How does linking to OS C libraries under Windows and Linux work?

Question

I understand Linux ships with a c library, which implements the ISO C functions and system call functions, and that this library is there to be linked against when developing C. However, different c compilers do not necessarily produce linkable code (e.g. one might pad datastructures used in function arguments differently from another). How is the built-in c library meant to be linked to when I could use any compiler to compile my C? Is the story any different for static versus dynamic linking?

Under Windows on the other hand, each compiler provides its own standard library, which solves part of the problem, but system calls are still in a single set of DLLs. How are C applications linked to these DLLs successfully? How about different languages? (The same DLLs can be used by pre-.Net Visual Basic, etc.)

Answer 1

Each platform has some "calling conventions" that each C implementation must adhere to in order to be able to talk to the operating system correctly. For Windows, for example, all OS-based functions have to be called using stdcall convention, as opposed to the default C convention of cdecl.

In Linux, since the standard C library (and kernel) is compiled using GCC, any other compilers for Linux must make sure their calling conventions are compatible to the one used by GCC.

Answer 2

Compilers do come with their implementations of the standard library. It's just that under Linux it's assumed that any compiler will follow the same conventions the version of GCC that compiled the library had.

As of interoperability, it can be easier than you think. There are established calling conventions that will allow compilers to produce a valid call to a function, even if the function wasn't compiled with the same software.

As of structures and padding, you'll notice that most frameworks work with opaque types, that is, pointers to structures. Often, the structure's layout isn't even available to clients. As such, they never works with the actual data, only pointers to the data, which clears the padding issue.