Why does the order in which libraries are linked s

Question

Why does the order in which libraries are linked sometimes cause errors in GCC?

Answer 1

Here's an example to make it clear how things work with GCC when static libraries are involved. So let's assume we have the following scenario:

• myprog.o - containing main() function, dependent on libmysqlclient
• libmysqlclient - static, for the sake of the example (you'd prefer the shared library, of course, as the libmysqlclient is huge); in /usr/local/lib; and dependent on stuff from libz
• libz (dynamic)

How do we link this? (Note: examples from compiling on Cygwin using gcc 4.3.4)

gcc -L/usr/local/lib -lmysqlclient myprog.o
# undefined reference to `_mysql_init'
# myprog depends on libmysqlclient
# so myprog has to come earlier on the command line

gcc myprog.o -L/usr/local/lib -lmysqlclient
# undefined reference to `_uncompress'
# we have to link with libz, too

gcc myprog.o -lz -L/usr/local/lib -lmysqlclient
# undefined reference to `_uncompress'
# libz is needed by libmysqlclient
# so it has to appear *after* it on the command line

gcc myprog.o -L/usr/local/lib -lmysqlclient -lz
# this works

Answer 2

A quick tip that tripped me up: if you're invoking the linker as "gcc" or "g++", then using "--start-group" and "--end-group" won't pass those options through to the linker -- nor will it flag an error. It will just fail the link with undefined symbols if you had the library order wrong.

You need to write them as "-Wl,--start-group" etc. to tell GCC to pass the argument through to the linker.

Answer 3

Another alternative would be to specify the list of libraries twice:

gcc prog.o libA.a libB.a libA.a libB.a -o prog.x

Doing this, you don't have to bother with the right sequence since the reference will be resolved in the second block.

Answer 4

I have seen this a lot, some of our modules link in excess of a 100 libraries of our code plus system & 3rd party libs.

Depending on different linkers HP/Intel/GCC/SUN/SGI/IBM/etc you can get unresolved functions/variables etc, on some platforms you have to list libraries twice.

For the most part we use structured hierarchy of libraries, core, platform, different layers of abstraction, but for some systems you still have to play with the order in the link command.

Once you hit upon a solution document it so the next developer does not have to work it out again.

My old lecturer used to say, "high cohesion & low coupling", it’s still true today.

Answer 5

I would imagine it is because some of those libraries have dependencies on other libraries, and if they have not been linked yet then you would get linker errors.

Answer 6

Link order certainly does matter, at least on some platforms. I have seen crashes for applications linked with libraries in wrong order (where wrong means A linked before B but B depends on A).