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 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 char-
       acters, 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 corresponding effective IDs of the calling process, and the object's permission bits are set  accord-
                  ing  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 guaranteed 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 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.

       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 trun-
       cate 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 3.25 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)