malloc for structs and arrays

This is a discussion on malloc for structs and arrays within the C Programming forums, part of the General Programming Boards category; Hey guys, First of all I know there have been previous threads like this and that there's also a faq ...

  1. #1
    1ST » R. vd Kooij
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    malloc for structs and arrays

    Hey guys,

    First of all I know there have been previous threads like this and that there's also a faq item about this.
    But I am unable to apply any of those examples to my problem, simply because I haven't done this before and I dont understand the instructions I've read. Therefore I hope you can help me with this specific problem.

    Here's my problem:
    I have a LINE_COUNT defined as 10000:

    Code:
    #define LINE_COUNT                    10000
    And I have some structs and arrays defined that use this LINE_COUNT value:
    Code:
       char buf[BUFSIZ], id_file[256], id_file_array[LINE_COUNT][30], id_array[LINE_COUNT][30];
    
       struct CAN_line
       {
          int cnt;
          int time;
          int identifier;
          int datalength;
          int db[8];
       };
       struct CAN_line linenr[LINE_COUNT];
    Now when I increase the LINE_COUNT to say 11000 the program hangs. I figured it would have to do with memory available and came across the malloc function as a possible solution for this.
    My question is, how should I use malloc in this situation?
    Do I request memory for every array and the struct that use this LINE_COUNT? Or maybe I can simply request an amount of space when I launch the program?
    Anyway, I have tried the following line but I haven't got a clue if it's right or not and when it is I wouldn't even know how to check if it actually works...

    Code:
    q = (char *) malloc(2 * sizeof(struct CAN_line));
    ...
    free(q);
    Sorry for the noob level of this question It's just so not clear for me how this works.

    Any help would be greatly appreciated!

    René
    http://www.f1rstracing.nl/
    OS: Windows XP
    Compiler: Dev-C++

  2. #2
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    malloc will return a NULL pointer if the size of the block to be allocated is zero, or if there was insufficient memory

  3. #3
    Just Lurking Dave_Sinkula's Avatar
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    Quote Originally Posted by rkooij
    My question is, how should I use malloc in this situation?
    Do I request memory for every array and the struct that use this LINE_COUNT?
    Yes. Something a bit like this.
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    
    #define LINE_COUNT 10000
    
    struct CAN_line
    {
       int cnt;
       int time;
       int identifier;
       int datalength;
       int db[8];
    };
    
    int main()
    {
       char buf[BUFSIZ], id_file[256];
       char **id_file_array;
       char **id_array;
       struct CAN_line *linenr;
       int i;
    
       id_file_array = malloc(LINE_COUNT * sizeof *id_file_array);
       if ( id_file_array != NULL )
       {
          for ( i = 0; i < LINE_COUNT; ++i )
          {
             id_file_array[i] = malloc(30 * sizeof *id_file_array[i]);
             if ( id_file_array[i] == NULL )
             {
                /* ADD CLEANUP */
                return 0;
             }
          }
       }
    
       id_array = malloc(LINE_COUNT * sizeof *id_array);
       if ( id_array != NULL )
       {
          for ( i = 0; i < LINE_COUNT; ++i )
          {
             id_array[i] = malloc(30 * sizeof *id_array[i]);
             if ( id_array[i] == NULL )
             {
                /* ADD CLEANUP */
                return 0;
             }
          }
       }
    
       linenr = malloc(LINE_COUNT * sizeof *linenr);
       if ( linenr == NULL )
       {
          /* ADD CLEANUP */
          return 0;
       }
    
       /* ... */
    
       for ( i = 0; i < LINE_COUNT; ++i )
       {
          free(id_file_array[i]);
          free(id_array[i]);
       }
       free(id_file_array);
       free(id_array);
       free(linenr);
       return 0;
    }
    Quote Originally Posted by rkooij
    Or maybe I can simply request an amount of space when I launch the program?
    Possibly. Are these variable declared within a function? Could they instead be global?
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  4. #4
    1ST » R. vd Kooij
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    Thank you very much for the example. I have tried it but I get an error message:

    invalid conversion from `void*' to `char**'
    For example on this line:

    Code:
       id_file_array = malloc(LINE_COUNT * sizeof *id_file_array);
    Have I done something wrong?

    Thanks again, René
    http://www.f1rstracing.nl/
    OS: Windows XP
    Compiler: Dev-C++

  5. #5
    ZuK
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    Looks like you try to compile this c-code as c++.
    In C a void* is compatible with any pointertype.
    Kurt

  6. #6
    1ST » R. vd Kooij
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    Quote Originally Posted by ZuK
    Looks like you try to compile this c-code as c++.
    In C a void* is compatible with any pointertype.
    Kurt
    Ok, I changed to a C file but now I get looooads of other errors... is 'bool' as a definition not C compatible? What can I do about that?

    syntax error before "exit_flag"
    Code:
    bool exit_flag=FALSE, error_flag=FALSE, file_flag=FALSE, capture_flag=FALSE;
    I use Dev-C++ 4.9.9.2 by the way...

    Thanks!
    http://www.f1rstracing.nl/
    OS: Windows XP
    Compiler: Dev-C++

  7. #7
    ZuK
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    no there is no bool in c.
    if you want to compile your code as c++ you have to use new instead of malloc ( the preferred way ) or cast the return of malloc to the right type.
    but if it's just the bool that makes troubles you could simply put a
    Code:
    typedef int bool;
    in your code
    Kurt

  8. #8
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    Quote Originally Posted by ZuK
    no there is no bool in c.
    if you want to compile your code as c++ you have to use new instead of malloc ( the preferred way ) or cast the return of malloc to the right type.
    but if it's just the bool that makes troubles you could simply put a
    Code:
    typedef int bool;
    in your code
    Kurt
    or if you have a c99 compatible compiler

    #include <stdbool.h>


    int main(void)
    {
    bool b = false;

    return 0;
    }

  9. #9
    1ST » R. vd Kooij
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    Dave_sinkula thanks so much for the example and the rest of you guys for the tips regarding boolean. I used Laserve's tip to include stdbool.h and it's all working perfectly fine now!
    Have on question left though... in the example I see the comment line:
    Code:
    /* ADD CLEANUP */
    What exactly is meant with that line? I dont have a clue what to add...

    Thanks again, this was the only thing left for me to create a stable base for my program.
    http://www.f1rstracing.nl/
    OS: Windows XP
    Compiler: Dev-C++

  10. #10
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    For every address returned by malloc :
    In order to free memory use the function free().

    like
    Code:
    char  *p;
    
    if ( ! ( p = malloc(20)) )
       {
       printf("malloc error\n");
       return;
       }
    
    /* do things with p */
    
    
    free(p); /* Let the program reuse this memory space
                     for other malloc calls */

  11. #11
    1ST » R. vd Kooij
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    Quote Originally Posted by dude543
    For every address returned by malloc :
    In order to free memory use the function free().

    like
    Code:
    char  *p;
    
    if ( ! ( p = malloc(20)) )
       {
       printf("malloc error\n");
       return;
       }
    
    /* do things with p */
    
    
    free(p); /* Let the program reuse this memory space
                     for other malloc calls */
    Hehe, errmm sorry... is that an answer to the question in my last post?
    http://www.f1rstracing.nl/
    OS: Windows XP
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  12. #12
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    Yes. Remember to /*add cleanup*/ by free()-ing all your memory when you're done.
    Code:
    #include <stdio.h>
    
    void J(char*a){int f,i=0,c='1';for(;a[i]!='0';++i)if(i==81){
    puts(a);return;}for(;c<='9';++c){for(f=0;f<9;++f)if(a[i-i%27+i%9
    /3*3+f/3*9+f%3]==c||a[i%9+f*9]==c||a[i-i%9+f]==c)goto e;a[i]=c;J(a);a[i]
    ='0';e:;}}int main(int c,char**v){int t=0;if(c>1){for(;v[1][
    t];++t);if(t==81){J(v[1]);return 0;}}puts("sudoku [0-9]{81}");return 1;}

  13. #13
    1ST » R. vd Kooij
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    Quote Originally Posted by jafet
    Yes. Remember to /*add cleanup*/ by free()-ing all your memory when you're done.
    Sorry guys, I still don't understand what to add. The memory request works perfectly as it is now, and I use free() at the end of the function to clear the memory space. I can see that it works in the task manager.
    What do you mean with cleanup? Is there something I forget or is the above sufficient for the right functionality of my program?

    Edit: hmmmm, do you mean I need to free the memory there as well...? So when "if ( id_file_array[i] == NULL )" is true, that means the memory allocation failed?

    Sorry, for the noobness
    Last edited by rkooij; 05-03-2006 at 01:35 AM.
    http://www.f1rstracing.nl/
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  14. #14
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    You've added the free() already? Well... pat yourself on the back, that's whap "cleanup" means
    Code:
    #include <stdio.h>
    
    void J(char*a){int f,i=0,c='1';for(;a[i]!='0';++i)if(i==81){
    puts(a);return;}for(;c<='9';++c){for(f=0;f<9;++f)if(a[i-i%27+i%9
    /3*3+f/3*9+f%3]==c||a[i%9+f*9]==c||a[i-i%9+f]==c)goto e;a[i]=c;J(a);a[i]
    ='0';e:;}}int main(int c,char**v){int t=0;if(c>1){for(;v[1][
    t];++t);if(t==81){J(v[1]);return 0;}}puts("sudoku [0-9]{81}");return 1;}

  15. #15
    1ST » R. vd Kooij
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    Quote Originally Posted by jafet
    You've added the free() already? Well... pat yourself on the back, that's whap "cleanup" means
    Hahahaha! Yay!
    I haven't added free() on that line but at the end of the function, my guess according to the above is that I have to put it at the /* Add Cleanup */ line as well
    Thanks for the help
    http://www.f1rstracing.nl/
    OS: Windows XP
    Compiler: Dev-C++

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