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mtrace.3: Complete rewrite of page, adding much more detail
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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man3/mtrace.3
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man3/mtrace.3
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.\" Copyright 2002 Walter Harms (walter.harms@informatik.uni-oldenburg.de)
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.\" Distributed under GPL
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.\" the glibc-info pages are very helpful here
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.TH MTRACE 3 2002-07-20 "GNU" "Linux Programmer's Manual"
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.\" Copyright (c) 2012 by Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.TH MTRACE 3 2012-03-22 "GNU" "Linux Programmer's Manual"
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.SH NAME
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mtrace, muntrace \- malloc debugging
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mtrace, muntrace \- malloc tracing
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.SH SYNOPSIS
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.B "#include <mcheck.h>"
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.sp
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.sp
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.B "void muntrace(void);"
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.SH DESCRIPTION
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The function
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The
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.BR mtrace ()
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installs handlers for
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.BR malloc (3),
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function installs hook functions for the memory-allocation functions
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.RB ( malloc (3),
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.BR realloc (3)
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and
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.BR free (3).
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The function
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.BR memalign (3),
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.BR free (3)).
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These hook functions record tracing information about memory allocation
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and deallocation.
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The tracing information can be used to discover memory leaks and
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attempts to free nonallocated memory in a program.
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The
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.BR muntrace ()
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disables these handlers.
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.br
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The environment variable
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.B MALLOC_TRACE
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defines a file where
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function disables the hook functions installed by
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.BR mtrace (),
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so that tracing information is no longer recorded
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for the memory-allocation functions.
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If no hook functions were successfully installed by
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.BR mtrace (),
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.BR muntrace ()
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does nothing.
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When
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.BR mtrace (3)
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is called, it checks the value of the environment variable
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.BR MALLOC_TRACE ,
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which should contain the pathname of a file in which
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the tracing information is to be recorded.
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If the pathname is successfully opened, it is truncated to zero length.
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If
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.BR MALLOC_TRACE
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is not set,
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or the pathname it specifies is invalid or not writable,
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then no hook functions are installed, and
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.BR mtrace ()
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writes its output.
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This file must be writable to the user or
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has no effect.
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In set-user-ID and set-group-ID programs,
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.BR MALLOC_TRACE
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is ignored, and
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.BR mtrace ()
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will do nothing.
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If the file is not empty it will be truncated.
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has no effect.
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.SH "CONFORMING TO"
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These are GNU extensions.
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These functions are GNU extensions.
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.SH NOTES
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The output of
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In normal usage,
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.BR mtrace ()
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will be ASCII but not in a friendly format.
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So glibc comes with a perl-script called mtrace to make sense of it.
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is called once at the start of execution of a program, and
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.BR muntrace ()
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is never called.
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The tracing output produced after a call to
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.BR mtrace ()
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is textual, but not designed to be human readable.
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The GNU C library provides a Perl script,
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.BR mtrace (1),
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that interprets the trace log and produces human-readable output.
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For best results,
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the traced program should be compiled with debugging enabled,
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so that line-number information is recorded in the executable.
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The tracing performed by
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.BR mtrace ()
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incurs a performance penalty (if
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.B MALLOC_TRACE
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points to a valid, writable pathname).
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.SH BUGS
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The line-number information produced by
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.BR mtrace (1)
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is not always precise:
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the line number references may refer to the previous or following (non-blank)
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line of the source code.
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.SH EXAMPLE
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The shell session below demonstrates the use of the
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.BR mtrace ()
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function and the
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.BR mtrace (1)
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command in a program that has memory leaks at two different locations.
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The demonstration uses the following program:
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.in +4
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.nf
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.RB "$ " "cat t_mtrace.c"
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#include <mcheck.h>
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#include <stdlib.h>
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#include <stdio.h>
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int
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main(int argc, char *argv[])
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{
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int j;
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mtrace();
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for (j = 0; j < 2; j++)
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malloc(100); /* Never freed\-\-a memory leak */
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calloc(16, 16); /* Never freed\-\-a memory leak */
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exit(EXIT_SUCCESS);
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}
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.fi
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.in
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When we run the program as follows, we see that
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.BR mtrace ()
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diagnosed memory leaks at two different locations in the program:
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.in +4n
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.nf
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.RB "$ " "cc \-g t_mtrace.c \-o t_mtrace"
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.RB "$ " "export MALLOC_TRACE=/tmp/t"
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.RB "$ " "./t_mtrace"
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.RB "$ " "mtrace ./t_mtrace $MALLOC_TRACE"
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Memory not freed:
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-----------------
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Address Size Caller
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0x084c9378 0x64 at /home/cecilia/t_mtrace.c:12
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0x084c93e0 0x64 at /home/cecilia/t_mtrace.c:12
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0x084c9448 0x100 at /home/cecilia/t_mtrace.c:16
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.fi
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.in
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The first two messages about unfreed memory correspond to the two
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.BR malloc (3)
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calls inside the
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.I for
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loop.
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The final message corresponds to the call to
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.BR calloc (3)
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(which in turn calls
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.BR malloc (3)).
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.SH "SEE ALSO"
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.BR mtrace (1),
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.BR malloc (3),
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.BR malloc_hook (3)
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