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Assuming the following header file corresponding to, for example, a shared library. The exported function takes a pointer to a custom structure defined in this header:
// lib.h
typedef struct {
char c;
double d;
int i;
} A;
DLL_EXPORT void f(A* p);
If the shared library is built using one compiler and then is used from C code built with another compiler it might not work because of a different memory alignment, as Memory alignment in C-structs suggests. So, is there a way to make my structure definition portable across different compilers on the same platform?
I am interested specifically in Windows platform (apparently it does not have a well-defined ABI), though would be curious to learn about other platforms as well.
811
TL;DR in practice you should be fine.
The C standard does not define this but a platform ABI generally does. That is, for a given CPU architecture and operating system, there can be a definition for how C maps to assembly that allows different compilers to interoperate.
Struct alignment isn't the only thing that a platform ABI has to define, you also have function calling conventions and stuff like that.
C++ makes it even more complex and the ABI has to specify vtables, exceptions, name mangling, etc.
On Windows I think there are multiple C++ ABIs depending on compiler but C is mostly compatible across compilers. I could be wrong, not a Windows expert.
Some links:
Anyway the bottom line is that you're looking for your guarantee in the platform/compiler ABI spec, not the C standard.
111
The only way to know for sure is to consult the documentation of the compilers in question. However, it is usually the case that C struct layout (except, as you say, for bitfields) is defined by an ABI description for the environment you're using, and C compilers will tend to follow the native ABI.
22
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