SHM_OPEN(3)               Linux Programmer's Manual              SHM_OPEN(3)

NAME         top

       shm_open,  shm_unlink  -  create/open  or  unlink POSIX shared memory

SYNOPSIS         top

       #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         top

       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

       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 be mmap(2)ed only 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

       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 fail (unless O_CREAT was
       specified, in which case a new, distinct object is created).

RETURN VALUE         top

       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         top

       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

       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 per-process limit on the number of open file descriptors
              has been reached.

              The length of name exceeds PATH_MAX.

       ENFILE The system-wide limit on the total number of open files 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

VERSIONS         top

       These functions are provided in glibc 2.2 and later.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface                Attribute     Value          │
       │shm_open(), shm_unlink() │ Thread safety │ MT-Safe locale │

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008.

       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".  POSIX.1-2008 says that the group
       ownership may be set to either the calling process's effective group
       ID or, if the object is visible in the filesystem, the group ID of
       the parent directory.

NOTES         top

       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 makes use of a
       dedicated tmpfs(5) filesystem that is normally mounted under

SEE ALSO         top

       close(2), fchmod(2), fchown(2), fcntl(2), fstat(2), ftruncate(2),
       memfd_create(2), mmap(2), open(2), umask(2), shm_overview(7)

COLOPHON         top

       This page is part of release 5.05 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2017-09-15                      SHM_OPEN(3)

Pages that refer to this page: execve(2)memfd_create(2)mmap2(2)mmap(2)umask(2)sem_init(3)tmpfs(5)file-hierarchy(7)sem_overview(7)shm_overview(7)