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
       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 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 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 correspond-
		  ing 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 ftruncate(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 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 permis-
	      sion 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 process's  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 3.44 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								    2009-02-25							       SHM_OPEN(3)