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PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of
       this interface may differ (consult the corresponding Linux  manual  page  for  details  of
       Linux behavior), or the interface may not be implemented on Linux.

NAME

       sem_open — initialize and open a named semaphore

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_timedwait(),
       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 shall be set  to  the  effective  user  ID  of  the
                 process. The group ID of the semaphore shall be set to the effective group ID of
                 the process; however, if the name argument is visible in the  file  system,  the
                 group  ID may be set to the group ID of the containing directory. 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, except
       that the interpretation of <slash> characters other than the leading <slash> character  in
       name  is  implementation-defined,  and  that  the  length limits for the name argument are
       implementation-defined and need not be the same as  the  pathname  limits  {PATH_MAX}  and
       {NAME_MAX}.   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.

       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, and  at  least  one  previous
       successful  sem_open()  call  for  this  semaphore has not been matched with a sem_close()
       call.

       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.

       ENFILE Too many semaphores are currently open in the system.

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

       ENOMEM There is insufficient memory for the creation of the new named semaphore.

       ENOSPC There is insufficient space on a storage device for the creation of the  new  named
              semaphore.

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

       ENAMETOOLONG
              The length of the name argument exceeds {_POSIX_PATH_MAX} on systems  that  do  not
              support  the  XSI  option  or  exceeds  {_XOPEN_PATH_MAX}  on XSI systems, or has a
              pathname component that is longer than {_POSIX_NAME_MAX} on  systems  that  do  not
              support the XSI option or longer than {_XOPEN_NAME_MAX} on XSI systems.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       None.

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

       A future version might require the sem_open() and sem_unlink() functions to have semantics
       similar to normal file system operations.

SEE ALSO

       semctl(), semget(), semop(), sem_close(), sem_post(), sem_timedwait(), sem_trywait(),
       sem_unlink()

       The Base Definitions volume of POSIX.1‐2017, <semaphore.h>

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1-2017, Standard for Information Technology -- Portable  Operating  System  Interface
       (POSIX),  The  Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       Any  typographical  or  formatting errors that appear in this page are most likely to have
       been introduced during the conversion of the source files to man page  format.  To  report
       such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .