NAME

     shm_open, shm_unlink — shared memory object operations

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

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

     POSIX specifications state that 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.  However,
     the FreeBSD kernel implementation explicitly includes support for read(2) and write(2).

     FreeBSD also supports zero-copy transmission of data from shared memory objects with
     sendfile(2).

     Neither shared memory objects nor their contents persist across reboots.

     Writes do not extend shared memory objects, so ftruncate(2) must be called before any data
     can be written.  See EXAMPLES.

EXAMPLES

     This example fails without the call to ftruncate(2):

             uint8_t buffer[getpagesize()];
             ssize_t len;
             int fd;

             fd = shm_open(SHM_ANON, O_RDWR | O_CREAT, 0600);
             if (fd < 0)
                     err(EX_OSERR, "%s: shm_open", __func__);
             if (ftruncate(fd, getpagesize()) < 0)
                     err(EX_IOERR, "%s: ftruncate", __func__);
             len = pwrite(fd, buffer, getpagesize(), 0);
             if (len < 0)
                     err(EX_IOERR, "%s: pwrite", __func__);
             if (len != getpagesize())
                     errx(EX_IOERR, "%s: pwrite length mismatch", __func__);

ERRORS

     shm_open() fails with these error codes for these conditions:

     [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 does
                        exist.

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

     shm_unlink() fails with these error codes for these conditions:

     [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), fstat(2), ftruncate(2), mmap(2), munmap(2), sendfile(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 8.0.

AUTHORS

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

     Matthew Dillon <dillon@FreeBSD.org> (MAP_NOSYNC)