Index for Section 3

Alphabetical listing for S

Bottom of page

shm_open(3)
NAME
  shm_open - Opens a shared memory object, creating the object if necessary
  (P1003.1b)

SYNOPSIS
  #include <sys/mman.h>

  int shm_open (
  const char *name,
  int ooption,
  mode_t mode);

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

PARAMETERS
  *name
      Points to a string naming the shared memory object.

  ooption
      Specifies options that define file status and file access modes. This
      argument is constructed from the bitwise inclusive OR of zero or more
      of the options defined in the <fcntl.h> header file.

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

DESCRIPTION
  The shm_open function establishes a connection between a shared memory
  object and a file descriptor. It creates an open file descriptor that
  refers to the shared memory object and a file descriptor that refers to
  that open file descriptor. This file descriptor is used by other functions
  to refer to the shared memory object. The name points to a string naming a
  shared memory object. The name can be a pathname, in which case other
  processes referring to the same pathname refer to the same shared memory
  object.

  When a shared memory object is created, its state and all data associated
  with it persist until the shared memory is unlinked.

  The shm_open function returns a file descriptor that is the lowest numbered
  file descriptor not currently open for that process.	File status options
  and access modes are set according to the ooption argument. These options
  are defined in the <fcntl.h> header file and can have zero or more of the
  following values:

    ·  O_RDONLY - Open for read access only.

    ·  O_RDWR - Open for read or write access.

    ·  O_CREAT - Create the shared memory if the memory object does not exist
       already.	 The user ID and group ID of the shared memory object are
       identical to those of the calling process.  The shared memory object's
       permission bits are set to the value of mode, except those set in the
       file mode creation mask of the process.

    ·  O_EXCL - Prevent the opening of a shared memory object if O_CREAT is
       set and the shared memory object already exists.	 Use this option only
       in combination with O_CREAT.

    ·  O_TRUNC - Truncate the shared memory object to zero length if it is
       successfully opened for read or write access (O_RDWR).

  The initial contents of the shared memory object are binary zeros.

RETURN VALUES
  On a successful call to shm_open, a nonnegative integer is returned that
  represents the lowest-numbered unused file descriptor.  The file descriptor
  points to the shared memory object. Otherwise, -1 is returned and errno is
  set to indicate the error.

ERRORS
  The shm_open function fails under the following conditions:

  [EACCES]
      Permission to create the shared memory object is denied. The shared
      memory object exists and the permissions specified by ooption are
      denied. O_TRUNC is specified and write permission is denied.

  [EEXIST]
      O_CREAT and O_EXCL are set, but the named shared memory object already
      exists.

  [EINTR]
      A signal has interrupted the shm_open operation.

  [EINVAL]
      The shm_open operation is not supported for the given name.

  [EMFILE]
      Too many file descriptors are currently in use by this process.

  [ENAMETOOLONG]
      The length of the path argument exceeds PATH_MAX, or a pathname
      component is longer than NAME_MAX while _POSIX_NO_TRUNC is in effect.

  [ENFILE]
      Too many shared memory objects are currently open in the system.

  [ENOENT]
      O_CREAT is not set and the named shared memory object does not exist.

  [ENOSPC]
      Memory space for creation of the new shared memory object is
      insufficient.

SEE ALSO
  Functions: close(2), dup(2), exec(2), fcntl(2), fstat(2), mmap(2),
  umask(2), shm_unlink(3)

  Guide to Realtime Programming

Index for Section 3

Alphabetical listing for S

Top of page