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NAME

     shm_open, shm_unlink - shared memory object operations

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <sys/types.h>
     #include <sys/mman.h>

     int
     shm_open(const char *path, int flags, mode_t mode);

     int
     shm_unlink(const char *path);

DESCRIPTION

     The shm_open() system call opens (or optionally creates) a POSIX shared
     memory object named path.  The flags argument contains a subset of the
     flags used by open(2).  An access mode of either O_RDONLY or O_RDWR must
     be included in flags.  The optional flags O_CREAT, O_EXCL, and O_TRUNC
     may also be specified.

     If O_CREAT is specified, then a new shared memory object named path will
     be created if it does not exist.  In this case, the shared memory object
     is created with mode mode subject to the process’ umask value.  If both
     the O_CREAT and O_EXCL flags are specified and a shared memory object
     named path already exists, then shm_open() will fail with EEXIST.

     Newly created objects start off with a size of zero.  If an existing
     shared memory object is opened with O_RDWR and the O_TRUNC flag is
     specified, then the shared memory object will be truncated to a size of
     zero.  The size of the object can be adjusted via ftruncate(2) and
     queried via fstat(2).

     The new descriptor is set to close during execve(2) system calls; see
     close(2) and fcntl(2).

     As a FreeBSD extension, the constant SHM_ANON may be used for the path
     argument to shm_open().  In this case, an anonymous, unnamed shared
     memory object is created.  Since the object has no name, it cannot be
     removed via a subsequent call to shm_unlink().  Instead, the shared
     memory object will be garbage collected when the last reference to the
     shared memory object is removed.  The shared memory object may be shared
     with other processes by sharing the file descriptor via fork(2) or
     sendmsg(2).  Attempting to open an anonymous shared memory object with
     O_RDONLY will fail with EINVAL.  All other flags are ignored.

     The shm_unlink() system call removes a shared memory object named path.

RETURN VALUES

     If successful, shm_open() returns a non-negative integer, and
     shm_unlink() returns zero.  Both functions return -1 on failure, and set
     errno to indicate the error.

COMPATIBILITY

     The path argument does not necessarily represent a pathname (although it
     does in most other implementations).  Two processes opening the same path
     are guaranteed to access the same shared memory object if and only if
     path begins with a slash (‘/’) character.

     Only the O_RDONLY, O_RDWR, O_CREAT, O_EXCL, and O_TRUNC flags may be used
     in portable programs.

     The result of using open(2), read(2), or write(2) on a shared memory
     object, or on the descriptor returned by shm_open(), is undefined.  It is
     also undefined whether the shared memory object itself, or its contents,
     persist across reboots.

     In FreeBSD, read(2) and write(2) on a shared memory object will fail with
     EOPNOTSUPP and neither shared memory objects nor their contents persist
     across reboots.

ERRORS

     The following errors are defined for shm_open():

     [EINVAL]           A flag other than O_RDONLY, O_RDWR, O_CREAT, O_EXCL,
                        or O_TRUNC was included in flags.

     [EMFILE]           The process has already reached its limit for open
                        file descriptors.

     [ENFILE]           The system file table is full.

     [EINVAL]           O_RDONLY was specified while creating an anonymous
                        shared memory object via SHM_ANON.

     [EFAULT]           The path argument points outside the process’
                        allocated address space.

     [ENAMETOOLONG]     The entire pathname exceeded 1023 characters.

     [EINVAL]           The path does not begin with a slash (‘/’) character.

     [ENOENT]           O_CREAT is specified and the named shared memory
                        object does not exist.

     [EEXIST]           O_CREAT and O_EXCL are specified and the named shared
                        memory object dies exist.

     [EACCES]           The required permissions (for reading or reading and
                        writing) are denied.

     The following errors are defined for shm_unlink():

     [EFAULT]           The path argument points outside the process’
                        allocated address space.

     [ENAMETOOLONG]     The entire pathname exceeded 1023 characters.

     [ENOENT]           The named shared memory object does not exist.

     [EACCES]           The required permissions are denied.  shm_unlink()
                        requires write permission to the shared memory object.

SEE ALSO

     close(2), ftruncate(2), fstat(2), mmap(2), munmap(2)

STANDARDS

     The shm_open() and shm_unlink() functions are believed to conform to IEEE
     Std 1003.1b-1993 (“POSIX.1”).

HISTORY

     The shm_open() and shm_unlink() functions first appeared in FreeBSD 4.3.
     The functions were reimplemented as system calls using shared memory
     objects directly rather than files in FreeBSD 7.0.

AUTHORS

     Garrett A. Wollman 〈wollman@FreeBSD.org〉 (C library support and this
     manual page)

     Matthew Dillon 〈dillon@FreeBSD.org〉 (MAP_NOSYNC)