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251 lines
12 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This manual describes how to install and use the GNU multiple precision
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arithmetic library, version 6.1.0.
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Copyright 1991, 1993-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document under
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the terms of the GNU Free Documentation License, Version 1.3 or any later
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version published by the Free Software Foundation; with no Invariant Sections,
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with the Front-Cover Texts being "A GNU Manual", and with the Back-Cover
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Texts being "You have freedom to copy and modify this GNU Manual, like GNU
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software". A copy of the license is included in
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GNU Free Documentation License. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Debugging (GNU MP 6.1.0)</title>
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<meta name="description" content="How to install and use the GNU multiple precision arithmetic library, version 6.1.0.">
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<meta name="keywords" content="Debugging (GNU MP 6.1.0)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="GMP-Basics.html#GMP-Basics" rel="up" title="GMP Basics">
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<link href="Profiling.html#Profiling" rel="next" title="Profiling">
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<link href="Efficiency.html#Efficiency" rel="prev" title="Efficiency">
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<style type="text/css">
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Debugging"></a>
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<div class="header">
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<p>
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Next: <a href="Profiling.html#Profiling" accesskey="n" rel="next">Profiling</a>, Previous: <a href="Efficiency.html#Efficiency" accesskey="p" rel="prev">Efficiency</a>, Up: <a href="GMP-Basics.html#GMP-Basics" accesskey="u" rel="up">GMP Basics</a> [<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Debugging-1"></a>
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<h3 class="section">3.12 Debugging</h3>
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<a name="index-Debugging"></a>
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<dl compact="compact">
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<dt>Stack Overflow</dt>
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<dd><a name="index-Stack-overflow-1"></a>
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<a name="index-Segmentation-violation"></a>
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<a name="index-Bus-error"></a>
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<p>Depending on the system, a segmentation violation or bus error might be the
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only indication of stack overflow. See ‘<samp>--enable-alloca</samp>’ choices in
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<a href="Build-Options.html#Build-Options">Build Options</a>, for how to address this.
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</p>
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<p>In new enough versions of GCC, ‘<samp>-fstack-check</samp>’ may be able to ensure an
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overflow is recognised by the system before too much damage is done, or
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‘<samp>-fstack-limit-symbol</samp>’ or ‘<samp>-fstack-limit-register</samp>’ may be able to
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add checking if the system itself doesn’t do any (see <a href="http://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#Code-Gen-Options">Options for Code Generation</a> in <cite>Using the GNU Compiler Collection (GCC)</cite>).
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These options must be added to the ‘<samp>CFLAGS</samp>’ used in the GMP build
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(see <a href="Build-Options.html#Build-Options">Build Options</a>), adding them just to an application will have no
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effect. Note also they’re a slowdown, adding overhead to each function call
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and each stack allocation.
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</p>
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</dd>
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<dt>Heap Problems</dt>
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<dd><a name="index-Heap-problems"></a>
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<a name="index-Malloc-problems"></a>
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<p>The most likely cause of application problems with GMP is heap corruption.
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Failing to <code>init</code> GMP variables will have unpredictable effects, and
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corruption arising elsewhere in a program may well affect GMP. Initializing
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GMP variables more than once or failing to clear them will cause memory leaks.
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</p>
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<a name="index-Malloc-debugger"></a>
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<p>In all such cases a <code>malloc</code> debugger is recommended. On a GNU or BSD
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system the standard C library <code>malloc</code> has some diagnostic facilities,
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see <a href="http://www.gnu.org/software/libc/manual/html_node/Allocation-Debugging.html#Allocation-Debugging">Allocation Debugging</a> in <cite>The GNU C Library
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Reference Manual</cite>, or ‘<samp>man 3 malloc</samp>’. Other possibilities, in no
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particular order, include
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</p>
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<div class="display">
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<pre class="display"><a href="http://www.inf.ethz.ch/personal/biere/projects/ccmalloc/">http://www.inf.ethz.ch/personal/biere/projects/ccmalloc/</a>
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<a href="http://dmalloc.com/">http://dmalloc.com/</a>
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<a href="http://www.perens.com/FreeSoftware/">http://www.perens.com/FreeSoftware/</a> (electric fence)
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<a href="http://packages.debian.org/stable/devel/fda">http://packages.debian.org/stable/devel/fda</a>
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<a href="http://www.gnupdate.org/components/leakbug/">http://www.gnupdate.org/components/leakbug/</a>
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<a href="http://people.redhat.com/~otaylor/memprof/">http://people.redhat.com/~otaylor/memprof/</a>
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<a href="http://www.cbmamiga.demon.co.uk/mpatrol/">http://www.cbmamiga.demon.co.uk/mpatrol/</a>
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</pre></div>
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<p>The GMP default allocation routines in <samp>memory.c</samp> also have a simple
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sentinel scheme which can be enabled with <code>#define DEBUG</code> in that file.
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This is mainly designed for detecting buffer overruns during GMP development,
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but might find other uses.
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</p>
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</dd>
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<dt>Stack Backtraces</dt>
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<dd><a name="index-Stack-backtrace"></a>
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<p>On some systems the compiler options GMP uses by default can interfere with
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debugging. In particular on x86 and 68k systems ‘<samp>-fomit-frame-pointer</samp>’
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is used and this generally inhibits stack backtracing. Recompiling without
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such options may help while debugging, though the usual caveats about it
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potentially moving a memory problem or hiding a compiler bug will apply.
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</p>
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</dd>
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<dt>GDB, the GNU Debugger</dt>
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<dd><a name="index-GDB"></a>
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<a name="index-GNU-Debugger"></a>
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<p>A sample <samp>.gdbinit</samp> is included in the distribution, showing how to call
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some undocumented dump functions to print GMP variables from within GDB. Note
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that these functions shouldn’t be used in final application code since they’re
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undocumented and may be subject to incompatible changes in future versions of
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GMP.
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</p>
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</dd>
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<dt>Source File Paths</dt>
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<dd><p>GMP has multiple source files with the same name, in different directories.
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For example <samp>mpz</samp>, <samp>mpq</samp> and <samp>mpf</samp> each have an
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<samp>init.c</samp>. If the debugger can’t already determine the right one it may
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help to build with absolute paths on each C file. One way to do that is to
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use a separate object directory with an absolute path to the source directory.
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</p>
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<div class="example">
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<pre class="example">cd /my/build/dir
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/my/source/dir/gmp-6.1.0/configure
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</pre></div>
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<p>This works via <code>VPATH</code>, and might require GNU <code>make</code>.
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Alternately it might be possible to change the <code>.c.lo</code> rules
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appropriately.
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</p>
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</dd>
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<dt>Assertion Checking</dt>
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<dd><a name="index-Assertion-checking-1"></a>
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<p>The build option <samp>--enable-assert</samp> is available to add some consistency
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checks to the library (see <a href="Build-Options.html#Build-Options">Build Options</a>). These are likely to be of
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limited value to most applications. Assertion failures are just as likely to
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indicate memory corruption as a library or compiler bug.
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</p>
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<p>Applications using the low-level <code>mpn</code> functions, however, will benefit
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from <samp>--enable-assert</samp> since it adds checks on the parameters of most
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such functions, many of which have subtle restrictions on their usage. Note
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however that only the generic C code has checks, not the assembly code, so
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<samp>--disable-assembly</samp> should be used for maximum checking.
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</p>
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</dd>
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<dt>Temporary Memory Checking</dt>
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<dd><p>The build option <samp>--enable-alloca=debug</samp> arranges that each block of
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temporary memory in GMP is allocated with a separate call to <code>malloc</code> (or
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the allocation function set with <code>mp_set_memory_functions</code>).
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</p>
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<p>This can help a malloc debugger detect accesses outside the intended bounds,
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or detect memory not released. In a normal build, on the other hand,
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temporary memory is allocated in blocks which GMP divides up for its own use,
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or may be allocated with a compiler builtin <code>alloca</code> which will go
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nowhere near any malloc debugger hooks.
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</p>
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</dd>
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<dt>Maximum Debuggability</dt>
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<dd><p>To summarize the above, a GMP build for maximum debuggability would be
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</p>
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<div class="example">
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<pre class="example">./configure --disable-shared --enable-assert \
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--enable-alloca=debug --disable-assembly CFLAGS=-g
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</pre></div>
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<p>For C++, add ‘<samp>--enable-cxx CXXFLAGS=-g</samp>’.
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</p>
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</dd>
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<dt>Checker</dt>
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<dd><a name="index-Checker"></a>
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<a name="index-GCC-Checker"></a>
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<p>The GCC checker (<a href="https://savannah.nongnu.org/projects/checker/">https://savannah.nongnu.org/projects/checker/</a>) can be
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used with GMP. It contains a stub library which means GMP applications
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compiled with checker can use a normal GMP build.
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</p>
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<p>A build of GMP with checking within GMP itself can be made. This will run
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very very slowly. On GNU/Linux for example,
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</p>
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<a name="index-checkergcc"></a>
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<div class="example">
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<pre class="example">./configure --disable-assembly CC=checkergcc
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</pre></div>
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<p><samp>--disable-assembly</samp> must be used, since the GMP assembly code doesn’t
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support the checking scheme. The GMP C++ features cannot be used, since
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current versions of checker (0.9.9.1) don’t yet support the standard C++
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library.
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</p>
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</dd>
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<dt>Valgrind</dt>
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<dd><a name="index-Valgrind"></a>
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<p>Valgrind (<a href="http://valgrind.org/">http://valgrind.org/</a>) is a memory checker for x86, ARM, MIPS,
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PowerPC, and S/390. It translates and emulates machine instructions to do
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strong checks for uninitialized data (at the level of individual bits), memory
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accesses through bad pointers, and memory leaks.
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</p>
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<p>Valgrind does not always support every possible instruction, in particular
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ones recently added to an ISA. Valgrind might therefore be incompatible with
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a recent GMP or even a less recent GMP which is compiled using a recent GCC.
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</p>
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<p>GMP’s assembly code sometimes promotes a read of the limbs to some larger size,
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for efficiency. GMP will do this even at the start and end of a multilimb
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operand, using naturally aligned operations on the larger type. This may lead
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to benign reads outside of allocated areas, triggering complaints from
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Valgrind. Valgrind’s option ‘<samp>--partial-loads-ok=yes</samp>’ should help.
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</p>
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</dd>
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<dt>Other Problems</dt>
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<dd><p>Any suspected bug in GMP itself should be isolated to make sure it’s not an
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application problem, see <a href="Reporting-Bugs.html#Reporting-Bugs">Reporting Bugs</a>.
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</p></dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Profiling.html#Profiling" accesskey="n" rel="next">Profiling</a>, Previous: <a href="Efficiency.html#Efficiency" accesskey="p" rel="prev">Efficiency</a>, Up: <a href="GMP-Basics.html#GMP-Basics" accesskey="u" rel="up">GMP Basics</a> [<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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