C 错误与使用 else 宏“预期参数声明符”

Question

I wrote the following C code (according to the C99 standard) and it ran with no problems:

#include <stdio.h>

#ifdef _WIN32
printf("Running on Windows");
#endif

void test(int x);

int main() {
    return 0;
}

but adding else caused so much errors (around 12) what's the problem with the new code:

#ifdef _WIN32
printf("Running on Windows");
#else
printf("Running on Windows");
#endif

Some of the errors:

error: expected parameter declarator
expected ')'
warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
error: conflicting types for 'printf'

Answer 1

When you use conditional compilation the preprocessor adds code to your program before the compile step. So, if the symbol _WIN32 exists, then you are effectively saying

printf("Running on Windows");

void test(int x);

int main() {
    return 0;
}

and that is syntactically incorrect because you have executable code (a call to printf) outside all functions.

If you didn't have a problem before you added the '#else' then that was because the symbol _WIN32 doesn't exist and the preprocessor wasn't adding the printf statement to your code.