gcc 如何查找子程序 subprogram, collect2 如何查找 ld nm

GCC 如何查找子程序

For each subprogram to be run

1. the compiler driver first tries the -B prefix, if any. 

2. If that name is not found, or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. 

3. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable.

Collect2 如何查找 ld nm

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.

-Bprefix

   This option specifies where to find the executables, libraries, include files, and data files of the compiler itself.

The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without ‘machine/version/’ (see Target Options).

For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable.

The compiler checks to see if the path provided by the -B refers to a directory, and if necessary it adds a directory separator character at the end of the path.

-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends ‘include’ to the prefix.

The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means.

Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX. See Environment Variables.

As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it is replaced by [dir/]include. This is to help with boot-strapping the compiler.