man2/brk.2

   1 .\" Copyright (c) 1993 Michael Haardt
   2 .\" (michael@moria.de),
   3 .\" Fri Apr  2 11:32:09 MET DST 1993
   4 .\"
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  26 .\" Modified Wed Jul 21 19:52:58 1993 by Rik Faith <faith@cs.unc.edu>
  27 .\" Modified Sun Aug 21 17:40:38 1994 by Rik Faith <faith@cs.unc.edu>
  28 .\"
  29 .TH BRK 2 2015-07-23 "Linux" "Linux Programmer's Manual"
  30 .SH NAME
  31 brk, sbrk \- change data segment size
  32 .SH SYNOPSIS
  33 .B #include <unistd.h>
  34 .sp
  35 .BI "int brk(void *" addr );
  36 .sp
  37 .BI "void *sbrk(intptr_t " increment );
  38 .sp
  39 .in -4n
  40 Feature Test Macro Requirements for glibc (see
  41 .BR feature_test_macros (7)):
  42 .in
  43 .sp
  44 .BR brk (),
  45 .BR sbrk ():
  46 .ad l
  47 .RS 4
  48 .PD 0
  49 .TP 4
  50 Since glibc 2.19:
  51 .nf
  52 _DEFAULT_SOURCE ||
  53     (_XOPEN_SOURCE\ >=\ 500) &&
  54 .\"    (_XOPEN_SOURCE\ >=\ 500 ||
  55 .\"        _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED) &&
  56     ! (_POSIX_C_SOURCE\ >=\ 200112L)
  57 .fi
  58 .TP 4
  59 From glibc 2.12 to 2.19:
  60 .nf
  61 _BSD_SOURCE || _SVID_SOURCE ||
  62     (_XOPEN_SOURCE\ >=\ 500) &&
  63 .\"    (_XOPEN_SOURCE\ >=\ 500 ||
  64 .\"        _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED) &&
  65     ! (_POSIX_C_SOURCE\ >=\ 200112L)
  66 .fi
  67 .TP 4
  68 Before glibc 2.12:
  69 _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE\ >=\ 500
  70 .\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  71 .PD
  72 .RE
  73 .ad b
  74 .SH DESCRIPTION
  75 .BR brk ()
  76 and
  77 .BR sbrk ()
  78 change the location of the
  79 .IR "program break" ,
  80 which defines the end of the process's data segment
  81 (i.e., the program break is the first location after the end of the
  82 uninitialized data segment).
  83 Increasing the program break has the effect of
  84 allocating memory to the process;
  85 decreasing the break deallocates memory.
  86
  87 .BR brk ()
  88 sets the end of the data segment to the value specified by
  89 .IR addr ,
  90 when that value is reasonable, the system has enough memory,
  91 and the process does not exceed its maximum data size (see
  92 .BR setrlimit (2)).
  93
  94 .BR sbrk ()
  95 increments the program's data space by
  96 .I increment
  97 bytes.
  98 Calling
  99 .BR sbrk ()
 100 with an
 101 .I increment
 102 of 0 can be used to find the current location of the program break.
 103 .SH RETURN VALUE
 104 On success,
 105 .BR brk ()
 106 returns zero.
 107 On error, \-1 is returned, and
 108 .I errno
 109 is set to
 110 .BR ENOMEM .
 111
 112 On success,
 113 .BR sbrk ()
 114 returns the previous program break.
 115 (If the break was increased,
 116 then this value is a pointer to the start of the newly allocated memory).
 117 On error,
 118 .I "(void\ *)\ \-1"
 119 is returned, and
 120 .I errno
 121 is set to
 122 .BR ENOMEM .
 123 .SH CONFORMING TO
 124 4.3BSD; SUSv1, marked LEGACY in SUSv2, removed in POSIX.1-2001.
 125 .\"
 126 .\" .BR brk ()
 127 .\" and
 128 .\" .BR sbrk ()
 129 .\" are not defined in the C Standard and are deliberately excluded from the
 130 .\" POSIX.1-1990 standard (see paragraphs B.1.1.1.3 and B.8.3.3).
 131 .SH NOTES
 132 Avoid using
 133 .BR brk ()
 134 and
 135 .BR sbrk ():
 136 the
 137 .BR malloc (3)
 138 memory allocation package is the
 139 portable and comfortable way of allocating memory.
 140
 141 Various systems use various types for the argument of
 142 .BR sbrk ().
 143 Common are \fIint\fP, \fIssize_t\fP, \fIptrdiff_t\fP, \fIintptr_t\fP.
 144 .\" One sees
 145 .\" \fIint\fP (e.g., XPGv4, DU 4.0, HP-UX 11, FreeBSD 4.0, OpenBSD 3.2),
 146 .\" \fIssize_t\fP (OSF1 2.0, Irix 5.3, 6.5),
 147 .\" \fIptrdiff_t\fP (libc4, libc5, ulibc, glibc 2.0, 2.1),
 148 .\" \fIintptr_t\fP (e.g., XPGv5, AIX, SunOS 5.8, 5.9, FreeBSD 4.7, NetBSD 1.6,
 149 .\" Tru64 5.1, glibc2.2).
 150 .SS C library/kernel differences
 151 The return value described above for
 152 .BR brk ()
 153 is the behavior provided by the glibc wrapper function for the Linux
 154 .BR brk ()
 155 system call.
 156 (On most other implementations, the return value from
 157 .BR brk ()
 158 is the same; this return value was also specified in SUSv2.)
 159 However,
 160 the actual Linux system call returns the new program break on success.
 161 On failure, the system call returns the current break.
 162 The glibc wrapper function does some work
 163 (i.e., checks whether the new break is less than
 164 .IR addr )
 165 to provide the 0 and \-1 return values described above.
 166
 167 On Linux,
 168 .BR sbrk ()
 169 is implemented as a library function that uses the
 170 .BR brk ()
 171 system call, and does some internal bookkeeping so that it can
 172 return the old break value.
 173 .SH SEE ALSO
 174 .BR execve (2),
 175 .BR getrlimit (2),
 176 .BR end (3),
 177 .BR malloc (3)