#ifndef explanation?

This is a discussion on #ifndef explanation? within the C Programming forums, part of the General Programming Boards category; I was wondering how #ifndef worked. to me, the following should compile. (i get syntax errors) any hints are greatly ...

  1. #1
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    #ifndef explanation?

    I was wondering how #ifndef worked. to me, the following should compile. (i get syntax errors) any hints are greatly appreciated!

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <time.h>
    
    #ifndef a
    #define a int
    #endif
    
    #ifndef b
    #define b int
    #endif
    
    #ifndef LT
    #define LT (a,b)(a<b)
    #endif
    
    main() {
    	int i;
    
    	while(i = 0; LT( i, 9); i++; )
    		printf("Compiled/n");
    //or
                    LT( 5, 9){
    		printf("Compiled/n");
    	}
    }
    my reference manual claims "if true, the code that is between one of the preceding and an #endif will be compiled"

  2. #2
    and the hat of wrongness Salem's Avatar
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    gcc -E prog.c

    Then open prog.i in your editor, and see what the pre-processor has turned your code into.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
    and the Hat of Guessing tabstop's Avatar
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    Also, while doesn't take three things in a loop guard, but for does.

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