SHM_OPEN(3)



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/types.h>
       #include <sys/mman.h>
       #include <fcntl.h>           /* For O_* constants */

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

       int shm_unlink(const char *name);

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
                  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).

                  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
                  initialised 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.

       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.

NOTES
       These functions are provided in glibc 2.2 and  later.   Programs  using
       these  functions  must  specify  the  -lrt  flag to cc in order to link
       against the required ("realtime") library.

       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 Unices.

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

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"

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

Linux 2.6.9                       2004-12-17                       SHM_OPEN(3)

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