1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
|
.\" Copyright (c) 1980, 1991, 1993
.\" The Regents of the University of California. All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" the American National Standards Committee X3, on Information
.\" Processing Systems.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" This product includes software developed by the University of
.\" California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" @(#)malloc.3 8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.Dt MALLOC 3
.Os BSD 4
.Sh NAME
.Nm malloc ,
.Nd general memory allocation function
.Pp
.Nm free
.Nd free up memory allocated with malloc, calloc or realloc
.Pp
.Nm realloc
.Nd reallocation of memory function
.Sh SYNOPSIS
.Fd #include <stdlib.h>
.Ft void *
.Fn malloc "size_t size"
.Ft void
.Fn free "void *ptr"
.Ft void *
.Fn realloc "void *ptr" "size_t size"
.Sh DESCRIPTION
The
.Fn malloc
function allocates uninitialized space for an object whose
size is specified by
.Fa size .
The
.Fn malloc
function maintains multiple lists of free blocks according to size, allocating
space from the appropriate list.
.Pp
The allocated space is
suitably aligned (after possible pointer
coercion) for storage of any type of object. If the space is of
.Em pagesize
or larger, the memory returned will be page-aligned.
.Pp
The
.Fn free
function causes the space pointed to by
.Fa ptr
to be deallocated, that is, at least made available for further allocation,
but if possible, it will passed back to the kernel with
.Xr sbrk 2 .
If
.Fa ptr
is a null pointer, no action occurs.
.Pp
The
.Fn realloc
function changes the size of the object pointed to by
.Fa ptr
to the size specified by
.Fa size .
The contents of the object are unchanged up to the lesser
of the new and old sizes.
If the new size is larger, the value of the newly allocated portion
of the object is indeterminate.
If
.Fa ptr
is a null pointer, the
.Fn realloc
function behaves like the
.Fn malloc
function for the specified size.
If the space cannot be allocated, the object
pointed to by
.Fa ptr
is unchanged.
If
.Fa size
is zero and
.Fa ptr
is not a null pointer, the object it points to is freed.
.Pp
.Sh ENVIRONMENT
This malloc will check the environment for a variable called
.Em MALLOC_OPTIONS
and scan it for flags.
Flags are single letters, uppercase means on, lowercase means off.
.Bl -tag -width indent
.It A
``abort'' malloc will coredump the process, rather than tolerate failure.
This is a very handy debugging aid, since the core file will represent the
time of failure,
rather than when the NULL pointer was accessed.
.It D
``dump'' malloc will dump statistics in a file called ``malloc.out'' at exit.
.It J
``junk'' fill some junk into the area allocated.
Currently junk is bytes of 0xd0, this is pronounced ``Duh'' :-)
.It R
``realloc'' always reallocate when
.Fn realloc
is called, even if the initial allocation was big enough.
This can substantially aid in compacting memory.
.It Z
``zero'' fill some junk into the area allocated (see ``J''),
except for the exact length the user asked for, which is zeroed.
.El
.Pp
The ``J'' and ``Z'' is mostly for testing and debugging,
if a program changes behavior if either of these options are used,
it is buggy.
.Sh RETURN VALUES
The
.Fn malloc
function returns
a pointer to the allocated space if successful; otherwise
a null pointer is returned.
.Pp
The
.Fn free
function returns no value.
.Pp
The
.Fn realloc
function returns either a null pointer or a pointer
to the possibly moved allocated space.
.Sh SEE ALSO
.Xr brk 2 ,
.Xr alloca 3 ,
.Xr calloc 3 ,
.Xr getpagesize 3 ,
.Xr memory 3
.Sh STANDARDS
The
.Fn malloc
function conforms to
.St -ansiC .
.Sh HISTORY
The present implementation of malloc started out as a filesystem on a drum
attached to a 20bit binary challenged computer built with discrete germanium
transistors, and it has since graduated to handle primary storage rather than
secondary.
.Pp
The main difference from other malloc implementations are believed to be that
the free pages are not accessed until allocated.
Most malloc implementations will store a data structure containing a,
possibly double-, linked list in the free chunks of memory, used to tie
all the free memory together.
That is a quite suboptimal thing to do.
Every time the free-list is traversed, all the otherwise unused, and very
likely paged out, pages get faulted into primary memory, just to see what
lies after them in the list.
.Pp
On systems which are paging, this can make a factor five in difference on the
page-faults of a process.
|
