C Library (libc.a)
#include <sys/mman.h>msemaphore *msem_init ( Sem, InitialValue)
msemaphore *Sem;
int InitialValue;The msem_init subroutine allocates a new binary semaphore and initializes the state of the new semaphore.
If the value of the InitialValue parameter is MSEM_LOCKED, the new semaphore is initialized in the locked state. If the value of the InitialValue parameter is MSEM_UNLOCKED, the new semaphore is initialized in the unlocked state.
The msemaphore structure is located within a mapped file or shared memory region created by a successful call to the mmap subroutine and having both read and write access.
Whether a semaphore is created in a mapped file or in an anonymous shared memory region, any reference by a process that has mapped the same file or shared region, using an msemaphore structure pointer that resolved to the same file or start of region offset, is taken as a reference to the same semaphore.
Any previous semaphore state stored in the msemaphore structure is ignored and overwritten.
| Item | Description |
|---|---|
| Sem | Points to an msemaphore structure in which the state of the semaphore is stored. |
| Initial Value | Determines whether the semaphore is locked or unlocked at allocation. |
When successful, the msem_init subroutine returns a pointer to the initialized msemaphore structure. Otherwise, it returns a null value and sets the errno global variable to indicate the error.
If the msem_init subroutine is unsuccessful, the errno global variable is set to one of the following values:
| Item | Description |
|---|---|
| EINVAL | Indicates the InitialValue parameter is not valid. |
| ENOMEM | Indicates a new semaphore could not be created. |