Why is gcc/ld ignore a -L setting?

Question

According to the manual page for ld (and gcc used for linking by extension), if a -L option appears on the command line, it applies to all libraries specified by -l and takes precedence over the default search locations. However, that is not working in my link step. I have this on the command line:

-L /users/me/mylib -lpcre -lz

and /users/me/mylib contains (copies) of libpcre.so and libz.so

These libraries exist in other locations on the system (although not necessarily the same versions) and what I see (with ldd on Linux and otool on Mac) is a path that references the libraries in those locations. Some of those locations are on the LD_LIBRARY_PATH (which I cannot control in the build environment I am running in) and it appears that somehow those locations are being picked up in preference to my explicit setting with -L.

Just to be clear, this a link step problem and not a runtime problem. There is a lot of info on the web on how to affect/override library locations when executing and I am familiar with all that. In some sense what I am trying to do with the -L is create a completely specified setup. I know I can fix things up with install_name_tool on MacOS but I'd really like to understand why -L isn't doing what it claims to.

One thing I learned using gcc -Wl,-v is that gcc appears to forward all the LD_LIBRARY_PATH directories to ld. However, it places them after the ones explicitly listed by me and man ld says they are searched in order they appear on the line.

Answer 1

Just to be clear, this a link step problem and not a runtime problem.

From what you describe as the problem, I don't think you are right about this - it sounds like a runtime problem for which you are (justifiably) looking for a solution that you can employ while linking that will solve the problem you have at runtime.

The reason I say it does not appear to be a problem with linking is that it sounds like your linking is working as it is intended. LD (or GCC) are not complaining about the linking, and your linked executables are being produced just fine. The issue you are having is that when you subsequently go to run those executables, the loader is finding libraries other than the ones you intend. The purpose of the -L flag during linking is to let the linker know where it can find suitable libraries to use in preparing the linked binary. That is completely separate from where the loader will search for the required libraries at runtime.

As you say, you are already aware that there are ways you could employ at runtime (such as changing LD_LIBRARY_PATH) that would avoid the issue by changing the set of paths that the loader searches for libraries, but you'd rather not have to do that because for whatever reason you won't necessarily have control over the runtime environment, which is fair enough.

Luckily, there is a facility that I believe will get you what you want. Take a look at the ld option called -rpath (see the GNU ld man page for full documentation). Basically, if you add paths during linking using the -rpath option, those paths gets stored in the linked executable as preferred locations to find the libraries at runtime, in much the same way they would be searched if included in LD_LIBRARY_PATH. This should work on Linux or Mac OS X (at least since 10.5).

Passing the -rpath option to ld via gcc requires using the -Wl option to pass the flag through. To obtain an ld command line that contains ld -rpath /custom/path/to/libs requires a gcc invocation something like: gcc -Wl,-rpath,/custom/path/to/libs

In short, try replacing what you currently have: -L/users/me/mylib -lpcre -lz

With: -L/users/me/mylib -Wl,-rpath,/users/me/mylib -lpcre -lz

The resulting executable (or library) will then have /users/me/mylib stored as the place to go to find libraries, and it should find libpcre.so and libz.so there without needing to control LD_LIBRARY_PATH.