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posix-shm

POSIX Shared Memory API.

Documentation

The posix-shm allows the creation and access of POSIX shared memory objects. A POSIX shared memory object is in effect a file handle which can be used by several processes to access the same region of shared memory.

posix-shm?procedure

#t if the current platform supports POSIX shared memory, #f otherwise.

shm-open:procedure

shm-open is a wrapper around shm_open, which analogous to the UNIX system call open(2).

Argument PATH specifies the shared memory object to be created or opened. For portable use, name should have an initial slash (/) and contain no embedded slashes.

OFLAGS is a list of bit masks from the posix unit which will be ORed together and passed to shm_open. It must contains exactly one of open/rdonly or open/rdwr and any of the other flags listed here:

open/creat
Creates the shared memory object if it does not exist. The user and group ownership of the object are taken from the corresponding effective IDs of the calling process, and the object's permission bits are set according to the low-order 9 bits of mode, except that those bits set in the process file mode creation mask (see umask(2)) are cleared for the new object. A new shared memory object initially has zero length. The size of the object can be set using file-truncate. The newly allocated bytes of a shared memory object are automatically initialised to 0.
open/excl
If open/create was also specified, and a shared memory object with the given name already exists, returns an error.
open/trunc
If the shared memory object already exists, truncate it to zero bytes.

MODE should be a bitmask composed of one or more permission values like perm/irusr and is only relevant when a new file is created. The default mode is perm/irwxu | perm/irgrp | perm/iroth.

On successful completion shm-open returns a new file descriptor referring to the shared memory object.

The operation of shm-unlink is analogous to unlink(2): it removes a shared memory object name, and, once all processes have unmapped the object, de-allocates and destroys the contents of the associated memory region. After a successful shm-unlink, attempts to shm-open an object with the same name will fail (unless open/creat was specified, in which case a new, distinct object is created).

Examples

 (let* ((str "Hello, world!")
        (path (sprintf "/shmtest~A" (random 100)))
        (fd (shm-open path (list open/rdwr open/creat))))
   (file-truncate fd (string-length str))
   (file-write fd str)
   (file-close fd)
   (let ((fd (shm-open path (list open/rdonly open/excl ))))
     (file-read fd (string-length str))
     (file-close fd)
     (shm-unlink path)))

Author

Ivan Raikov

Version history

1.1
Fix in test script [thanks to Mario]
1.0
Initial release

License

Copyright 2011-2014 Ivan Raikov

 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are
 met:
 
 Redistributions of source code must retain the above copyright
 notice, this list of conditions and the following disclaimer.
 
 Redistributions in binary form must reproduce the above copyright
 notice, this list of conditions and the following disclaimer in the
 documentation and/or other materials provided with the distribution.
 
 Neither the name of the author nor the names of its contributors may
 be used to endorse or promote products derived from this software
 without specific prior written permission.
 
 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
 INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
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