Why does the library linker flag sometimes have to go at the end using GCC?

As the linker processes each module (be it a library or a object file), it attempts to resolve each undefined symbol while potentially adding to its list of undefined symbols. When it gets to the end of the list of modules, it either has resolved all undefined symbols and is successful or it reports undefined symbols.

In your case, when it processed librt, it had no undefined symbols. Processing proc resulted in clock_gettime being an undefined symbol. gcc will not go back and look in librt for the undefined symbols.

For that reason, you should always have your code first, followed by your libraries, followed by platform provided libraries.

Hope this helps.

From the ld (the GNU linker) documentation (http://sourceware.org/binutils/docs/ld/Options.html#Options):

The linker will search an archive only once, at the location where it is specified on the command line. If the archive defines a symbol which was undefined in some object which appeared before the archive on the command line, the linker will include the appropriate file(s) from the archive. However, an undefined symbol in an object appearing later on the command line will not cause the linker to search the archive again.

So if you specify the library too early, the linker will scan it, but not find anything of interest. Then the linker moves on to the object file produced by the compiler and finds references that need to be resolved, but it has already scanned the library and won't bother looking there again.