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+.\" manual page may be incorrect or out-of-date.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