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<title>GNU Compiler Collection (GCC) Internals: Collect2</title>
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<a name="Collect2"></a>
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<p>
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Next: <a href="Header-Dirs.html#Header-Dirs" accesskey="n" rel="next">Header Dirs</a>, Previous: <a href="Fragments.html#Fragments" accesskey="p" rel="prev">Fragments</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<a name="collect2"></a>
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<h2 class="chapter">20 <code>collect2</code></h2>
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<p>GCC uses a utility called <code>collect2</code> on nearly all systems to arrange
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to call various initialization functions at start time.
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</p>
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<p>The program <code>collect2</code> works by linking the program once and
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looking through the linker output file for symbols with particular names
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indicating they are constructor functions. If it finds any, it
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creates a new temporary ‘<samp>.c</samp>’ file containing a table of them,
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compiles it, and links the program a second time including that file.
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</p>
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<a name="index-_005f_005fmain"></a>
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<a name="index-constructors_002c-automatic-calls"></a>
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<p>The actual calls to the constructors are carried out by a subroutine
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called <code>__main</code>, which is called (automatically) at the beginning
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of the body of <code>main</code> (provided <code>main</code> was compiled with GNU
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CC). Calling <code>__main</code> is necessary, even when compiling C code, to
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allow linking C and C++ object code together. (If you use
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<samp>-nostdlib</samp>, you get an unresolved reference to <code>__main</code>,
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since it’s defined in the standard GCC library. Include <samp>-lgcc</samp> at
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the end of your compiler command line to resolve this reference.)
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</p>
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<p>The program <code>collect2</code> is installed as <code>ld</code> in the directory
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where the passes of the compiler are installed. When <code>collect2</code>
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needs to find the <em>real</em> <code>ld</code>, it tries the following file
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names:
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</p>
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<ul>
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<li> a hard coded linker file name, if GCC was configured with the
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<samp>--with-ld</samp> option.
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</li><li> <samp>real-ld</samp> in the directories listed in the compiler’s search
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directories.
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</li><li> <samp>real-ld</samp> in the directories listed in the environment variable
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<code>PATH</code>.
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</li><li> The file specified in the <code>REAL_LD_FILE_NAME</code> configuration macro,
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if specified.
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</li><li> <samp>ld</samp> in the compiler’s search directories, except that
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<code>collect2</code> will not execute itself recursively.
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</li><li> <samp>ld</samp> in <code>PATH</code>.
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</li></ul>
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<p>“The compiler’s search directories” means all the directories where
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<code>gcc</code> searches for passes of the compiler. This includes
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directories that you specify with <samp>-B</samp>.
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</p>
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<p>Cross-compilers search a little differently:
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</p>
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<ul>
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<li> <samp>real-ld</samp> in the compiler’s search directories.
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</li><li> <samp><var>target</var>-real-ld</samp> in <code>PATH</code>.
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</li><li> The file specified in the <code>REAL_LD_FILE_NAME</code> configuration macro,
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if specified.
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</li><li> <samp>ld</samp> in the compiler’s search directories.
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</li><li> <samp><var>target</var>-ld</samp> in <code>PATH</code>.
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</li></ul>
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<p><code>collect2</code> explicitly avoids running <code>ld</code> using the file name
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under which <code>collect2</code> itself was invoked. In fact, it remembers
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up a list of such names—in case one copy of <code>collect2</code> finds
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another copy (or version) of <code>collect2</code> installed as <code>ld</code> in a
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second place in the search path.
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</p>
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<p><code>collect2</code> searches for the utilities <code>nm</code> and <code>strip</code>
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using the same algorithm as above for <code>ld</code>.
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</p>
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<hr>
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Next: <a href="Header-Dirs.html#Header-Dirs" accesskey="n" rel="next">Header Dirs</a>, Previous: <a href="Fragments.html#Fragments" accesskey="p" rel="prev">Fragments</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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