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