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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 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
                  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
       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  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

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       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.