Index for Section 3

Alphabetical listing for S

Bottom of page

sem_open(3)
NAME
  sem_open - Opens/creates a named semaphore for use by a process (P1003.1b)

LIBRARY
  Realtime Library (librt.so, librt.a)

SYNOPSIS
  #include <semaphore.h>

  sem_t *sem_open (
  const char *name,
  int oflag,
  mode_t mode,
  unsigned int value);

PARAMETERS
  name	  Points to a string naming the semaphore object.

  oflag	  Specifies whether the semaphore is to be created (O_CREAT flag bit
	  set) or only opened (O_CREAT flag bit clear).	 If O_CREAT is set,
	  the O_EXCL flag bit may additionally be set to specify that the
	  call should fail if a semaphore of the same name already exists.
	  The O_CREAT and O_EXCL flags are defined in the <fcntl.h> header
	  file.

  mode	  Specifies the semaphore's permission bits.  This argument is used
	  only when the semaphore is being created.

  value	  Specifies the initial value to be given to the semaphore.  This
	  argument is used only when the semaphore is being created.

DESCRIPTION
  Use the sem_open function to establish the connection between a named
  semaphore and a process.  Subsequently, the calling process can reference
  the semaphore by using the address returned from the call.  The semaphore
  is available in subsequent calls to sem_wait, sem_trywait, sem_post, and
  sem_getvalue functions.  The semaphore remains usable by the process until
  the semaphore is closed by a successful call to the sem_close function.

  The O_CREAT flag bit in the oflag parameter controls whether the semaphore
  is created or only opened by the call to sem_open.

  A created semaphore's user ID is set to the user ID of the calling process
  and its group ID is set to a system default group or to the group ID of the
  process.  The semaphore's permission bits are set to the value of the mode
  argument, except for those set in the file mode creation mask of the
  process.

  After a semaphore is created, other processes can open the semaphore by
  calling sem_open with the same value for the name argument.

RETURN VALUES
  On successful completion, this function opens the semaphore and returns the
  semaphore's descriptor.  Otherwise, the function returns -1 and sets errno
  to indicate the error.

ERRORS
  The sem_open function fails under the following conditions:

  [EACCES]	      The named semaphore exists and the permissions
		      specified by oflag are denied, or the named semaphore
		      does not exist and the permissions specified by oflag
		      are 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 string exceeds {PATH_MAX}, or a
		      pathname component is longer than {NAME_MAX} while
		      {_POSIX_NO_TRUNC} is in effect.

  [ENFILE]	      Too many semaphores are currently open in the system.

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

  [ENOSPC]	      Insufficient space exists for the creation of a new
		      named semaphore.

RELATED INFORMATION
  Functions: sem_close(3), sem_post(3), sem_trywait(3), sem_unlink(3),
  sem_wait(3), mkfifo(3)

  Guide to Realtime Programming

Index for Section 3

Alphabetical listing for S

Top of page