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Linux 2.6 - man page for shm_open (linux section 3)

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

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

       #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.

       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 con-
       verse operation, 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, 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  own-
		  ership  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 cre-
		  ation mask (see umask(2)) are cleared for the new object.  A set of macro  con-
		  stants  which can be used to define mode is listed in open(2).  (Symbolic defi-
		  nitions 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 guaranteed to be the lowest-numbered file descrip-
       tor 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 ftruncate(2) (for a newly cre-
       ated object) and mmap(2).  After a call to mmap(2) the file descriptor may be closed with-
       out 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 con-
       tents  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).

       On  success,  shm_open()  returns  a  nonnegative file descriptor.  On failure, shm_open()
       returns -1.  shm_unlink() returns 0 on success, or -1 on error.

       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 permission 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.

	      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

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

       These functions are provided in glibc 2.2 and later.


       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  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.

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

       This  page  is  part of release 3.55 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at

Linux					    2009-02-25				      SHM_OPEN(3)

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