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'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".

Answer 3

The ABI for C is platform specific - it covers issues such as register allocation and calling conventions, which are obviously specific to a particular processor. Here are some examples:

• The ARM ABI (includes C++)
• The PowerPC Embedded ABI
• The several ABIs of x86

x86 has had many calling conventions, which extensions under Windows to declare which one is used. Platform ABIs for embedded Linux have also changed over time, leading to incompatible user space. See some history of the ARM Linux port here, which shows the problems in the transition to a newer ABI.

Answer 4

An ABI, even for C, has parts which are quite platform independent, parts which depend on the processor (which registers should be saved, which are used for passing parameters,...) and parts which depend on the OS (more or less the same factors as for the processor as some choices are not imposed by the architecture but are the result of trade-offs, plus some OS's have a language independent notion of exception and so a compiler for any language has to generate the right thing to handle those, handling of threads may also impose things on the ABI -- if a register points to TLS, you can't use it for what you want).

In theory, every compiler may have its own ABI. But usually, for a couple processor/OS, the ABI is fixed by the OS vendor which often also provide a C compiler and common libraries which use that ABI and competitors prefer to be compatible. (I'd not be surprised if there are exceptions for some OS for which C isn't a major programming language).

But the OS vendor may switch ABI for one reason or the other (new versions of processors may have features that you want to use in the ABI for one - for instance some have asked for a 32bit ABI for x86_64 allowing to use all the registers). During the migration phase - which may be for a very long time - you may have to handle two ABI.

Answer 5

neither does C, right?
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.
Pretty much might refer to architecture-specific defaults chosen by C compiler vendors being adapted within other languages. So if Keil's ARM C compiler will use left to right little endian parameter ordering and stack to pass arguments and some predetermined register for return value, then extern "C" from other compilers will assume compatibility with such scheme.

While such agreement maybe considered part of ABI, unlike managed execution context such as JVM browser sandbox, this is far from being complete standard ABI by itself.

Answer 6

Although several attempts have been made at defining a single ABI for a given architecture across multiple operating systems (Particularly for i386 on Unix Systems), the efforts have not met with such success. Instead, operating systems tend to define their own ABIs ...

Quoting ... Linux System Programming page 4.

Answer 7

C does not have a standard ABI. This is easily illustrated by all the calling conventions (cdecl, fastcall and stdcall) that are used out there. Each is a different ABI.

Answer 8

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.