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.

       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 does 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)

Debian                                           March 20, 2007                                      SHM_OPEN(2)