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| <a name="collect2"></a> |
| <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> |
| <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>.c</samp>’ file containing a table of them, |
| compiles it, and links the program a second time including that file. |
| </p> |
| <a name="index-_005f_005fmain"></a> |
| <a name="index-constructors_002c-automatic-calls"></a> |
| <p>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>-nostdlib</samp>, you get an unresolved reference to <code>__main</code>, |
| since it’s defined in the standard GCC library. Include <samp>-lgcc</samp> at |
| the end of your compiler command line to resolve this reference.) |
| </p> |
| <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: |
| </p> |
| <ul> |
| <li> a hard coded linker file name, if GCC was configured with the |
| <samp>--with-ld</samp> option. |
| |
| </li><li> <samp>real-ld</samp> in the directories listed in the compiler’s search |
| directories. |
| |
| </li><li> <samp>real-ld</samp> in the directories listed in the environment variable |
| <code>PATH</code>. |
| |
| </li><li> The file specified in the <code>REAL_LD_FILE_NAME</code> configuration macro, |
| if specified. |
| |
| </li><li> <samp>ld</samp> in the compiler’s search directories, except that |
| <code>collect2</code> will not execute itself recursively. |
| |
| </li><li> <samp>ld</samp> in <code>PATH</code>. |
| </li></ul> |
| |
| <p>“The compiler’s search directories” means all the directories where |
| <code>gcc</code> searches for passes of the compiler. This includes |
| directories that you specify with <samp>-B</samp>. |
| </p> |
| <p>Cross-compilers search a little differently: |
| </p> |
| <ul> |
| <li> <samp>real-ld</samp> in the compiler’s search directories. |
| |
| </li><li> <samp><var>target</var>-real-ld</samp> in <code>PATH</code>. |
| |
| </li><li> The file specified in the <code>REAL_LD_FILE_NAME</code> configuration macro, |
| if specified. |
| |
| </li><li> <samp>ld</samp> in the compiler’s search directories. |
| |
| </li><li> <samp><var>target</var>-ld</samp> in <code>PATH</code>. |
| </li></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> |
| <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>. |
| </p> |
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