Question

I know that probably there is no concept of scope for macros, but please help me understand the following output – which seems to suggest that macros are local to functions:

#include<stdio.h>
#include<stdlib.h>
#define A 100
void fun();
int main()
{

    fun();
    printf("%d\n",A);
    system("pause");
    return 0;
}
void fun()
{
    #undef A

}

The output of the program is 100 though according to me it should have been a compiler error. Please explain why?

Answer 1

The pre-processor works on the text of your source code and it does it before the compiler proper ever starts to run.

In essence your compiler works on a file that looks like

/* Lots of code from the included files omitted */
void fun();
int main()
{

    fun();
    printf("%d\n",100);
    system("pause");
    return 0;
}
void fun()
{

}

So running and printing 100 is exactly what you would expect.

Notice that all the pre-processor directives are gone, and that all instances of A between the define and the undef have been replaced with 100.

The thing to remember is:

• The pre-processor runs, changing the text
• Then the compiler runs on the result