GNU Compiler Collection (GCC) Internals: Collect2

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## 20 `collect2`

GCC uses a utility called `collect2` on nearly all systems to arrange to call various initialization 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 file names:

- a hard coded linker file name, if GCC was configured with the `--with-ld` option.
- `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:

- `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 file name 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 above for `ld`.

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