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\input texinfo  @c                                  -*- Texinfo -*-

@setfilename mmalloc.info

@ifinfo

@format

START-INFO-DIR-ENTRY

* Mmalloc: (mmalloc).           The GNU mapped-malloc package.

END-INFO-DIR-ENTRY

@end format

This file documents the GNU mmalloc (mapped-malloc) package, written by

fnf@@cygnus.com, based on GNU malloc written by mike@@ai.mit.edu.

Copyright (C) 1992 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of

this manual provided the copyright notice and this permission notice

are preserved on all copies.

@ignore

Permission is granted to process this file through Tex and print the

results, provided the printed document carries copying permission

notice identical to this one except for the removal of this paragraph

(this paragraph not being relevant to the printed manual).

@end ignore

Permission is granted to copy and distribute modified versions of this

manual under the conditions for verbatim copying, provided also that the

entire resulting derived work is distributed under the terms of a

permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual

into another language, under the above conditions for modified versions.

@end ifinfo

@iftex

@c @finalout

@setchapternewpage odd

@settitle MMALLOC, the GNU memory-mapped malloc package

@titlepage

@title mmalloc

@subtitle The GNU memory-mapped malloc package

@author Fred Fish

@author Cygnus Support

@author Mike Haertel

@author Free Software Foundation

@page

@tex

\def\$#1${{#1}}  % Kluge: collect RCS revision info without $...$

\xdef\manvers{\$Revision: 1.2 $}  % For use in headers, footers too

{\parskip=0pt

\hfill Cygnus Support\par

\hfill fnf\@cygnus.com\par

\hfill {\it MMALLOC, the GNU memory-mapped malloc package}, \manvers\par

\hfill \TeX{}info \texinfoversion\par

}

@end tex

@vskip 0pt plus 1filll

Copyright @copyright{} 1992 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of

this manual provided the copyright notice and this permission notice

are preserved on all copies.

Permission is granted to copy and distribute modified versions of this

manual under the conditions for verbatim copying, provided also that

the entire resulting derived work is distributed under the terms of a

permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual

into another language, under the above conditions for modified versions.

@end titlepage

@end iftex

@ifinfo

@node Top, Overview, (dir), (dir)

@top mmalloc

This file documents the GNU memory-mapped malloc package mmalloc.

@menu

* Overview::                    Overall Description

* Implementation::              Implementation

 --- The Detailed Node Listing ---

Implementation

* Compatibility::               Backwards Compatibility

* Functions::                   Function Descriptions

@end menu

@end ifinfo

@node Overview, Implementation, Top, Top

@chapter Overall Description

This is a heavily modified version of GNU @code{malloc}.  It uses

@code{mmap} as the basic mechanism for obtaining memory from the

system, rather than @code{sbrk}.  This gives it several advantages over the

more traditional malloc:

@itemize @bullet

@item

Several different heaps can be used, each of them growing

or shinking under control of @code{mmap}, with the @code{mmalloc} functions

using a specific heap on a call by call basis.

@item

By using @code{mmap}, it is easy to create heaps which are intended to

be persistent and exist as a filesystem object after the creating

process has gone away.

@item

Because multiple heaps can be managed, data used for a

specific purpose can be allocated into its own heap, making

it easier to allow applications to ``dump'' and ``restore'' initialized

malloc-managed memory regions.  For example, the ``unexec'' hack popularized

by GNU Emacs could potentially go away.

@end itemize

@node Implementation,  , Overview, Top

@chapter Implementation

The @code{mmalloc} functions contain no internal static state.  All

@code{mmalloc} internal data is allocated in the mapped in region, along

with the user data that it manages.  This allows it to manage multiple

such regions and to ``pick up where it left off'' when such regions are

later dynamically mapped back in.

In some sense, malloc has been ``purified'' to contain no internal state

information and generalized to use multiple memory regions rather than a

single region managed by @code{sbrk}.  However the new routines now need an

extra parameter which informs @code{mmalloc} which memory region it is dealing

with (along with other information).  This parameter is called the

@dfn{malloc descriptor}.

The functions initially provided by @code{mmalloc} are:

@example

void *mmalloc_attach (int fd, void *baseaddr);

void *mmalloc_detach (void *md);

int mmalloc_errno (void *md);

int mmalloc_setkey (void *md, int keynum, void *key);

void *mmalloc_getkey (void *md, int keynum);

void *mmalloc (void *md, size_t size);

void *mrealloc (void *md, void *ptr, size_t size);

void *mvalloc (void *md, size_t size);

void mfree (void *md, void *ptr);

@end example

@menu

* Compatibility::               Backwards Compatibility

* Functions::                   Function Descriptions

@end menu

@node Compatibility, Functions, Implementation, Implementation

@section Backwards Compatibility

To allow a single malloc package to be used in a given application,

provision is made for the traditional @code{malloc}, @code{realloc}, and

@code{free} functions to be implemented as special cases of the

@code{mmalloc} functions.  In particular, if any of the functions that

expect malloc descriptors are called with a @code{NULL} pointer rather than a

valid malloc descriptor, then they default to using an @code{sbrk} managed

region.

The @code{mmalloc} package provides compatible @code{malloc}, @code{realloc},

and @code{free} functions using this mechanism internally.

Applications can avoid this extra interface layer by simply including the

following defines:

@example

#define malloc(size)            mmalloc ((void *)0, (size))

#define realloc(ptr,size)       mrealloc ((void *)0, (ptr), (size));

#define free(ptr)               mfree ((void *)0, (ptr))

@end example

@noindent

or replace the existing @code{malloc}, @code{realloc}, and @code{free}

calls with the above patterns if using @code{#define} causes problems.

@node Functions,  , Compatibility, Implementation

@section Function Descriptions

These are the details on the functions that make up the @code{mmalloc}

package.

@table @code

@item void *mmalloc_attach (int @var{fd}, void *@var{baseaddr});

Initialize access to a @code{mmalloc} managed region.

If @var{fd} is a valid file descriptor for an open file, then data for the

@code{mmalloc} managed region is mapped to that file.   Otherwise

@file{/dev/zero} is used and the data will not exist in any filesystem object.

If the open file corresponding to @var{fd} is from a previous use of

@code{mmalloc} and passes some basic sanity checks to ensure that it is

compatible with the current @code{mmalloc} package, then its data is

mapped in and is immediately accessible at the same addresses in

the current process as the process that created the file.

If @var{baseaddr} is not @code{NULL}, the mapping is established

starting at the specified address in the process address space.  If

@var{baseaddr} is @code{NULL}, the @code{mmalloc} package chooses a

suitable address at which to start the mapped region, which will be the

value of the previous mapping if opening an existing file which was

previously built by @code{mmalloc}, or for new files will be a value

chosen by @code{mmap}.

Specifying @var{baseaddr} provides more control over where the regions

start and how big they can be before bumping into existing mapped

regions or future mapped regions.

On success, returns a malloc descriptor which is used in subsequent

calls to other @code{mmalloc} package functions.  It is explicitly

@samp{void *} (@samp{char *} for systems that don't fully support

@code{void}) so that users of the package don't have to worry about the

actual implementation details.

On failure returns @code{NULL}.

@item void *mmalloc_detach (void *@var{md});

Terminate access to a @code{mmalloc} managed region identified by the

descriptor @var{md}, by closing the base file and unmapping all memory

pages associated with the region.

Returns @code{NULL} on success.

Returns the malloc descriptor on failure, which can subsequently

be used for further action (such as obtaining more information about

the nature of the failure).

@item void *mmalloc (void *@var{md}, size_t @var{size});

Given an @code{mmalloc} descriptor @var{md}, allocate additional memory of

@var{size} bytes in the associated mapped region.

@item *mrealloc (void *@var{md}, void *@var{ptr}, size_t @var{size});

Given an @code{mmalloc} descriptor @var{md} and a pointer to memory

previously allocated by @code{mmalloc} in @var{ptr}, reallocate the

memory to be @var{size} bytes long, possibly moving the existing

contents of memory if necessary.

@item void *mvalloc (void *@var{md}, size_t @var{size});

Like @code{mmalloc} but the resulting memory is aligned on a page boundary.

@item void mfree (void *@var{md}, void *@var{ptr});

Given an @code{mmalloc} descriptor @var{md} and a pointer to memory previously

allocated by @code{mmalloc} in @var{ptr}, free the previously allocated memory.

@item int mmalloc_errno (void *@var{md});

Given a @code{mmalloc} descriptor, if the last @code{mmalloc} operation

failed for some reason due to a system call failure, then

returns the associated @code{errno}.  Returns 0 otherwise.

(This function is not yet implemented).

@end table

@bye