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       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.

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 .