Question

I was asked of this question when mentor an entry-level programmer, I was thinking of this compile + link process so official and usual that I never think about why.

One thing I could think of is to improve the development productivity, but should there be any other more compiler-related reasons?

Answer 1

Efficiency.
When you compile a program you create an object file for each source file, if you change a source file you only need to recompile that module and then relink (relinking is cheap).
If the compiler did everything in one pass it would have to recompile everything for every change.

It also fits with the unix philosophy of small programs that do one thing, so you have a pre-processor, a compiler, a linker, a library creator. These steps might now be different modes of the same tool.

However there are reasons why you want the compiler to link in one step, there are some optimizations you can do if you allow the compiler to change object files at link time – most modern compilers allow this but it requires them to put extra info into the object files at compile time.

It would be better if the compiler could store the entire project in a single database, rather than the mess of sources, resources, browse info files, object files etc – but developers are very conservative!