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

DESCRIPTION

     The shared memory object referenced by name is opened for reading and/or
     writing as specified by the argument flags and the file descriptor
     returned to the calling process.  The returned file descriptor will be
     the lowest non-open file descriptor for the calling process, and is not
     shared with any other processes, as it is a new file descriptor. The new
     file descriptor will have the FD_CLOEXEC flag set.  Repeated calls to
     shm_open with the same string value for name() will return a file
     descriptor referring to the same shared memory object, provided that the
     object has not been unlinked by a call to shm_unlink().  The flags argu-
     ment may indicate the file is to be created if it does not exist (by
     specifying the O_CREAT flag), in which case the file is created with mode
     mode as described in chmod(2) and modified by the process' umask value
     (see umask(2)).

     The flags specified are formed by or'ing the following values:

           O_RDONLY        open for reading only
           O_RDWR          open for reading and writing
           O_CREAT         create object if it does not exist
           O_EXCL          error if create and object exists
           O_TRUNC         truncate size to 0

     Exactly one of O_RDONLY or O_RDWR must be specified.

     If O_TRUNC is specified and the file exists, the file is truncated to
     zero length.  If O_EXCL is set with O_CREAT and the file already exists,
     shm_open() returns an error.  This may be used to implement a simple
     exclusive access locking mechanism.

     If successful, shm_open() returns a non-negative integer, termed a file
     descriptor.  It returns -1 and sets errno on failure.  The file pointer
     used to mark the current position within the memory object is set to the
     beginning of the object.

     When a new shared memory object is created it is given the owner and
     group corresponding to the effective user and group of the calling
     process. There is no visible entry in the file system for the created
     object in this implementation.

     When a shared memory object is created, it persists until it it unlinked
     and all other references are gone. Objects do not persist across a system
     reboot.

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

     The system imposes a limit on the number of file descriptors open simul-

     [EINTR]            The shm_open() operation was interrupted by a signal.

     [EINVAL]           The shm_open() operation is not supported.

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

     [ENAMETOOLONG]     name exceeded SHM_NAME_MAX characters.

     [ENFILE]           The system file table is full.

     [ENOENT]           O_CREAT is not set and the named object does not
                        exist.

     [ENOSPC]           O_CREAT is specified, the file does not exist, and
                        there is insufficient space available to create the
                        object.

SEE ALSO

     chmod(2), close(2), getdtablesize(2), mmap(2), shm_unlink(2), umask(2)

HISTORY

     shm_open() is specified in the POSIX Realtime Extension
     (1003.1b-1993/1003.1i-1995).

Darwin                        September 20, 1999                        Darwin