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collect2

Many target systems do not have support in the assembler and linker for “constructors”—initialization functions to be called before the official “start” of main. On such systems, GNU CC uses a utility called collect2 to arrange to call these functions at start time.

The program collect2 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 ‘.c’ file containing a table of them, compiles it, and links the program a second time including that file.

The actual calls to the constructors are carried out by a subroutine called __main, which is called (automatically) at the beginning of the body of main (provided main was compiled with GNU CC).

Calling __main is necessary, even when compiling C code, to allow linking C and C++ object code together. (If you use ‘-nostdlib’, you get an unresolved reference to __main, since it’s defined in the standard GCC library. Include
‘-lgcc’ at the end of your compiler command line to resolve this reference.)

The program, collect2, is installed as ‘ld’ in the directory where the passes of the compiler are installed. When collect2 needs to find the real ‘ld’, it tries the following filenames.

- ‘real-ld’ in the directories listed in the compiler’s search directories.
- ‘real-ld’ in the directories listed in the environment variable PATH.
- The file specified in the REAL_LD_FILE_NAME configuration macro, if specified.
- ‘ld’ in the compiler’s search directories, except that collect2 will not execute itself recursively.
- ‘ld’ in PATH.

“The compiler’s search directories” means all the directories where gcc searches for passes of the compiler. This includes directories that you specify with ‘-B’.

Cross-compilers search a little differently, using the following filenames.

- ‘real-ld’ in the compiler’s search directories.
- ‘target-real-ld’ in PATH.
- The file specified in the REAL_LD_FILE_NAME configuration macro, if specified.
- ‘ld’ in the compiler’s search directories.
- ‘target-ld’ in PATH.

collect2 explicitly avoids running ld using the filename under which collect2 itself was invoked. In fact, it remembers up a list of such names—in case one copy of collect2 finds another copy (or version) of collect2 installed as ‘ld’ in a second place in the search path.

collect2 searches for the utilities, nm and strip, using the same algorithm as previous installation for ‘ld’.