man3/mtrace.3

   1 .\" Copyright (c) 2012 by Michael Kerrisk <mtk.manpages@gmail.com>
   2 .\"
   3 .\" %%%LICENSE_START(VERBATIM)
   4 .\" Permission is granted to make and distribute verbatim copies of this
   5 .\" manual provided the copyright notice and this permission notice are
   6 .\" preserved on all copies.
   7 .\"
   8 .\" Permission is granted to copy and distribute modified versions of this
   9 .\" manual under the conditions for verbatim copying, provided that the
  10 .\" entire resulting derived work is distributed under the terms of a
  11 .\" permission notice identical to this one.
  12 .\"
  13 .\" Since the Linux kernel and libraries are constantly changing, this
  14 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
  15 .\" responsibility for errors or omissions, or for damages resulting from
  16 .\" the use of the information contained herein.  The author(s) may not
  17 .\" have taken the same level of care in the production of this manual,
  18 .\" which is licensed free of charge, as they might when working
  19 .\" professionally.
  20 .\"
  21 .\" Formatted or processed versions of this manual, if unaccompanied by
  22 .\" the source, must acknowledge the copyright and authors of this work.
  23 .\" %%%LICENSE_END
  24 .\"
  25 .TH MTRACE 3 2015-03-02 "GNU" "Linux Programmer's Manual"
  26 .SH NAME
  27 mtrace, muntrace \- malloc tracing
  28 .SH SYNOPSIS
  29 .B "#include <mcheck.h>"
  30 .sp
  31 .B "void mtrace(void);"
  32 .sp
  33 .B "void muntrace(void);"
  34 .SH DESCRIPTION
  35 The
  36 .BR mtrace ()
  37 function installs hook functions for the memory-allocation functions
  38 .RB ( malloc (3),
  39 .BR realloc (3)
  40 .BR memalign (3),
  41 .BR free (3)).
  42 These hook functions record tracing information about memory allocation
  43 and deallocation.
  44 The tracing information can be used to discover memory leaks and
  45 attempts to free nonallocated memory in a program.
  46
  47 The
  48 .BR muntrace ()
  49 function disables the hook functions installed by
  50 .BR mtrace (),
  51 so that tracing information is no longer recorded
  52 for the memory-allocation functions.
  53 If no hook functions were successfully installed by
  54 .BR mtrace (),
  55 .BR muntrace ()
  56 does nothing.
  57
  58 When
  59 .BR mtrace ()
  60 is called, it checks the value of the environment variable
  61 .BR MALLOC_TRACE ,
  62 which should contain the pathname of a file in which
  63 the tracing information is to be recorded.
  64 If the pathname is successfully opened, it is truncated to zero length.
  65
  66 If
  67 .BR MALLOC_TRACE
  68 is not set,
  69 or the pathname it specifies is invalid or not writable,
  70 then no hook functions are installed, and
  71 .BR mtrace ()
  72 has no effect.
  73 In set-user-ID and set-group-ID programs,
  74 .BR MALLOC_TRACE
  75 is ignored, and
  76 .BR mtrace ()
  77 has no effect.
  78 .SH ATTRIBUTES
  79 For an explanation of the terms used in this section, see
  80 .BR attributes (7).
  81 .TS
  82 allbox;
  83 lbw20 lb lb
  84 l l l.
  85 Interface       Attribute       Value
  86 T{
  87 .BR mtrace (),
  88 .BR muntrace ()
  89 .\" FIXME: the following marking is different from which in glibc manual,
  90 .\" markings in glibc manual are more detailed.
  91 .\" mtrace: MT-Unsafe env race:mtrace const:malloc_hooks init
  92 .\" muntrace: MT-Unsafe race:mtrace const:malloc_hooks locale
  93 .\" But there is something wrong in glibc manual, for example:
  94 .\" glibc manual says muntrace should have marking locale because it calls
  95 .\" fprintf(), but muntrace does not execute area which cause locale problem.
  96 T}      Thread safety   MT-Unsafe
  97 .TE
  98 .SH CONFORMING TO
  99 These functions are GNU extensions.
 100 .SH NOTES
 101 In normal usage,
 102 .BR mtrace ()
 103 is called once at the start of execution of a program, and
 104 .BR muntrace ()
 105 is never called.
 106
 107 The tracing output produced after a call to
 108 .BR mtrace ()
 109 is textual, but not designed to be human readable.
 110 The GNU C library provides a Perl script,
 111 .BR mtrace (1),
 112 that interprets the trace log and produces human-readable output.
 113 For best results,
 114 the traced program should be compiled with debugging enabled,
 115 so that line-number information is recorded in the executable.
 116
 117 The tracing performed by
 118 .BR mtrace ()
 119 incurs a performance penalty (if
 120 .B MALLOC_TRACE
 121 points to a valid, writable pathname).
 122 .SH BUGS
 123 The line-number information produced by
 124 .BR mtrace (1)
 125 is not always precise:
 126 the line number references may refer to the previous or following (nonblank)
 127 line of the source code.
 128 .SH EXAMPLE
 129 The shell session below demonstrates the use of the
 130 .BR mtrace ()
 131 function and the
 132 .BR mtrace (1)
 133 command in a program that has memory leaks at two different locations.
 134 The demonstration uses the following program:
 135 .in +4
 136 .nf
 137
 138 .RB "$ " "cat t_mtrace.c"
 139 #include <mcheck.h>
 140 #include <stdlib.h>
 141 #include <stdio.h>
 142
 143 int
 144 main(int argc, char *argv[])
 145 {
 146     int j;
 147
 148     mtrace();
 149
 150     for (j = 0; j < 2; j++)
 151         malloc(100);            /* Never freed\-\-a memory leak */
 152
 153     calloc(16, 16);             /* Never freed\-\-a memory leak */
 154     exit(EXIT_SUCCESS);
 155 }
 156
 157 .fi
 158 .in
 159 When we run the program as follows, we see that
 160 .BR mtrace ()
 161 diagnosed memory leaks at two different locations in the program:
 162 .in +4n
 163 .nf
 164
 165 .RB "$ " "cc \-g t_mtrace.c \-o t_mtrace"
 166 .RB "$ " "export MALLOC_TRACE=/tmp/t"
 167 .RB "$ " "./t_mtrace"
 168 .RB "$ " "mtrace ./t_mtrace $MALLOC_TRACE"
 169 Memory not freed:
 170 -----------------
 171    Address     Size     Caller
 172 0x084c9378     0x64  at /home/cecilia/t_mtrace.c:12
 173 0x084c93e0     0x64  at /home/cecilia/t_mtrace.c:12
 174 0x084c9448    0x100  at /home/cecilia/t_mtrace.c:16
 175 .fi
 176 .in
 177
 178 The first two messages about unfreed memory correspond to the two
 179 .BR malloc (3)
 180 calls inside the
 181 .I for
 182 loop.
 183 The final message corresponds to the call to
 184 .BR calloc (3)
 185 (which in turn calls
 186 .BR malloc (3)).
 187 .SH SEE ALSO
 188 .BR mtrace (1),
 189 .BR malloc (3),
 190 .BR malloc_hook (3),
 191 .BR mcheck (3)