SHMOPEN(2)                 BSD System Calls Manual                SHMOPEN(2)

NAME
     shmopen -- open a shared memory object

SYNOPSIS
     ##include <>

     int
     shmopen(const char *name, int flags, modet 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 FDCLOEXEC flag set.  Repeated calls to
     shmopen 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 shmunlink().  The flags argu-
     ment may indicate the file is to be created if it does not exist (by
     specifying the OCREAT 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:

           ORDONLY        open for reading only
           ORDWR          open for reading and writing
           OCREAT         create object if it does not exist
           OEXCL          error if create and object exists
           OTRUNC         truncate size to 0

     Exactly one of ORDONLY or ORDWR must be specified.

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

     If successful, shmopen() 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-
     taneously by one process.  Getdtablesize(2) returns the current system
     limit.

ERORS
     The named object is opened unless:

     [EACES]           The required permissions (for reading and/or writing)
                        are denied for the given flags.

     [EACES]           OCREAT is specified, the object does not exist, and
                        permission to create the object is denied.

     [EXIST]           OCREAT and OEXCL were specified and the object
                        exists.

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

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

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

     [ENAMETOLONG]     name exceeded SHMNAMEMAX characters.

     [ENFILE]           The system file table is full.

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

     [ENOSPC]           OCREAT 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), shmunlink(2), umask(2)

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

Darwin                        September 20, 1999                        Darwin