Using a macro in function declaration in C

Question

I have come across this code in the source of CMake:

https://fossies.org/windows/misc/cmake-3.17.0.zip/cmake-3.17.0/Utilities/cmzlib/compress.c

int ZEXPORT compress (dest, destLen, source, sourceLen) Bytef *dest; uLongf *destLen; const Bytef *source; uLong sourceLen; { return compress2(dest, destLen, source, sourceLen, Z_DEFAULT_COMPRESSION); }

Why is ZEXPORT used in the function, and how does it even compile?

If I change ZEXPORT to a random integer, like 5:

int 5 compress (dest, destLen, source, sourceLen)

code won't even compile anymore.

Here are possible expansions:

define ZEXPORT WINAPI define ZEXPORT __declspec(dllexport)

How does zlib.h (which is #included in your link) #define ZEXPORT? It is unlikely to be #define ZEXPORT 5 — Weather Vane
– Weather Vane, Commented Apr 1, 2020 at 19:11
ZEXPORT is defined in zconf.h to a compiler-specific way to "export" functions. — Some programmer dude
– Some programmer dude, Commented Apr 1, 2020 at 19:16

S.S. Anne · Accepted Answer · 2020-04-01 19:18:09Z

If Windows is used, this macro compiles to:

WINAPI

This is another macro that most likely expands to:

__stdcall

This makes the Microsoft compiler use a calling convention where the callee (rather than the caller) cleans up the stack.

On BEOS, this is defined to either:

__declspec(dllimport)

or:

__declspec(dllexport)

depending on whether the header file is being used in a user application or the library itself, respectively.

If any other operating system other than Windows or BEOS is used, then this macro is defined to nothing.

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