484 lines
14 KiB
Groff
484 lines
14 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
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.Sh NAME
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.Nm malloc , calloc , realloc , free , reallocf
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.Nd general purpose memory allocation functions
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In 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 calloc "size_t number" "size_t size"
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.Ft void *
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.Fn realloc "void *ptr" "size_t size"
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.Ft void *
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.Fn reallocf "void *ptr" "size_t size"
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.Ft void
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.Fn free "void *ptr"
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.Ft char *
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.Va _malloc_options
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.Ft void
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.Fn \*(lp*_malloc_message\*(rp "char *p1" "char *p2" "char *p3" "char *p4"
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.Sh DESCRIPTION
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The
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.Fn malloc
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function allocates
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.Fa size
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bytes of memory.
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The allocated space is suitably aligned (after possible pointer coercion)
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for storage of any type of object.
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If the space is at least
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.Em pagesize
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bytes in length (see
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.Xr getpagesize 3 ) ,
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the returned memory will be page boundary aligned as well.
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If
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.Fn malloc
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fails, a
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.Dv NULL
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pointer is returned.
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.Pp
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Note that
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.Fn malloc
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does
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.Em NOT
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normally initialize the returned memory to zero bytes.
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.Pp
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The
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.Fn calloc
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function allocates space for
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.Fa number
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objects,
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each
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.Fa size
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bytes in length.
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The result is identical to calling
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.Fn malloc
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with an argument of
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.Dq "number * size" ,
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with the exception that the allocated memory is explicitly initialized
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to zero bytes.
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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 previously allocated memory referenced by
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.Fa ptr
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to
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.Fa size
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bytes.
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The contents of the memory are unchanged up to the lesser of the new and
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old sizes.
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If the new size is larger,
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the value of the newly allocated portion of the memory is undefined.
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If the requested memory cannot be allocated,
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.Dv NULL
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is returned and
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the memory referenced by
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.Fa ptr
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is valid and unchanged.
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If
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.Fa ptr
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is
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.Dv NULL ,
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the
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.Fn realloc
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function behaves identically to
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.Fn malloc
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for the specified size.
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.Pp
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The
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.Fn reallocf
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function call is identical to the realloc function call, except that it
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will free the passed pointer when the requested memory cannot be allocated.
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This is a
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.Fx
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specific API designed to ease the problems with traditional coding styles
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for realloc causing memory leaks in libraries.
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.Pp
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The
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.Fn free
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function causes the allocated memory referenced by
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.Fa ptr
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to be made available for future allocations.
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If
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.Fa ptr
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is
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.Dv NULL ,
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no action occurs.
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.Sh TUNING
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Once, when the first call is made to one of these memory allocation
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routines, various flags will be set or reset, which affect the
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workings of this allocation implementation.
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.Pp
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The ``name'' of the file referenced by the symbolic link named
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.Pa /etc/malloc.conf ,
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the value of the environment variable
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.Ev MALLOC_OPTIONS ,
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and the string pointed to by the global variable
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.Va _malloc_options
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will be interpreted, in that order, character by character as flags.
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.Pp
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Most flags are single letters,
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where uppercase indicates that the behavior is set, or on,
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and lowercase means that the behavior is not set, or off.
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.Bl -tag -width indent
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.It A
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All warnings (except for the warning about unknown
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flags being set) become fatal.
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The process will call
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.Xr abort 3
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in these cases.
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.It J
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Each byte of new memory allocated by
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.Fn malloc ,
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.Fn realloc
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or
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.Fn reallocf
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as well as all memory returned by
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.Fn free ,
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.Fn realloc
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or
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.Fn reallocf
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will be initialized to 0xd0.
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This options also sets the
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.Dq R
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option.
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This is intended for debugging and will impact performance negatively.
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.It H
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Pass a hint to the kernel about pages unused by the allocation functions.
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This will help performance if the system is paging excessively. This
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option is off by default.
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.It R
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Causes the
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.Fn realloc
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and
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.Fn reallocf
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functions to always reallocate memory even if the initial allocation was
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sufficiently large.
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This can substantially aid in compacting memory.
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.It U
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Generate
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.Dq utrace
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entries for
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.Xr ktrace 1 ,
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for all operations.
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Consult the source for details on this option.
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.It V
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Attempting to allocate zero bytes will return a
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.Dv NULL
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pointer instead of
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a valid pointer.
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(The default behavior is to make a minimal allocation and return a
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pointer to it.)
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This option is provided for System V compatibility.
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This option is incompatible with the
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.Dq X
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option.
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.It X
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Rather than return failure for any allocation function,
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display a diagnostic message on stderr and cause the program to drop
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core (using
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.Xr abort 3 ) .
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This option should be set at compile time by including the following in
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the source code:
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.Bd -literal -offset indent
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_malloc_options = "X";
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.Ed
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.It Z
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This option implicitly sets the
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.Dq J
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and
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.Dq R
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options, and then zeros out the bytes that were requested.
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This is intended for debugging and will impact performance negatively.
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.It <
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Reduce the size of the cache by a factor of two.
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The default cache size is 16 pages.
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This option can be specified multiple times.
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.It >
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Double the size of the cache by a factor of two.
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The default cache size is 16 pages.
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This option can be specified multiple times.
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.El
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.Pp
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The
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.Dq J
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and
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.Dq Z
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options are intended for testing and debugging.
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An application which changes its behavior when these options are used
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is flawed.
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.Sh EXAMPLES
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To set a systemwide reduction of cache size, and to dump core whenever
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a problem occurs:
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.Pp
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.Bd -literal -offset indent
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ln -s 'A<' /etc/malloc.conf
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.Ed
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.Pp
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To specify in the source that a program does no return value checking
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on calls to these functions:
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.Bd -literal -offset indent
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_malloc_options = "X";
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.Ed
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.Sh ENVIRONMENT
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The following environment variables affect the execution of the allocation
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functions:
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.Bl -tag -width ".Ev MALLOC_OPTIONS"
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.It Ev MALLOC_OPTIONS
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If the environment variable
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.Ev MALLOC_OPTIONS
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is set, the characters it contains will be interpreted as flags to the
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allocation functions.
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.El
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.Sh RETURN VALUES
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The
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.Fn malloc
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and
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.Fn calloc
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functions return a pointer to the allocated memory if successful; otherwise
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a
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.Dv NULL
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pointer is returned and
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.Va errno
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is set to
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.Er ENOMEM .
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.Pp
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The
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.Fn realloc
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and
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.Fn reallocf
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functions return a pointer, possibly identical to
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.Fa ptr ,
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to the allocated memory
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if successful; otherwise a
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.Dv NULL
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pointer is returned, in which case the
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memory referenced by
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.Fa ptr
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is still available and intact.
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In the case of memory allocation failure,
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.Va errno
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is set to
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.Er ENOMEM .
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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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.Sh DEBUGGING MALLOC PROBLEMS
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The major difference between this implementation and other allocation
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implementations is that the free pages are not accessed unless allocated,
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and are aggressively returned to the kernel for reuse.
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.Bd -ragged -offset indent
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Most allocation implementations will store a data structure containing a
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linked list in the free chunks of memory,
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used to tie all the free memory together.
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That can be suboptimal,
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as every time the free-list is traversed,
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the otherwise unused, and likely paged out,
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pages are faulted into primary memory.
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On systems which are paging,
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this can result in a factor of five increase in the number of page-faults
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done by a process.
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.Ed
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.Pp
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A side effect of this architecture is that many minor transgressions on
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the interface which would traditionally not be detected are in fact
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detected. As a result, programs that have been running happily for
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years may suddenly start to complain loudly, when linked with this
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allocation implementation.
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.Pp
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The first and most important thing to do is to set the
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.Dq A
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option.
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This option forces a coredump (if possible) at the first sign of trouble,
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rather than the normal policy of trying to continue if at all possible.
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.Pp
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It is probably also a good idea to recompile the program with suitable
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options and symbols for debugger support.
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.Pp
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If the program starts to give unusual results, coredump or generally behave
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differently without emitting any of the messages listed in the next
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section, it is likely because it depends on the storage being filled with
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zero bytes. Try running it with
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.Dq Z
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option set;
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if that improves the situation, this diagnosis has been confirmed.
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If the program still misbehaves,
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the likely problem is accessing memory outside the allocated area,
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more likely after than before the allocated area.
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.Pp
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Alternatively, if the symptoms are not easy to reproduce, setting the
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.Dq J
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option may help provoke the problem.
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.Pp
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In truly difficult cases, the
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.Dq U
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option, if supported by the kernel, can provide a detailed trace of
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all calls made to these functions.
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.Pp
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Unfortunately this implementation does not provide much detail about
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the problems it detects, the performance impact for storing such information
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would be prohibitive.
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There are a number of allocation implementations available on the 'Net
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which focus on detecting and pinpointing problems by trading performance
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for extra sanity checks and detailed diagnostics.
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.Sh DIAGNOSTIC MESSAGES
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If
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.Fn malloc ,
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.Fn calloc ,
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.Fn realloc
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or
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.Fn free
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detect an error or warning condition,
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a message will be printed to file descriptor STDERR_FILENO.
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Errors will result in the process dumping core.
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If the
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.Dq A
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option is set, all warnings are treated as errors.
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.Pp
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The
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.Va _malloc_message
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variable allows the programmer to override the function which emits
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the text strings forming the errors and warnings if for some reason
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the stderr filedescriptor is not suitable for this.
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Please note that doing anything which tries to allocate memory in
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this function will assure death of the process.
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.Pp
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The following is a brief description of possible error messages and
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their meanings:
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.Pp
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.Bl -diag
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.It "(ES): mumble mumble mumble"
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The allocation functions were compiled with
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.Dq EXTRA_SANITY
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defined, and an error was found during the additional error checking.
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Consult the source code for further information.
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.It "mmap(2) failed, check limits"
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This most likely means that the system is dangerously overloaded or that
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the process' limits are incorrectly specified.
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.It "freelist is destroyed"
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The internal free-list has been corrupted.
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.It "out of memory"
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The
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.Dq X
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option was specified and an allocation of memory failed.
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.El
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.Pp
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The following is a brief description of possible warning messages and
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their meanings:
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.Bl -diag
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.It "chunk/page is already free"
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The process attempted to
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.Fn free
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memory which had already been freed.
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.It "junk pointer, ..."
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A pointer specified to one of the allocation functions points outside the
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bounds of the memory of which they are aware.
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.It "malloc() has never been called"
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No memory has been allocated,
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yet something is being freed or
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realloc'ed.
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.It "modified (chunk-/page-) pointer"
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The pointer passed to
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.Fn free
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or
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.Fn realloc
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has been modified.
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.It "pointer to wrong page"
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The pointer that
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.Fn free ,
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.Fn realloc ,
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or
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.Fn reallocf
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is trying to free does not reference a possible page.
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.It "recursive call"
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A process has attempted to call an allocation function recursively.
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This is not permitted. In particular, signal handlers should not
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attempt to allocate memory.
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.It "unknown char in MALLOC_OPTIONS"
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An unknown option was specified.
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Even with the
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.Dq A
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option set, this warning is still only a warning.
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.El
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.Sh SEE ALSO
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.Xr brk 2 ,
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.Xr mmap 2 ,
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.Xr alloca 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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.Fn calloc ,
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.Fn realloc
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and
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.Fn free
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functions conform to
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.St -isoC .
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.Sh HISTORY
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The present allocation implementation started out as a filesystem for a
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drum attached to a 20bit binary challenged computer which was built
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with discrete germanium transistors. It has since graduated to
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handle primary storage rather than secondary.
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It first appeared in its new shape and ability in
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.Fx 2.2 .
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.Pp
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The
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.Xr reallocf 3
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function first appeared in
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.Fx 3.0 .
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.Sh AUTHORS
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.An Poul-Henning Kamp Aq phk@FreeBSD.org
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.Sh BUGS
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The messages printed in case of problems provide no detail about the
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actual values.
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.Pp
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It can be argued that returning a
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.Dv NULL
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pointer when asked to
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allocate zero bytes is a silly response to a silly question.
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