man2/brk.2

   1 .\" Copyright (c) 1993 Michael Haardt
   2 .\" (michael@moria.de),
   3 .\" Fri Apr  2 11:32:09 MET DST 1993
   4 .\"
   5 .\" This is free documentation; you can redistribute it and/or
   6 .\" modify it under the terms of the GNU General Public License as
   7 .\" published by the Free Software Foundation; either version 2 of
   8 .\" the License, or (at your option) any later version.
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  10 .\" The GNU General Public License's references to "object code"
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  15 .\" This manual is distributed in the hope that it will be useful,
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  20 .\" You should have received a copy of the GNU General Public
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  23 .\" USA.
  24 .\"
  25 .\" Modified Wed Jul 21 19:52:58 1993 by Rik Faith <faith@cs.unc.edu>
  26 .\" Modified Sun Aug 21 17:40:38 1994 by Rik Faith <faith@cs.unc.edu>
  27 .\"
  28 .TH BRK 2 2010-09-20 "Linux" "Linux Programmer's Manual"
  29 .SH NAME
  30 brk, sbrk \- change data segment size
  31 .SH SYNOPSIS
  32 .B #include <unistd.h>
  33 .sp
  34 .BI "int brk(void *" addr );
  35 .sp
  36 .BI "void *sbrk(intptr_t " increment );
  37 .sp
  38 .in -4n
  39 Feature Test Macro Requirements for glibc (see
  40 .BR feature_test_macros (7)):
  41 .in
  42 .sp
  43 .BR brk (),
  44 .BR sbrk ():
  45 .ad l
  46 .RS 4
  47 .PD 0
  48 .TP 4
  49 Since glibc 2.12:
  50 .nf
  51 _BSD_SOURCE || _SVID_SOURCE ||
  52     (_XOPEN_SOURCE\ >=\ 500 ||
  53         _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED) &&
  54     !(_POSIX_C_SOURCE\ >=\ 200112L || _XOPEN_SOURCE\ >=\ 600)
  55 .TP 4
  56 .fi
  57 Before glibc 2.12:
  58 _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
  59 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  60 .PD
  61 .RE
  62 .ad b
  63 .SH DESCRIPTION
  64 .BR brk ()
  65 and
  66 .BR sbrk ()
  67 change the location of the
  68 .IR "program break" ,
  69 which defines the end of the process's data segment
  70 (i.e., the program break is the first location after the end of the
  71 uninitialized data segment).
  72 Increasing the program break has the effect of
  73 allocating memory to the process;
  74 decreasing the break deallocates memory.
  75
  76 .BR brk ()
  77 sets the end of the data segment to the value specified by
  78 .IR addr ,
  79 when that value is reasonable, the system has enough memory,
  80 and the process does not exceed its maximum data size (see
  81 .BR setrlimit (2)).
  82
  83 .BR sbrk ()
  84 increments the program's data space by
  85 .I increment
  86 bytes.
  87 Calling
  88 .BR sbrk ()
  89 with an
  90 .I increment
  91 of 0 can be used to find the current location of the program break.
  92 .SH RETURN VALUE
  93 On success,
  94 .BR brk ()
  95 returns zero.
  96 On error, \-1 is returned, and
  97 .I errno
  98 is set to
  99 .BR ENOMEM .
 100 (But see \fILinux Notes\fP below.)
 101
 102 On success,
 103 .BR sbrk ()
 104 returns the previous program break.
 105 (If the break was increased,
 106 then this value is a pointer to the start of the newly allocated memory).
 107 On error,
 108 .I "(void\ *)\ \-1"
 109 is returned, and
 110 .I errno
 111 is set to
 112 .BR ENOMEM .
 113 .SH CONFORMING TO
 114 4.3BSD; SUSv1, marked LEGACY in SUSv2, removed in POSIX.1-2001.
 115 .\"
 116 .\" .BR brk ()
 117 .\" and
 118 .\" .BR sbrk ()
 119 .\" are not defined in the C Standard and are deliberately excluded from the
 120 .\" POSIX.1-1990 standard (see paragraphs B.1.1.1.3 and B.8.3.3).
 121 .SH NOTES
 122 Avoid using
 123 .BR brk ()
 124 and
 125 .BR sbrk ():
 126 the
 127 .BR malloc (3)
 128 memory allocation package is the
 129 portable and comfortable way of allocating memory.
 130
 131 Various systems use various types for the argument of
 132 .BR sbrk ().
 133 Common are \fIint\fP, \fIssize_t\fP, \fIptrdiff_t\fP, \fIintptr_t\fP.
 134 .\" One sees
 135 .\" \fIint\fP (e.g., XPGv4, DU 4.0, HP-UX 11, FreeBSD 4.0, OpenBSD 3.2),
 136 .\" \fIssize_t\fP (OSF1 2.0, Irix 5.3, 6.5),
 137 .\" \fIptrdiff_t\fP (libc4, libc5, ulibc, glibc 2.0, 2.1),
 138 .\" \fIintptr_t\fP (e.g., XPGv5, AIX, SunOS 5.8, 5.9, FreeBSD 4.7, NetBSD 1.6,
 139 .\" Tru64 5.1, glibc2.2).
 140 .SS Linux notes
 141 The return value described above for
 142 .BR brk ()
 143 is the behavior provided by the glibc wrapper function for the Linux
 144 .BR brk ()
 145 system call.
 146 (On most other implementations, the return value from
 147 .BR brk ()
 148 is the same; this return value was also specified in SUSv2.)
 149 However,
 150 the actual Linux system call returns the new program break on success.
 151 On failure, the system call returns the current break.
 152 The glibc wrapper function does some work
 153 (i.e., checks whether the new break is less than
 154 .IR addr )
 155 to provide the 0 and \-1 return values described above.
 156
 157 On Linux,
 158 .BR sbrk ()
 159 is implemented as a library function that uses the
 160 .BR brk ()
 161 system call, and does some internal bookkeeping so that it can
 162 return the old break value.
 163 .SH SEE ALSO
 164 .BR execve (2),
 165 .BR getrlimit (2),
 166 .BR end (3),
 167 .BR malloc (3)