SHM_OPEN(3)		   Linux Programmer's Manual		   SHM_OPEN(3)



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

SYNOPSIS
       #include <sys/mman.h>
       #include <sys/stat.h>	    /* For mode constants */
       #include <fcntl.h>	    /* 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 porta-
       ble use, a shared memory object should be identified by a name  of  the
       form  /somename;	 that  is,  a null-terminated string of up to NAME_MAX
       (i.e., 255) characters consisting of an initial slash, followed by  one
       or more characters, none of which are 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	be  mmap(2)ed  only  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
		  object's  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 <sys/stat.h>.)

		  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
       <fcntl.h>.

       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 nonnegative file descriptor.  On fail-
       ure,  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 per-process limit on the number of open file descriptors has
	      been reached.

       ENAMETOOLONG
	      The length of name exceeds PATH_MAX.

       ENFILE The system-wide limit on the total number of open files 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.

ATTRIBUTES
       For  an	explanation  of	 the  terms  used   in	 this	section,   see
       attributes(7).

       +-------------------------+---------------+----------------+
       |Interface		 | Attribute	 | Value	  |
       +-------------------------+---------------+----------------+
       |shm_open(), shm_unlink() | Thread safety | MT-Safe locale |
       +-------------------------+---------------+----------------+

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008.

       POSIX.1-2001  says  that	 the group ownership of a newly created shared
       memory object is set to either the calling process's effective group ID
       or  "a system default group ID".	 POSIX.1-2008 says that the group own-
       ership may be set to either the calling process's  effective  group  ID
       or,  if	the  object  is visible in the filesystem, the group ID of the
       parent directory.

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 filesystem, which is normally mounted under /dev/shm.

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

COLOPHON
       This page is part of release 4.10 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2015-12-28			   SHM_OPEN(3)