Does C have a standard ABI?

Question

From a discussion somewhere else:

C++ has no standard ABI

But neither does C, right?

On any given platform it pretty much does. It wouldn\'

Answer 1

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".

Answer 2

Prior to the C89 Standard, C compilers for many platforms used essentially the same ABI, save for variations in data sizes. For machines whose stack grows downward, code which calls a function would push the arguments on the stack in order from right to left and then call the function (pushing the return address in the process). A called function would leave its arguments on the stack, and the caller would at its leisure adjust the stack pointer to remove them [or, on some architectures, might adjust the stacked values in place]. While <stdarg.h> made it unnecessary for most programs to rely upon that convention, it remained in use for many years because it was simple and worked pretty well. While there was no "official" document establishing that as a cross-platform "standard", most compilers targeting machines with downward-growing stacks worked that way, leading to a greater level of consistency than exists today.