man3/mtrace.3

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  25 .TH MTRACE 3 2017-09-15 "GNU" "Linux Programmer's Manual"
  26 .SH NAME
  27 mtrace, muntrace \- malloc tracing
  28 .SH SYNOPSIS
  29 .B "#include <mcheck.h>"
  30 .PP
  31 .B "void mtrace(void);"
  32 .PP
  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 .PP
  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 .PP
  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 .PP
  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 T}      Thread safety   MT-Unsafe
  90 .TE
  91 .\" FIXME: The marking is different from that in the glibc manual,
  92 .\" markings in glibc manual are more detailed:
  93 .\"
  94 .\"      mtrace: MT-Unsafe env race:mtrace const:malloc_hooks init
  95 .\"      muntrace: MT-Unsafe race:mtrace const:malloc_hooks locale
  96 .\"
  97 .\" But there is something wrong in glibc manual, for example:
  98 .\" glibc manual says muntrace should have marking locale because it calls
  99 .\" fprintf(), but muntrace does not execute area which cause locale problem.
 100 .SH CONFORMING TO
 101 These functions are GNU extensions.
 102 .SH NOTES
 103 In normal usage,
 104 .BR mtrace ()
 105 is called once at the start of execution of a program, and
 106 .BR muntrace ()
 107 is never called.
 108 .PP
 109 The tracing output produced after a call to
 110 .BR mtrace ()
 111 is textual, but not designed to be human readable.
 112 The GNU C library provides a Perl script,
 113 .BR mtrace (1),
 114 that interprets the trace log and produces human-readable output.
 115 For best results,
 116 the traced program should be compiled with debugging enabled,
 117 so that line-number information is recorded in the executable.
 118 .PP
 119 The tracing performed by
 120 .BR mtrace ()
 121 incurs a performance penalty (if
 122 .B MALLOC_TRACE
 123 points to a valid, writable pathname).
 124 .SH BUGS
 125 The line-number information produced by
 126 .BR mtrace (1)
 127 is not always precise:
 128 the line number references may refer to the previous or following (nonblank)
 129 line of the source code.
 130 .SH EXAMPLE
 131 The shell session below demonstrates the use of the
 132 .BR mtrace ()
 133 function and the
 134 .BR mtrace (1)
 135 command in a program that has memory leaks at two different locations.
 136 The demonstration uses the following program:
 137 .PP
 138 .in +4
 139 .EX
 140 .RB "$ " "cat t_mtrace.c"
 141 #include <mcheck.h>
 142 #include <stdlib.h>
 143 #include <stdio.h>
 144
 145 int
 146 main(int argc, char *argv[])
 147 {
 148     int j;
 149
 150     mtrace();
 151
 152     for (j = 0; j < 2; j++)
 153         malloc(100);            /* Never freed\-\-a memory leak */
 154
 155     calloc(16, 16);             /* Never freed\-\-a memory leak */
 156     exit(EXIT_SUCCESS);
 157 }
 158 .EE
 159 .in
 160 .PP
 161 When we run the program as follows, we see that
 162 .BR mtrace ()
 163 diagnosed memory leaks at two different locations in the program:
 164 .PP
 165 .in +4n
 166 .EX
 167 .RB "$ " "cc \-g t_mtrace.c \-o t_mtrace"
 168 .RB "$ " "export MALLOC_TRACE=/tmp/t"
 169 .RB "$ " "./t_mtrace"
 170 .RB "$ " "mtrace ./t_mtrace $MALLOC_TRACE"
 171 Memory not freed:
 172 -----------------
 173    Address     Size     Caller
 174 0x084c9378     0x64  at /home/cecilia/t_mtrace.c:12
 175 0x084c93e0     0x64  at /home/cecilia/t_mtrace.c:12
 176 0x084c9448    0x100  at /home/cecilia/t_mtrace.c:16
 177 .EE
 178 .in
 179 .PP
 180 The first two messages about unfreed memory correspond to the two
 181 .BR malloc (3)
 182 calls inside the
 183 .I for
 184 loop.
 185 The final message corresponds to the call to
 186 .BR calloc (3)
 187 (which in turn calls
 188 .BR malloc (3)).
 189 .SH SEE ALSO
 190 .BR mtrace (1),
 191 .BR malloc (3),
 192 .BR malloc_hook (3),
 193 .BR mcheck (3)