| Allegro CL version 9.0 Minimal update since 9.0 release. 8.2 version | ||||||||||
This simple-stream subclass
is a subclass of both file-simple-stream and direct-simple-stream. Similar to
file-simple-stream but
opening a mapped file allows for much faster access to the file, with
much less kernel overhead. Note: most operating systems do not allow
the automatic extension of mapped files, so this class does not allow
the extending operation either. Writes beyond the end of the file may
cause an error.
A mapped file must be a simple-stream which has
:mapped t specified to the operator (like
to open) which creates the
stream. Recall that a simple stream is created when the :element-type
is not specified (see the
beginning of streams.htm).
The direction argument
to open should be
:input or :io. The file should
have permissions consistent with the value specified
to direction: if :input is
specified, the file must be readable; if :io is
specified, it must be both readable and writable.
The handle of a
mapped file can be released at stream creation time if the
option :release-handle t is
specified. open will then
return a stream which has been mapped to a file but for which the file
number (the handle) has been closed. The mapping is not removed, but
the file handle is made available for use in other file stream
situations. The mapping is removed when cl:close is called on the stream.
After a mapped-file-simple-stream is opened, the
excl::buffer slot contains an aligned pointer (see
the section Aligned
pointers in ftype.htm). This value can
be used with any accessors that support aligned pointers,
e.g. fslot-value-typed with
an :aligned allocation.
The options when creating an instance of this class are:
See device-open for a description of the input-handle, output-handle, and mapped options.
For the offset and length arguments, no attempt is made to validate their values; they are passed to the underlying mapping implementation. For offset the system is likely to require a page alignment; the mapping may fail if the offset is not some multiple of a page size, typically 4096. For length page alignment may be needed here as well but that is less likely. Often the length argument is more lax, because the system simply rounds that argument up to the next page boundary. If the length is not page aligned, however, it would be dangerous to use any memory between the length and the page boundary.
See streams.htm for general information on simple-streams in Allegro CL.
Copyright (c) 1998-2019, Franz Inc. Oakland, CA., USA. All rights reserved.
This page was not revised from the 8.2 page.
Created 2012.5.30.
| Allegro CL version 9.0 Minimal update since 9.0 release. 8.2 version | ||||||||||