137 lines
6.9 KiB
HTML
137 lines
6.9 KiB
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<html lang="en">
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<head>
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<title>Compiling - GNU gprof</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="GNU gprof">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="prev" href="Introduction.html#Introduction" title="Introduction">
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<link rel="next" href="Executing.html#Executing" title="Executing">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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This file documents the gprof profiler of the GNU system.
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Copyright (C) 1988-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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section entitled ``GNU Free Documentation License''.
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</head>
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<body>
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<div class="node">
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<a name="Compiling"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Executing.html#Executing">Executing</a>,
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Previous: <a rel="previous" accesskey="p" href="Introduction.html#Introduction">Introduction</a>,
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Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
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<hr>
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</div>
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<h2 class="chapter">2 Compiling a Program for Profiling</h2>
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<p>The first step in generating profile information for your program is
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to compile and link it with profiling enabled.
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<p>To compile a source file for profiling, specify the ‘<samp><span class="samp">-pg</span></samp>’ option when
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you run the compiler. (This is in addition to the options you normally
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use.)
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<p>To link the program for profiling, if you use a compiler such as <code>cc</code>
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to do the linking, simply specify ‘<samp><span class="samp">-pg</span></samp>’ in addition to your usual
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options. The same option, ‘<samp><span class="samp">-pg</span></samp>’, alters either compilation or linking
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to do what is necessary for profiling. Here are examples:
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<pre class="example"> cc -g -c myprog.c utils.c -pg
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cc -o myprog myprog.o utils.o -pg
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</pre>
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<p>The ‘<samp><span class="samp">-pg</span></samp>’ option also works with a command that both compiles and links:
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<pre class="example"> cc -o myprog myprog.c utils.c -g -pg
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</pre>
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<p>Note: The ‘<samp><span class="samp">-pg</span></samp>’ option must be part of your compilation options
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as well as your link options. If it is not then no call-graph data
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will be gathered and when you run <code>gprof</code> you will get an error
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message like this:
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<pre class="example"> gprof: gmon.out file is missing call-graph data
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</pre>
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<p>If you add the ‘<samp><span class="samp">-Q</span></samp>’ switch to suppress the printing of the call
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graph data you will still be able to see the time samples:
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<pre class="example"> Flat profile:
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Each sample counts as 0.01 seconds.
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% cumulative self self total
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time seconds seconds calls Ts/call Ts/call name
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44.12 0.07 0.07 zazLoop
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35.29 0.14 0.06 main
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20.59 0.17 0.04 bazMillion
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</pre>
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<p>If you run the linker <code>ld</code> directly instead of through a compiler
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such as <code>cc</code>, you may have to specify a profiling startup file
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<samp><span class="file">gcrt0.o</span></samp> as the first input file instead of the usual startup
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file <samp><span class="file">crt0.o</span></samp>. In addition, you would probably want to
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specify the profiling C library, <samp><span class="file">libc_p.a</span></samp>, by writing
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‘<samp><span class="samp">-lc_p</span></samp>’ instead of the usual ‘<samp><span class="samp">-lc</span></samp>’. This is not absolutely
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necessary, but doing this gives you number-of-calls information for
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standard library functions such as <code>read</code> and <code>open</code>. For
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example:
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<pre class="example"> ld -o myprog /lib/gcrt0.o myprog.o utils.o -lc_p
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</pre>
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<p>If you are running the program on a system which supports shared
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libraries you may run into problems with the profiling support code in
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a shared library being called before that library has been fully
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initialised. This is usually detected by the program encountering a
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segmentation fault as soon as it is run. The solution is to link
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against a static version of the library containing the profiling
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support code, which for <code>gcc</code> users can be done via the
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‘<samp><span class="samp">-static</span></samp>’ or ‘<samp><span class="samp">-static-libgcc</span></samp>’ command-line option. For
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example:
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<pre class="example"> gcc -g -pg -static-libgcc myprog.c utils.c -o myprog
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</pre>
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<p>If you compile only some of the modules of the program with ‘<samp><span class="samp">-pg</span></samp>’, you
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can still profile the program, but you won't get complete information about
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the modules that were compiled without ‘<samp><span class="samp">-pg</span></samp>’. The only information
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you get for the functions in those modules is the total time spent in them;
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there is no record of how many times they were called, or from where. This
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will not affect the flat profile (except that the <code>calls</code> field for
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the functions will be blank), but will greatly reduce the usefulness of the
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call graph.
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<p>If you wish to perform line-by-line profiling you should use the
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<code>gcov</code> tool instead of <code>gprof</code>. See that tool's manual or
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info pages for more details of how to do this.
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<p>Note, older versions of <code>gcc</code> produce line-by-line profiling
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information that works with <code>gprof</code> rather than <code>gcov</code> so
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there is still support for displaying this kind of information in
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<code>gprof</code>. See <a href="Line_002dby_002dline.html#Line_002dby_002dline">Line-by-line Profiling</a>.
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<p>It also worth noting that <code>gcc</code> implements a
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‘<samp><span class="samp">-finstrument-functions</span></samp>’ command-line option which will insert
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calls to special user supplied instrumentation routines at the entry
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and exit of every function in their program. This can be used to
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implement an alternative profiling scheme.
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</body></html>
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