Monday, October 22, 2012

GCC Linker - Library Flag Order

My recent ventures into the Linux world has brought about some experiences I have not encountered before in the Microsoft world.

One in particular is the behavior of GCC linker in treating the library flag parameters. It appears that the order of which is of high importance. Get it wrong and it would be as if the flag was never specified!

For example,
    g++ -shared -lfoo -o plugin.dll main.o
In the above example, main.o depends on libfoo.a. However, as main.o is specified *after* -lfoo, g++ will fail to find the necessary dependencies. Rather than keeping a list of all exported symbols as it encounters them, the linker throws the symbols away as soon as it finishes processing the module if nothing before it requires them. In the case of the example, libfoo.a does not depend on main.o -- it is the other way around. So, all of libfoo.a's export symbols get thrown away!

I do not even want to start postulating the reason behind this design, but I would imagine it had something to do with memory usage. Imagine if you'd done it the other way around (which MSVC and Borland both do), you'd end up with a huge export table consisting of all the export symbols from every single module. More than likely the linker would simply fail with an out of memory error (prime example: the notoriously problematic ilink32.exe).

So, some things are upside down in the Linux world (or you could argue vice versa for the Microsoft world depending on your POV) and it does take a little getting used to.

The correct linker command line for the above example? Here it is.
    g++ -shared -o plugin.dll main.o -lfoo

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