mtrace(3)

1mtrace(3)                  Library Functions Manual                  mtrace(3)
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NAME

6       mtrace, muntrace - malloc tracing
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <mcheck.h>
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14       void mtrace(void);
15       void muntrace(void);
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DESCRIPTION

18       The mtrace() function installs hook functions for the memory-allocation
19       functions (malloc(3), realloc(3)  memalign(3),  free(3)).   These  hook
20       functions  record  tracing  information about memory allocation and de‐
21       allocation.  The tracing information can be  used  to  discover  memory
22       leaks and attempts to free nonallocated memory in a program.
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24       The  muntrace()  function  disables  the  hook  functions  installed by
25       mtrace(), so that tracing information is no  longer  recorded  for  the
26       memory-allocation  functions.   If  no hook functions were successfully
27       installed by mtrace(), muntrace() does nothing.
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29       When mtrace() is called, it checks the value of the  environment  vari‐
30       able MALLOC_TRACE, which should contain the pathname of a file in which
31       the tracing information is to be recorded.  If the pathname is success‐
32       fully opened, it is truncated to zero length.
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34       If  MALLOC_TRACE is not set, or the pathname it specifies is invalid or
35       not writable, then no hook functions are installed, and mtrace() has no
36       effect.   In set-user-ID and set-group-ID programs, MALLOC_TRACE is ig‐
37       nored, and mtrace() has no effect.
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ATTRIBUTES

40       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
41       tributes(7).
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43       ┌──────────────────────────────────────────┬───────────────┬───────────┐
44       │Interface                                 │ Attribute     │ Value     │
45       ├──────────────────────────────────────────┼───────────────┼───────────┤
46       │mtrace(), muntrace()                      │ Thread safety │ MT-Unsafe │
47       └──────────────────────────────────────────┴───────────────┴───────────┘
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STANDARDS

50       GNU.
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NOTES

53       In normal usage, mtrace() is called once at the start of execution of a
54       program, and muntrace() is never called.
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56       The tracing output produced after a call to mtrace()  is  textual,  but
57       not  designed  to be human readable.  The GNU C library provides a Perl
58       script, mtrace(1), that interprets the trace log  and  produces  human-
59       readable  output.   For best results, the traced program should be com‐
60       piled with  debugging  enabled,  so  that  line-number  information  is
61       recorded in the executable.
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63       The tracing performed by mtrace() incurs a performance penalty (if MAL‐
64       LOC_TRACE points to a valid, writable pathname).
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BUGS

67       The line-number information produced by mtrace(1) is  not  always  pre‐
68       cise: the line number references may refer to the previous or following
69       (nonblank) line of the source code.
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EXAMPLES

72       The shell session below demonstrates the use of the  mtrace()  function
73       and  the  mtrace(1)  command  in a program that has memory leaks at two
74       different locations.  The demonstration uses the following program:
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76           $ cat t_mtrace.c
77           #include <mcheck.h>
78           #include <stdio.h>
79           #include <stdlib.h>
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81           int
82           main(void)
83           {
84               mtrace();
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86               for (unsigned int j = 0; j < 2; j++)
87                   malloc(100);            /* Never freed--a memory leak */
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89               calloc(16, 16);             /* Never freed--a memory leak */
90               exit(EXIT_SUCCESS);
91           }
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93       When we run the program as follows, we see that mtrace() diagnosed mem‐
94       ory leaks at two different locations in the program:
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96           $ cc -g t_mtrace.c -o t_mtrace
97           $ export MALLOC_TRACE=/tmp/t
98           $ ./t_mtrace
99           $ mtrace ./t_mtrace $MALLOC_TRACE
100           Memory not freed:
101           -----------------
102              Address     Size     Caller
103           0x084c9378     0x64  at /home/cecilia/t_mtrace.c:12
104           0x084c93e0     0x64  at /home/cecilia/t_mtrace.c:12
105           0x084c9448    0x100  at /home/cecilia/t_mtrace.c:16
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107       The  first two messages about unfreed memory correspond to the two mal‐
108       loc(3) calls inside the for loop.  The final message corresponds to the
109       call to calloc(3) (which in turn calls malloc(3)).
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