msem_init(2)							System Calls Manual						      msem_init(2)

NAME

       msem_init - initialize a semaphore in a mapped file or anonymous memory region

SYNOPSIS

DESCRIPTION

       allocates a new binary semaphore and initializes the state of the new semaphore.

       sem points to an structure in which the state of the semaphore is to be stored.

       If  initial_value  is  the  new	semaphore is initialized in the locked state.  If initial_value is the new semaphore is initialized in the
       unlocked state.

       The structure must be located within a mapped file or anonymous memory region created by a successful call to and have both read and  write
       access.

       If  a semaphore is created in a mapped file region, any reference by a process that has mapped the same file, using a pointer that resolves
       to the same file offset is interpreted as a reference to the same semaphore.  If a semaphore is created in an anonymous memory region,  any
       reference  by a process sharing the same region by use of a pointer that resolves to the same offset from the start of the region is inter-
       preted as a reference to the same semaphore.

       Any previous semaphore state stored in the structure will be ignored and overwritten.

   Implementation Notes
       In order to ensure that an structure is entirely contained in a single memory page, sem must be at an address that is an exact multiple	of
       The size of the structure is guaranteed to prevent semaphores that cross page boundaries given the above restriction.

       For a memory mapped file region, the system deallocates memory that corresponds to a range of the file that has been truncated with or If a
       semaphore is located in memory so deallocated, the effect is equivalent to an on the semaphore.

RETURN VALUE

       returns the address of the initialized structure; otherwise, it returns NULL and sets to indicate the error.

ERRORS

       fails if any of the following conditions are encountered:

	      sem	     points to an structure that is not located in a mapped region created by and with read  and  write  access,  or  ini-
			     tial_value is not valid.

	      A new semaphore could not be created.

	      sem	     is an invalid pointer.

AUTHOR

       was developed by HP and OSF.

SEE ALSO

       mmap(2), msem_lock(2), msem_remove(2), msem_unlock(2), mman(5).

STANDARDS CONFORMANCE

																      msem_init(2)