Like Tree1Likes
  • 1 Post By Click_here

What exactly does #ifdef DEBUG do?

This is a discussion on What exactly does #ifdef DEBUG do? within the C Programming forums, part of the General Programming Boards category; What does DEBUG do? My book talks about it, but I really don't understand what the point is in having ...

  1. #1
    Registered User
    Join Date
    Jan 2013
    Posts
    40

    What exactly does #ifdef DEBUG do?

    What does DEBUG do?
    My book talks about it, but I really don't understand what the point is in having the program debug.

  2. #2
    Registered User
    Join Date
    Sep 2008
    Location
    Toronto, Canada
    Posts
    1,831
    If you decide to define this variable then whatever is enclosed in the #ifdef / #endif blocks will be compiled. Otherwise not. You can define it by including #define DEBUG at the top (in the source code), or as a flag in the compile parameters I believe. So you don't have to physically alter the source file.

    Presumably these #ifdef / #endif blocks contain diagnostic code that's used to output descriptive messages to help trace and verify the program's execution.

  3. #3
    Registered User
    Join Date
    Jan 2009
    Posts
    1,485
    You can have debug code that is compiled, or not, depending on if the constant "DEBUG" is defined (for example printing debug info). The actual name is not important really the interesting part is the #ifdef part (short for #if defined) which just asks if the constant following is defined, if it is, the code until the #endif mark is also compiled.

    You can define this when you compile (for gcc and clang with the -D flag) and decide if you want to compile with or without the code included in these #ifdef clauses. Example:

    Code:
    #include <stdio.h>
    
    int main()
    {
        for(int i = 0; i < 10; i++) {
        #if defined RANDOM_NAME
            printf("%d\n", i);
        #endif
        }
    
        return 0;
    }
    If you compile this with (in case of gcc or clang at least): gcc -DRANDOM_NAME foo.c then the value of i will be printed, if you leave it out as in: gcc foo.c then nothing will get printed.

    So it allows you to compile two different versions depending on if you define the constant or not.
    Last edited by Subsonics; 01-21-2013 at 12:34 PM.

  4. #4
    Registered User
    Join Date
    Jan 2013
    Posts
    40
    Quote Originally Posted by nonoob View Post
    If you decide to define this variable then whatever is enclosed in the #ifdef / #endif blocks will be compiled. Otherwise not. You can define it by including #define DEBUG at the top (in the source code), or as a flag in the compile parameters I believe. So you don't have to physically alter the source file.

    Presumably these #ifdef / #endif blocks contain diagnostic code that's used to output descriptive messages to help trace and verify the program's execution.
    Ok this is probably a stupid question. But, if I don't want to compile a part of the program, while can't I just take that part out and keep it in another file?

    Does Debug actually tell you where the problem occurs?

  5. #5
    Registered User
    Join Date
    Nov 2012
    Posts
    1,057
    DEBUG sounds like it's a name of an arbitrary preprocessor macro being defined. To see how it works it's best to consider a simple example. This program calculates the area of a rectangular region, and prints "debugging messages" if the macro DEBUG is defined:

    Code:
    void printd(const char *msg) {
    #ifdef DEBUG
    	printf("%s", msg);
    #else
    	(void)msg; // ignore msg
    #endif
    }
    
    int main(void)
    {
    	printd("main: Starting the program\n");
    	printd("main: Calculating the area\n");
    	double area = 2.84 * 5.67;
    	printd("main: Printing the area\n");
    	printf("Area: %0.2f\n", area);
    	return EXIT_SUCCESS;
    }
    Most compilers let you compile with a specific symbol defined. So if you compile with gcc using

    gcc -DDEBUG -o printarea printarea.c

    The output will include the debugging messages. However if you decide you don't need the debugging messages anymore but want to keep them in the source code for reference purposes, then you simply recompile the same source file without defining that symbol

    gcc -o printarea printarea.c

    Of course, there are many ways to accomplish this. You mentioned putting one part in a separate source file. Yes, you could do that too. If you defined the "printd" function in two different versions, for example printd1.o and printd2.o, where one of them was the "do nothing version" and the other was the "debugging version", then linking your program with the version you want will produce a similar effect. In the end it all depends on how you want to organize your program. As programs grow you normally end up with several such defines and you tend to put them in a file called "config.h" or something similar. And then you modify that to change the build settings. DEBUG is just an arbitrary name chosen for one such setting.

  6. #6
    TEIAM - problem solved
    Join Date
    Apr 2012
    Location
    Melbourne Australia
    Posts
    1,343
    Quote Originally Posted by pyroknife View Post
    Ok this is probably a stupid question. But, if I don't want to compile a part of the program, while can't I just take that part out and keep it in another file?

    Does Debug actually tell you where the problem occurs?
    There would be a chance that you put it back in your code incorrectly if you are copying and pasting - Especially if you are modifying code as you go.

    There is something that does help you debug -> It's called "assert" and it works in a similar way.

    Basically, you include assert.h - When you are making your program, you put asserts in to make sure all your assumptions are correct. (For example, in another thread (I can't remember which one) I have suggested that the OP asserts that a variable is never 0)

    When the algorithm is working and you know it won't be 0, you define NDEBUG at the top of your code. All asserts are then optimised out of existence.

    I'll give you an example if you wanted to get it working - Basically I've put in a few errors for you to find using the asserts. This little mess of a program was supposed to find the element in the array before '9' and print it out.

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <assert.h>
    
    
    int get_preceding(int arr[], size_t arr_size, int element);
    
    
    int main(void)
    {
        int my_arr[10] = {3, 6, 1, 4, 5, 2, 7, 8, 9, 10};
        int i;
    
    
        for (i=0; i<15; i++)
        {
    
    
            int preceding_element;
            preceding_element = get_preceding(my_arr, sizeof(my_arr)/sizeof(*my_arr), i);
    
    
            if (my_arr[i]==9)
            {
                printf("%d", preceding_element);
            }
        }
    
    
    
    
        return EXIT_SUCCESS;
    }
    
    
    int get_preceding(int arr[], size_t arr_size, int element)
    {
    
        /* These asserts will automatically go when NDEBUG is defined */
    
        assert(arr != NULL);
        assert((element - 1) >= 0);
        assert(element < arr_size);
    
    
        /* This section has been manually designated to go 
            when NDEBUG is defined via the line below (finishes at #endif) */
    
    #ifndef NDEBUG
    
    
        // Once the program is running, I won't care about this...
        printf("(%d) ", arr[element - 1]);
    
    
    #endif
    
    
        return arr[element - 1];
    }
    When you are confident that the program is working, you can define NDEBUG and all the asserts are optimised out of your code.

    I like to make all my debug stuff around the define of NDEBUG -> That way it fits in nicely with the assert library.

    [edit]
    I forgot to say
    #ifndef -> "if not defined"
    [/edit]

    [edit]
    I reread the post and decided that it needed a few comments
    [/edit]
    Last edited by Click_here; 01-21-2013 at 07:38 PM.
    stahta01 likes this.
    Fact - Beethoven wrote his first symphony in C

Popular pages Recent additions subscribe to a feed

Similar Threads

  1. Help with #ifdef DEBUG #endif
    By lefti in forum C Programming
    Replies: 3
    Last Post: 01-09-2012, 06:26 PM
  2. #ifdef DEBUG
    By lehe in forum C++ Programming
    Replies: 4
    Last Post: 06-09-2009, 03:26 PM
  3. #ifdef - Can It Be Used With Logic Such as OR / AND?
    By dedham_ma_man in forum C Programming
    Replies: 3
    Last Post: 04-21-2006, 02:57 PM
  4. #ifdef and so on.
    By MipZhaP in forum C++ Programming
    Replies: 3
    Last Post: 02-16-2005, 04:34 PM
  5. How To use #ifdef ?
    By GaPe in forum Windows Programming
    Replies: 1
    Last Post: 11-01-2003, 11:57 AM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21