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1 perry 1 MALLOC
2 !!!MALLOC
3 NAME
4 SYNOPSIS
5 DESCRIPTION
6 RETURN VALUE
7 CONFORMING TO
8 SEE ALSO
9 NOTES
10 ----
11 !!NAME
12
13
14 calloc, malloc, free, realloc - Allocate and free dynamic memory
15 !!SYNOPSIS
16
17
18 __#include
19 __ ''nmemb''__, size_t__ ''size''__);
20 void *malloc(size_t__ ''size''__);
21 void free(void__ ''*ptr''__);
22 void *realloc(void__ ''*ptr''__, size_t__ ''size''__);
23 __
24 !!DESCRIPTION
25
26
27 __calloc()__ allocates memory for an array of
28 ''nmemb'' elements of ''size'' bytes each and returns
29 a pointer to the allocated memory. The memory is set to
30 zero.
31
32
33 __malloc()__ allocates ''size'' bytes and returns a
34 pointer to the allocated memory. The memory is not
35 cleared.
36
37
38 __free()__ frees the memory space pointed to by
39 ''ptr'', which must have been returned by a previous call
40 to __malloc()__, __calloc()__ or __realloc()__.
41 Otherwise, or if __free(__''ptr''__)__ has already
42 been called before, undefined behaviour occurs. If
43 ''ptr'' is __NULL__, no operation is
44 performed.
45
46
47 __realloc()__ changes the size of the memory block
48 pointed to by ''ptr'' to ''size'' bytes. The contents
49 will be unchanged to the minimum of the old and new sizes;
50 newly allocated memory will be uninitialized. If ''ptr''
51 is __NULL__, the call is equivalent to
52 __malloc(size)__; if size is equal to zero, the call is
53 equivalent to __free(__''ptr''__)__''.'' Unless
54 ''ptr'' is __NULL__, it must have been returned by an
55 earlier call to __malloc()__, __calloc()__ or
56 __realloc()__.
57 !!RETURN VALUE
58
59
60 For __calloc()__ and __malloc()__, the value returned
61 is a pointer to the allocated memory, which is suitably
62 aligned for any kind of variable, or __NULL__ if the
63 request fails.
64
65
66 __free()__ returns no value.
67
68
69 __realloc()__ returns a pointer to the newly allocated
70 memory, which is suitably aligned for any kind of variable
71 and may be different from ''ptr'', or __NULL__ if the
72 request fails or if size was equal to 0. If __realloc()__
73 fails the original block is left untouched - it is not freed
74 or moved.
75 !!CONFORMING TO
76
77
78 ANSI-C
79 !!SEE ALSO
80
81
82 brk(2)
83 !!NOTES
84
85
86 The Unix98 standard requires __malloc()__,
87 __calloc()__, and __realloc__() to set ''errno'' to
88 ENOMEM upon failure. Glibc assumes that this is done (and
89 the glibc versions of these routines do this); if you use a
90 private malloc implementation that does not set
91 ''errno'', then certain library routines may fail without
92 having a reason in ''errno''.
93
94
95 Crashes in __malloc()__, __free()__ or
96 __realloc()__ are almost always related to heap
97 corruption, such as overflowing an allocated chunk or
98 freeing the same pointer twice.
99
100
101 Recent versions of Linux libc (later than 5.4.23) and GNU
102 libc (2.x) include a malloc implementation which is tunable
103 via environment variables. When __MALLOC_CHECK___ is set,
104 a special (less efficient) implementation is used which is
105 designed to be tolerant against simple errors, such as
106 double calls of __free()__ with the same argument, or
107 overruns of a single byte (off-by-one bugs). Not all such
108 errors can be proteced against, however, and memory leaks
109 can result. If __MALLOC_CHECK___ is set to 0, any
110 detected heap corruption is silently ignored; if set to 1, a
111 diagnostic is printed on stderr; if set to 2, __abort()__
112 is called immediately. This can be useful because otherwise
113 a crash may happen much later, and the true cause for the
114 problem is then very hard to track down.
115
116
117 Linux follows an optimistic memory allocation strategy. This
118 means that when __malloc()__ returns non-NULL there is no
119 guarantee that the memory really is available. In case it
120 turns out that the system is out of memory, one or more
121 processes will be killed by the infamous OOM
122 killer.
123 ----
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