NAME

       sem_open - initialize and open a named semaphore (REALTIME)

SYNOPSIS

       #include <semaphore.h>

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

DESCRIPTION

       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
       reference  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:

       O_CREAT
              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
              {SEM_VALUE_MAX}.

       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
              undefined.

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

       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
       implementation-defined.

       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.

RETURN VALUE

       Upon successful completion, the sem_open()  function  shall  return  the  address  of  the
       semaphore.  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.

ERRORS

       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
              denied.

       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
              specified in oflag and value was greater than {SEM_VALUE_MAX}.

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

       ENAMETOOLONG
              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.

EXAMPLES

       None.

APPLICATION USAGE

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

RATIONALE

       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.

FUTURE DIRECTIONS

       None.

SEE ALSO

       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,
       <semaphore.h>

COPYRIGHT

       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
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .