What's the difference between -rpath and -L?

Question

gcc and ld provide many ways to specify a search path for libraries—among them the -rpath and -L flags. The manpages reveal no differences between these two flags, effectively saying each flag adds a library to the library search path. Yet it seems strange that both flags do exactly the same thing. What are the differences, if any, between these two options?

Answer 1

You must be reading some outdated copies of the manpages (emphasis added):

-rpath=dir
      Add a directory to the runtime library search path. This is used
      when linking an ELF executable with shared objects. All -rpath
      arguments are concatenated and passed to the runtime linker, which
      uses them to locate shared objects at runtime.

vs.

-L searchdir
--library-path=searchdir
      Add path searchdir to the list of paths that ld will search for
      archive libraries and ld control scripts.

So, -L tells ld where to look for libraries to link against when linking. You use this (for example) when you're building against libraries in your build tree, which will be put in the normal system library paths by make install. --rpath, on the other hand, stores that path inside the executable, so that the runtime dynamic linker can find the libraries. You use this when your libraries are outside the system library search path.