msem_init(3)msem_init(3)NAMEmsem_init - Initialize a semaphore in a mapped file or shared memory
region
SYNOPSIS
#include <sys/mman.h>
msemaphore *msem_init(
msemaphore *sem ,
int initial_value );
LIBRARY
Standard C Library (libc)
PARAMETERS
Points to an msemaphore structure in which the state of the semaphore
is stored. Determines whether the semaphore is locked or unlocked at
allocation.
DESCRIPTION
The msem_init() function allocates a new binary semaphore and initial‐
izes the state of the new semaphore.
If the initial_value parameter is MSEM_LOCKED, the new semaphore is
initialized in the locked state. If the initial_value parameter is
MSEM_UNLOCKED, the new semaphore is initialized in the unlocked state.
The msemaphore structure is located within a mapped file or shared mem‐
ory region created by a successful call to the mmap() function and hav‐
ing both read and write access.
If a semaphore is created in a mapped file region, any reference by a
process which has mapped the same file, using a (struct msemaphore *)
pointer which resolves to the same file offset, is taken as a reference
to the same semaphore. If a semaphore is created in an anonymous shared
memory region, any reference by a process which shares the same region,
using a (struct msemaphore *) pointer which resolves to the same offset
from the start of the region, is taken as a reference to the same sema‐
phore.
Any previous semaphore state stored in the msemaphore structure is
ignored and overwritten.
NOTES
Trial use
RETURN VALUES
Upon successful completion, the msem_init() function returns a pointer
to the initialized msemaphore structure. On error, the msem_init()
function returns null and sets errno to indicate the error.
ERRORS
If the msem_init() function fails, errno may be set to one of the fol‐
lowing values: The initial_value parameter is not valid. A new sema‐
phore could not be created.
SEE ALSO
Functions: mmap(2), msem_lock(3), msem_remove(3), msem_unlock(3)msem_init(3)
