How does this C program compile and run with two main functions?

Question

Today, while working with one custom library, I found a strange behavior. A static library code contained a debug main() function. It wasn\'t inside a #define

Answer 1

After the linker loads any object files, it searches libraries for undefined symbols. If there are none, then no libraries need to be read. Since main has been defined, even if it finds a main in every library, there is no reason to load a second one.

Linkers have dramatically different behaviors, however. For example, if your library included an object file with both main () and foo () in it, and foo was undefined, you would very likely get an error for a multiply defined symbol main ().

Modern (tautological) linkers will omit global symbols from objects that are unreachable - e.g. AIX. Old style linkers like those found on Solaris, and Linux systems still behave like the unix linkers from the 1970s, loading all of the symbols from an object module, reachable or not. This can be a source of horrible bloat as well as excessive link times.

Also characteristic of *nix linkers is that they effectively search a library only once for each time it is listed. This puts a demand on the programmer to order the libraries on the command line to a linker or in a make file, in addition to writing a program. Not requiring an ordered listing of libraries is not modern. Older operating systems often had linkers that would search all libraries repeatedly until a pass failed to resolve a symbol.