man3/malloc.3

   1 .\" (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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  23 .\" Modified Sat Jul 24 19:00:59 1993 by Rik Faith (faith@cs.unc.edu)
  24 .\" Clarification concerning realloc, iwj10@cus.cam.ac.uk (Ian Jackson), 950701
  25 .\" Documented MALLOC_CHECK_, Wolfram Gloger (wmglo@dent.med.uni-muenchen.de)
  26 .\"
  27 .TH MALLOC 3  1993-04-04 "GNU" "Linux Programmer's Manual"
  28 .SH NAME
  29 calloc, malloc, free, realloc \- Allocate and free dynamic memory
  30 .SH SYNOPSIS
  31 .nf
  32 .B #include <stdlib.h>
  33 .sp
  34 .BI "void *calloc(size_t " "nmemb" ", size_t " "size" );
  35 .nl
  36 .BI "void *malloc(size_t " "size" );
  37 .nl
  38 .BI "void free(void " "*ptr" );
  39 .nl
  40 .BI "void *realloc(void " "*ptr" ", size_t "  "size" );
  41 .fi
  42 .SH DESCRIPTION
  43 .BR calloc ()
  44 allocates memory for an array of
  45 .I nmemb
  46 elements of
  47 .I size
  48 bytes each and returns a pointer to the allocated memory.
  49 The memory is set to zero.
  50 .PP
  51 .BR malloc ()
  52 allocates
  53 .I size
  54 bytes and returns a pointer to the allocated memory.
  55 The memory is not cleared.
  56 .PP
  57 .BR free ()
  58 frees the memory space pointed to by
  59 .IR ptr ,
  60 which must have been returned by a previous call to
  61 .BR malloc (),
  62 .BR calloc ()
  63 or
  64 .BR realloc ().
  65 Otherwise, or if
  66 .BI "free(" "ptr" )
  67 has already been called before, undefined behaviour occurs.
  68 If
  69 .I ptr
  70 is
  71 .BR NULL ,
  72 no operation is performed.
  73 .PP
  74 .BR realloc ()
  75 changes the size of the memory block pointed to by
  76 .I ptr
  77 to
  78 .I size
  79 bytes.
  80 The contents will be unchanged to the minimum of the old and new sizes;
  81 newly allocated memory will be uninitialized.
  82 If
  83 .I ptr
  84 is
  85 .BR NULL ,
  86 the call is equivalent to
  87 .BR malloc(size) ;
  88 if size is equal to zero,
  89 the call is equivalent to
  90 .BI "free(" "ptr" ) .
  91 Unless
  92 .I ptr
  93 is
  94 .BR NULL ,
  95 it must have been returned by an earlier call to
  96 .BR malloc (),
  97 .BR calloc ()
  98 or
  99 .BR realloc ().
 100 If the area pointed to was moved, a
 101 .BI "free(" "ptr" )
 102 is done.
 103 .SH "RETURN VALUE"
 104 For
 105 .BR calloc() " and " malloc() ,
 106 the value returned is a pointer to the allocated memory, which is suitably
 107 aligned for any kind of variable, or
 108 .B NULL
 109 if the request fails.
 110 .PP
 111 .BR free ()
 112 returns no value.
 113 .PP
 114 .BR realloc ()
 115 returns a pointer to the newly allocated memory, which is suitably
 116 aligned for any kind of variable and may be different from
 117 .IR ptr ,
 118 or
 119 .B NULL
 120 if the request fails. If
 121 .I size
 122 was equal to 0, either NULL or a pointer suitable to be passed to
 123 .IR free ()
 124 is returned.  If
 125 .BR realloc ()
 126 fails the original block is left untouched; it is not freed or moved.
 127 .SH "CONFORMING TO"
 128 ANSI-C
 129 .SH "SEE ALSO"
 130 .BR brk (2),
 131 .BR posix_memalign (3)
 132 .SH NOTES
 133 The Unix98 standard requires
 134 .BR malloc (),
 135 .BR calloc (),
 136 and
 137 .BR realloc ()
 138 to set
 139 .I errno
 140 to ENOMEM upon failure. Glibc assumes that this is done
 141 (and the glibc versions of these routines do this); if you
 142 use a private malloc implementation that does not set
 143 .IR errno ,
 144 then certain library routines may fail without having
 145 a reason in
 146 .IR errno .
 147 .LP
 148 Crashes in
 149 .BR malloc (),
 150 .BR free ()
 151 or
 152 .BR realloc ()
 153 are almost always related to heap corruption, such as overflowing
 154 an allocated chunk or freeing the same pointer twice.
 155 .PP
 156 Recent versions of Linux libc (later than 5.4.23) and GNU libc (2.x)
 157 include a malloc implementation which is tunable via environment
 158 variables.  When
 159 .BR MALLOC_CHECK_
 160 is set, a special (less efficient) implementation is used which
 161 is designed to be tolerant against simple errors, such as double
 162 calls of
 163 .BR free ()
 164 with the same argument, or overruns of a single byte (off-by-one
 165 bugs).  Not all such errors can be protected against, however, and
 166 memory leaks can result.
 167 If
 168 .BR MALLOC_CHECK_
 169 is set to 0, any detected heap corruption is silently ignored;
 170 if set to 1, a diagnostic is printed on stderr;
 171 if set to 2,
 172 .BR abort ()
 173 is called immediately.  This can be useful because otherwise
 174 a crash may happen much later, and the true cause for the problem
 175 is then very hard to track down.
 176 .SH BUGS
 177 By default, Linux follows an optimistic memory allocation strategy.
 178 This means that when
 179 .BR malloc ()
 180 returns non-NULL there is no guarantee that the memory really
 181 is available. This is a really bad bug.
 182 In case it turns out that the system is out of memory,
 183 one or more processes will be killed by the infamous OOM killer.
 184 In case Linux is employed under circumstances where it would be
 185 less desirable to suddenly lose some randomly picked processes,
 186 and moreover the kernel version is sufficiently recent,
 187 one can switch off this overcommitting behavior using a command like
 188 .RS
 189 # echo 2 > /proc/sys/vm/overcommit_memory
 190 .RE
 191 See also the kernel Documentation directory, files
 192 .I vm/overcommit-accounting
 193 and
 194 .IR sysctl/vm.txt .