8362cc2d1d
Various neat features added. More documentation in the manpage. If your machine has very little RAM, I guess that would be < 16M these days :-(, you may want to try this: ln -fs 'H<' /etc/malloc.conf check the manpage.
295 lines
9.1 KiB
Groff
295 lines
9.1 KiB
Groff
.\" Copyright (c) 1980, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" This code is derived from software contributed to Berkeley by
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.\" the American National Standards Committee X3, on Information
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.\" Processing Systems.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)malloc.3 8.1 (Berkeley) 6/4/93
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.\" $Id$
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.\"
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.Dd August 27, 1996
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.Dt MALLOC 3
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.Os FreeBSD 2
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.Sh NAME
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.Nm malloc ,
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.Nd general memory allocation function
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.Pp
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.Nm free
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.Nd free up memory allocated with malloc, calloc or realloc
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.Pp
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.Nm realloc
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.Nd reallocation of memory function
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.Sh SYNOPSIS
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.Fd #include <stdlib.h>
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.Ft void *
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.Fn malloc "size_t size"
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.Ft void
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.Fn free "void *ptr"
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.Ft void *
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.Fn realloc "void *ptr" "size_t size"
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.Ft char *
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.Va malloc_options
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.Sh DESCRIPTION
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The
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.Fn malloc
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function allocates uninitialized space for an object whose
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size is specified by
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.Fa size .
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The
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.Fn malloc
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function maintains multiple lists of free blocks according to size, allocating
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space from the appropriate list.
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.Pp
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The allocated space is
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suitably aligned (after possible pointer
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coercion) for storage of any type of object. If the space is of
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.Em pagesize
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or larger, the memory returned will be page-aligned.
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.Pp
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The
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.Fn free
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function causes the space pointed to by
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.Fa ptr
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to be deallocated, that is, at least made available for further allocation,
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but if possible, it will passed back to the kernel with
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.Xr sbrk 2 .
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If
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.Fa ptr
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is a null pointer, no action occurs.
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.Pp
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The
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.Fn realloc
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function changes the size of the object pointed to by
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.Fa ptr
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to the size specified by
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.Fa size .
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The contents of the object are unchanged up to the lesser
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of the new and old sizes.
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If the new size is larger, the value of the newly allocated portion
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of the object is indeterminate.
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If
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.Fa ptr
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is a null pointer, the
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.Fn realloc
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function behaves like the
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.Fn malloc
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function for the specified size.
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If the space cannot be allocated, the object
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pointed to by
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.Fa ptr
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is unchanged.
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If
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.Fa size
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is zero and
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.Fa ptr
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is not a null pointer, the object it points to is freed.
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.Pp
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Malloc will first look for a symbolic link called
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.Pa /etc/malloc.conf
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and next check the environment for a variable called
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.Ev MALLOC_OPTIONS
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and finally for the global variable
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.Va malloc_options
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and scan them for flags in that order.
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Flags are single letters, uppercase means on, lowercase means off.
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.Bl -tag -width indent
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.It A
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``abort'' malloc will coredump the process, rather than tolerate failure.
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This is a very handy debugging aid, since the core file will represent the
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time of failure,
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rather than when the NULL pointer was accessed.
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.It D
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``dump'' malloc will dump statistics in a file called ``malloc.out'' at exit.
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.It J
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``junk'' fill some junk into the area allocated.
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Currently junk is bytes of 0xd0, this is pronounced ``Duh'' :-)
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.It H
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``hint'' pass a hint to the kernel about pages we don't use. If the
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machine is paging a lot this may help a bit.
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.It R
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``realloc'' always reallocate when
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.Fn realloc
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is called, even if the initial allocation was big enough.
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This can substantially aid in compacting memory.
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.It U
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``utrace'' generate entries for ktrace(1) for all operations.
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Consult the source for this one.
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.It Z
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``zero'' fill some junk into the area allocated (see ``J''),
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except for the exact length the user asked for, which is zeroed.
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.It <
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``Half the cache size'' Reduce the size of the cache by a factor of two.
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.It >
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``Double the cache size'' Double the size of the cache by a factor of two.
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.El
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.Pp
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So to set a systemwide reduction of cache size and coredumps on problems
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one would:
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.Li ln -s 'A<' /etc/malloc.conf
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.Pp
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The ``J'' and ``Z'' is mostly for testing and debugging,
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if a program changes behavior if either of these options are used,
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it is buggy.
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.Pp
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The default cache size is 16 pages.
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.Sh ENVIRONMENT
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See above.
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.Sh RETURN VALUES
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The
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.Fn malloc
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function returns
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a pointer to the allocated space if successful; otherwise
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a null pointer is returned.
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.Pp
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The
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.Fn free
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function returns no value.
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.Pp
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The
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.Fn realloc
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function returns either a null pointer or a pointer
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to the possibly moved allocated space.
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.Sh MESSAGES
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If
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.Fn malloc ,
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.Fn free
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or
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.Fn realloc
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detects an error or warning condition,
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a message will be printed to filedescriptor
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2 (not using stdio).
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Errors will always result in the process being
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.Xr abort 2 'ed,
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If the ``A'' option has been specified, also warnings will
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.Xr abort 2
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the process.
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.Pp
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Here is a brief description of the error messages and what they mean:
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.Pp
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``(ES): mumble mumble mumble'':
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malloc have been compiled with -DEXTRA_SANITY and something looks
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fishy in there. Consult sources and or wizards.
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.Pp
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``allocation failed''
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if the ``A'' option is specified it is an error for
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.Fn malloc
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or
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.Fn realloc
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to return NULL.
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.Pp
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``mmap(2) failed, check limits.''
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This is a rather weird condition that is most likely to mean that
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the system is seriously overloaded or that your ulimits are sick.
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.Pp
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``freelist is destroyed.''
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mallocs internal freelist has been stomped on.
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.Pp
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Here is a brief description of the warning messages and what they mean:
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.Pp
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``chunk/page is already free.''
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A pointer to a free chunk is attempted freed again.
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.Pp
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``junk pointer, too high to make sense.''
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The pointer doesn't make sense. It's above the area of memory that
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malloc knows something about.
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This could be a pointer from some
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.Xr mmap 2 'ed
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memory.
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.Pp
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``junk pointer, too low to make sense.''
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The pointer doesn't make sense. It's below the area of memory that
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malloc knows something about.
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This pointer probably came from your data or bss segments.
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.Pp
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``malloc() has never been called.''
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Nothing has ever been allocated, yet something is being freed or
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realloc'ed.
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.Pp
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``modified (chunk-/page-) pointer.''
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The pointer passed to free or realloc has been modified.
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.Pp
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``pointer to wrong page.''
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The pointer that malloc is trying to free is not pointing to
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a sensible page.
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.Pp
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``recursive call.''
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You have tried to call recursively into these functions.
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I can only imagine this as happening if you call one of these
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functions from a signal function, which happens to be called
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while you're already in here.
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Well, sorry to say: that's not supported.
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If this is a problem for you I'd like to hear about it. It
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would be possible to add a sigblock() around this package,
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but it would have a performance penalty that is not acceptable
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as the default.
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.Pp
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``unknown char in MALLOC_OPTIONS''
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we found something we didn't understand.
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.Sh SEE ALSO
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.Xr brk 2 ,
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.Xr alloca 3 ,
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.Xr calloc 3 ,
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.Xr getpagesize 3 ,
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.Xr memory 3
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.Pa /usr/share/doc/papers/malloc.ascii.gz
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.Sh STANDARDS
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The
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.Fn malloc
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function conforms to
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.St -ansiC .
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.Sh HISTORY
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The present implementation of malloc started out as a filesystem on a drum
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attached to a 20bit binary challenged computer built with discrete germanium
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transistors, and it has since graduated to handle primary storage rather than
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secondary.
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.Pp
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The main difference from other malloc implementations are believed to be that
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the free pages are not accessed until allocated.
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Most malloc implementations will store a data structure containing a,
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possibly double-, linked list in the free chunks of memory, used to tie
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all the free memory together.
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That is a quite suboptimal thing to do.
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Every time the free-list is traversed, all the otherwise unused, and very
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likely paged out, pages get faulted into primary memory, just to see what
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lies after them in the list.
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.Pp
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On systems which are paging, this can make a factor five in difference on the
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page-faults of a process.
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