What is an application binary interface (ABI)?

Question

I never clearly understood what an ABI is. Please don\'t point me to a Wikipedia article. If I could understand it, I wouldn\'t be here posting such a lengthy post.

Answer 1

Functionality: A set of contracts which affect the compiler, assembly writers, the linker, and the operating system. The contracts specify how functions are laid out, where parameters are passed, how parameters are passed, how function returns work. These are generally specific to a (processor architecture, operating system) tuple.

Existing entities: parameter layout, function semantics, register allocation. For instance, the ARM architectures has numerous ABIs (APCS, EABI, GNU-EABI, never mind a bunch of historical cases) - using the a mixed ABI will result in your code simply not working when calling across boundaries.

Consumer: The compiler, assembly writers, operating system, CPU specific architecture.

Who needs these details? The compiler, assembly writers, linkers which do code generation (or alignment requirements), operating system (interrupt handling, syscall interface). If you did assembly programming, you were conforming to an ABI!

C++ name mangling is a special case - its a linker and dynamic linker centered issue - if name mangling is not standardized, then dynamic linking will not work. Henceforth, the C++ ABI is called just that, the C++ ABI. It is not a linker level issue, but instead a code generation issue. Once you have a C++ binary, it is not possible to make it compatible with another C++ ABI (name mangling, exception handling) without recompiling from source.

ELF is a file format for the use of a loader and dynamic linker. ELF is a container format for binary code and data, and as such specifies the ABI of a piece of code. I would not consider ELF to be an ABI in the strict sense, as PE executables are not an ABI.

All ABIs are instruction set specific. An ARM ABI will not make sense on an MSP430 or x86_64 processor.

Windows has several ABIs - for instance, fastcall and stdcall are two common use ABIs. The syscall ABI is different again.