SHM_OPEN(3)		   Linux Programmers Manual		  SHM_OPEN(3)



NAME
       shm_open,  shm_unlink  -  Create/open  or  unlink  POSIX  shared memory
       objects

SYNOPSIS
       #include 
       #include 	    /* For mode constants */
       #include 	    /* For O_* constants */

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

       int shm_unlink(const char *name);

       Link with -lrt.

DESCRIPTION
       shm_open() creates and opens a new, or opens an existing, POSIX	shared
       memory  object.	 A  POSIX  shared  memory object is in effect a handle
       which can be used by unrelated processes to mmap(2) the same region  of
       shared  memory.	The shm_unlink() function performs the converse opera
       tion, removing an object previously created by shm_open().

       The operation of shm_open() is analogous  to  that  of  open(2).   name
       specifies  the  shared  memory  object  to  be  created or opened.  For
       portable use, name should have an initial  slash  (/)  and  contain  no
       embedded slashes.

       oflag  is  a bit mask created by ORing together exactly one of O_RDONLY
       or O_RDWR and any of the other flags listed here:

       O_RDONLY   Open the object for read access.   A	shared	memory	object
		  opened   in	this  way  can	only  be  mmap(2)ed  for  read
		  (PROT_READ) access.

       O_RDWR	  Open the object for read-write access.

       O_CREAT	  Create the shared memory object if it does not  exist.   The
		  user	and  group  ownership of the object are taken from the
		  corresponding effective IDs of the calling process, and  the
		  objects  permission bits are set according to the low-order
		  9 bits of mode, except that those bits set  in  the  process
		  file	mode  creation mask (see umask(2)) are cleared for the
		  new object.  A set of macro constants which can be  used  to
		  define  mode is listed in open(2).  (Symbolic definitions of
		  these constants can be obtained by including	.)

		  A  new  shared memory object initially has zero length  the
		  size of the object can be set using ftruncate(2).  The newly
		  allocated  bytes of a shared memory object are automatically
		  initialized to 0.

       O_EXCL	  If O_CREAT was also specified, and a	shared	memory	object
		  with	the  given  name already exists, return an error.  The
		  check for the existence of the object, and its  creation  if
		  it does not exist, are performed atomically.

       O_TRUNC	  If  the  shared memory object already exists, truncate it to
		  zero bytes.

       Definitions  of	these  flag  values  can  be  obtained	by   including
       .

       On  successful  completion  shm_open()  returns	a  new file descriptor
       referring to the shared memory object.  This file descriptor is guaran
       teed  to  be  the lowest-numbered file descriptor not previously opened
       within the process.  The FD_CLOEXEC flag (see fcntl(2)) is set for  the
       file descriptor.

       The  file  descriptor  is  normally  used in subsequent calls to ftrun
       cate(2) (for a newly created object) and  mmap(2).   After  a  call  to
       mmap(2)	the file descriptor may be closed without affecting the memory
       mapping.

       The operation of shm_unlink() is analogous to unlink(2): it  removes  a
       shared  memory  object  name, and, once all processes have unmapped the
       object, de-allocates and destroys the contents of the associated memory
       region.	 After	a  successful  shm_unlink(), attempts to shm_open() an
       object with the same name will fail (unless O_CREAT was	specified,  in
       which case a new, distinct object is created).

RETURN VALUE
       On  success,  shm_open()  returns  a  non-negative file descriptor.  On
       failure, shm_open() returns -1.	shm_unlink() returns 0 on success,  or
       -1 on error.

ERRORS
       On  failure,  errno  is set to indicate the cause of the error.	Values
       which may appear in errno include the following:

       EACCES Permission to shm_unlink() the shared memory object was  denied.

       EACCES Permission  was denied to shm_open() name in the specified mode,
	      or O_TRUNC was specified and the caller does not have write per
	      mission on the object.

       EEXIST Both  O_CREAT  and  O_EXCL  were specified to shm_open() and the
	      shared memory object specified by name already exists.

       EINVAL The name argument to shm_open() was invalid.

       EMFILE The process already has the maximum number of files open.

       ENAMETOOLONG
	      The length of name exceeds PATH_MAX.

       ENFILE The limit on the total number of files open on  the  system  has
	      been reached.

       ENOENT An attempt was made to shm_open() a name that did not exist, and
	      O_CREAT was not specified.

       ENOENT An attempt was to made to shm_unlink()  a  name  that  does  not
	      exist.

VERSIONS
       These functions are provided in glibc 2.2 and later.

CONFORMING TO
       POSIX.1-2001.

       POSIX.1-2001  says  that  the group ownership of a newly created shared
       memory object is set to either the calling processs effective group ID
       or "a system default group ID".

NOTES
       POSIX  leaves  the  behavior of the combination of O_RDONLY and O_TRUNC
       unspecified.  On Linux, this will  successfully	truncate  an  existing
       shared memory object  this may not be so on other Unix systems.

       The POSIX shared memory object implementation on Linux 2.4 makes use of
       a dedicated file system, which is normally mounted under /dev/shm.

SEE ALSO
       close(2),  fchmod(2),  fchown(2),  fcntl(2),  fstat(2),	 ftruncate(2),
       mmap(2), open(2), umask(2), shm_overview(7)

COLOPHON
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux				  2008-04-25			   SHM_OPEN(3)