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<h2 class="chapter">20 <code>collect2</code></h2>
<p>GCC uses a utility called <code>collect2</code> on nearly all systems to arrange
to call various initialization functions at start time.
<p>The program <code>collect2</code> works by linking the program once and
looking through the linker output file for symbols with particular names
indicating they are constructor functions. If it finds any, it
creates a new temporary ‘<samp><span class="samp">.c</span></samp>’ file containing a table of them,
compiles it, and links the program a second time including that file.
<p><a name="index-g_t_005f_005fmain-5071"></a><a name="index-constructors_002c-automatic-calls-5072"></a>The actual calls to the constructors are carried out by a subroutine
called <code>__main</code>, which is called (automatically) at the beginning
of the body of <code>main</code> (provided <code>main</code> was compiled with GNU
CC). Calling <code>__main</code> is necessary, even when compiling C code, to
allow linking C and C++ object code together. (If you use
<samp><span class="option">-nostdlib</span></samp>, you get an unresolved reference to <code>__main</code>,
since it's defined in the standard GCC library. Include <samp><span class="option">-lgcc</span></samp> at
the end of your compiler command line to resolve this reference.)
<p>The program <code>collect2</code> is installed as <code>ld</code> in the directory
where the passes of the compiler are installed. When <code>collect2</code>
needs to find the <em>real</em> <code>ld</code>, it tries the following file
names:
<ul>
<li>a hard coded linker file name, if GCC was configured with the
<samp><span class="option">--with-ld</span></samp> option.
<li><samp><span class="file">real-ld</span></samp> in the directories listed in the compiler's search
directories.
<li><samp><span class="file">real-ld</span></samp> in the directories listed in the environment variable
<code>PATH</code>.
<li>The file specified in the <code>REAL_LD_FILE_NAME</code> configuration macro,
if specified.
<li><samp><span class="file">ld</span></samp> in the compiler's search directories, except that
<code>collect2</code> will not execute itself recursively.
<li><samp><span class="file">ld</span></samp> in <code>PATH</code>.
</ul>
<p>“The compiler's search directories” means all the directories where
<samp><span class="command">gcc</span></samp> searches for passes of the compiler. This includes
directories that you specify with <samp><span class="option">-B</span></samp>.
<p>Cross-compilers search a little differently:
<ul>
<li><samp><span class="file">real-ld</span></samp> in the compiler's search directories.
<li><samp><var>target</var><span class="file">-real-ld</span></samp> in <code>PATH</code>.
<li>The file specified in the <code>REAL_LD_FILE_NAME</code> configuration macro,
if specified.
<li><samp><span class="file">ld</span></samp> in the compiler's search directories.
<li><samp><var>target</var><span class="file">-ld</span></samp> in <code>PATH</code>.
</ul>
<p><code>collect2</code> explicitly avoids running <code>ld</code> using the file name
under which <code>collect2</code> itself was invoked. In fact, it remembers
up a list of such names—in case one copy of <code>collect2</code> finds
another copy (or version) of <code>collect2</code> installed as <code>ld</code> in a
second place in the search path.
<p><code>collect2</code> searches for the utilities <code>nm</code> and <code>strip</code>
using the same algorithm as above for <code>ld</code>.
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