1130b656e5
This will make a number of things easier in the future, as well as (finally!) avoiding the Id-smashing problem which has plagued developers for so long. Boy, I'm glad we're not using sup anymore. This update would have been insane otherwise.
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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.\" $FreeBSD$
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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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