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Linux 2.6 - man page for sem_open (linux section 3posix)

SEM_OPEN(P)			    POSIX Programmer's Manual			      SEM_OPEN(P)

       sem_open - initialize and open a named semaphore (REALTIME)

       #include <semaphore.h>

       sem_t *sem_open(const char *name, int oflag, ...);

       The  sem_open()	function  shall  establish  a  connection between a named semaphore and a
       process. Following a call to sem_open() with semaphore name name, the process  may  refer-
       ence  the  semaphore  associated  with name using the address returned from the call. This
       semaphore may be used in subsequent calls to sem_wait(),  sem_trywait(),  sem_post(),  and
       sem_close(). The semaphore remains usable by this process until the semaphore is closed by
       a successful call to sem_close(), _exit(), or one of the exec functions.

       The oflag argument controls whether the semaphore is created or	merely	accessed  by  the
       call to sem_open(). The following flag bits may be set in oflag:

	      This  flag  is used to create a semaphore if it does not already exist.  If O_CREAT
	      is set and the semaphore already exists, then O_CREAT  has  no  effect,  except  as
	      noted  under  O_EXCL.  Otherwise, sem_open() creates a named semaphore. The O_CREAT
	      flag requires a third and a fourth argument: mode, which is  of  type  mode_t,  and
	      value, which is of type unsigned. The semaphore is created with an initial value of
	      value.  Valid  initial  values  for  semaphores  are  less   than   or   equal   to

       The  user ID of the semaphore is set to the effective user ID of the process; the group ID
       of the semaphore is set to a system default group ID or to the effective group ID  of  the
       process.  The  permission  bits of the semaphore are set to the value of the mode argument
       except those set in the file mode creation mask of the process. When bits  in  mode  other
       than the file permission bits are specified, the effect is unspecified.

       After the semaphore named name has been created by sem_open() with the O_CREAT flag, other
       processes can connect to the semaphore by calling sem_open() with the same value of name.

       O_EXCL If O_EXCL and O_CREAT are set, sem_open() fails if the semaphore name  exists.  The
	      check  for  the  existence of the semaphore and the creation of the semaphore if it
	      does not exist are atomic with respect to other processes executing sem_open() with
	      O_EXCL  and  O_CREAT  set.  If  O_EXCL is set and O_CREAT is not set, the effect is

       If flags other than O_CREAT and O_EXCL are specified in the oflag parameter, the effect is

       The name argument points to a string naming a semaphore object.	It is unspecified whether
       the name appears in the file system and is visible to functions	that  take  pathnames  as
       arguments.   The  name argument conforms to the construction rules for a pathname. If name
       begins with the slash character, then processes calling sem_open() with the same value  of
       name  shall refer to the same semaphore object, as long as that name has not been removed.
       If name does not begin with the slash character, the effect is implementation-defined. The
       interpretation  of  slash  characters  other  than  the leading slash character in name is

       If a process makes multiple successful calls to sem_open() with the same value  for  name,
       the  same semaphore address shall be returned for each such successful call, provided that
       there have been no calls to sem_unlink() for this semaphore.

       References to copies of the semaphore produce undefined results.

       Upon successful completion, the sem_open() function shall return the address of the  sema-
       phore.  Otherwise,  it  shall  return  a value of SEM_FAILED and set errno to indicate the
       error. The symbol SEM_FAILED is defined in the <semaphore.h> header. No successful  return
       from sem_open() shall return the value SEM_FAILED.

       If  any of the following conditions occur, the sem_open() function shall return SEM_FAILED
       and set errno to the corresponding value:

       EACCES The named semaphore exists and the permissions specified by oflag  are  denied,  or
	      the  named semaphore does not exist and permission to create the named semaphore is

       EEXIST O_CREAT and O_EXCL are set and the named semaphore already exists.

       EINTR  The sem_open() operation was interrupted by a signal.

       EINVAL The sem_open() operation is not supported for the given name, or O_CREAT was speci-
	      fied in oflag and value was greater than {SEM_VALUE_MAX}.

       EMFILE Too  many  semaphore  descriptors  or file descriptors are currently in use by this

	      The length of the name argument exceeds  {PATH_MAX}  or  a  pathname  component  is
	      longer than {NAME_MAX}.

       ENFILE Too many semaphores are currently open in the system.

       ENOENT O_CREAT is not set and the named semaphore does not exist.

       ENOSPC There is insufficient space for the creation of the new named semaphore.

       The following sections are informative.


       The  sem_open() function is part of the Semaphores option and need not be available on all

       Early drafts required an error return value of -1 with the type sem_t * for the sem_open()
       function,  which is not guaranteed to be portable across implementations. The revised text
       provides the symbolic error code SEM_FAILED to eliminate the type conflict.


       semctl() , semget() , semop() , sem_close() , sem_post() , sem_timedwait() , sem_trywait()
       ,  sem_unlink() , sem_wait() , the Base Definitions volume of IEEE Std 1003.1-2001, <sema-

       Portions of this text are reprinted and	reproduced  in	electronic  form  from	IEEE  Std
       1003.1,	2003  Edition,	Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003  by
       the  Institute  of  Electrical  and  Electronics Engineers, Inc and The Open Group. In the
       event of any discrepancy between this version and the original IEEE  and  The  Open  Group
       Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003				      SEM_OPEN(P)

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