Does C have a standard ABI?

Question

From a discussion somewhere else:

C++ has no standard ABI (Application Binary Interface)

But neither does C, right?

On any given platform it pretty much does. It wouldn't be useful as the lingua franca for inter-language communication if it lacked one.

What's your take on this?

Answer 1

C defines no ABI. In fact, it bends over backwards to avoid defining an ABI. Those people, who like me, who have spent most of their programming lives programming in C on 16/32/64 bit architectures with 8 bit bytes, 2's complement arithmetic and flat address spaces, will usually be quite surprised on reading the convoluted language of the current C standard.

For example, read the stuff about pointers. The standard doesn't say anything so simple as "a pointer is an address" for that would be making an assumption about the ABI. In particular, it allows for pointers being in different address spaces and having varying width.

An ABI is a mapping from the execution model of the language to a particular machine/operating system/compiler combination. It makes no sense to define one in the language specification because that runs the risk of excluding C implementations on some architectures.

Answer 2

C has no standard ABI in principle, but in practice, this rarely matters: You do what your OS-vendor does.

Take the calling conventions on x86 Windows, for example: The Windows API uses the so-called 'standard' calling convention (stdcall). Thus, any compiler which wants to interface with the OS needs to implement it. However, stdcall doesn't support all C90 language features (eg calling functions without prototypes, variadic functions). As Microsoft provided a C compiler, a second calling convention was necessary, called the 'C' calling convention (cdecl). Most C compilers on Windows use this as their default calling convention, and thus are interoperable.

In principle, the same could have happened with C++, but as the C++ ABI (including the calling convention) is necessarily far more elaborate, compiler vendors did not agree on a single ABI, but could still interoperate by falling back to extern "C".