When will this execute?

问题

I have a C code:

... 
void caller() {
    #define YES 1
    #define NO 0
}
...

Will the both #define lines execute when caller is called/executed, or will they execute at compile-time only.

回答1:

The prerpcessor macros don't execute, they are just named fragments of the code which will be replaced by the preprocessor to theirs content if you use them. Read more about preprocessor macros here.

So, after preprocessing, your code will be:

void caller() {
}

Let assume you use the YES macro after you #define it:

#define YES 1
#define NO 0

void caller() {
    printf("My answer is: %d", YES);
}

After preprocessing the code above will be the following:

void caller() {
    printf("My answer is: %d", 1);
}

来源：https://stackoverflow.com/questions/43862311/when-will-this-execute