structs of structs

This is a discussion on structs of structs within the C Programming forums, part of the General Programming Boards category; Hi All, Sorry - this is my second nested data structure post today Code: struct base { char *string; }; ...

  1. #1
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    structs of structs

    Hi All,

    Sorry - this is my second nested data structure post today

    Code:
    struct base
    {
            char *string;
    };
    
    struct parent
    {
            struct base my_base;
    };
    
    int main()
    {
            struct parent *my_parent;
            char temp_string[] = "temp string for test";
    
            my_parent->my_base.string = temp_string;
    
            printf("String = %s\n",my_parent->my_base.string);
    
            return 0;
    }
    I want to point the base char array at an existing string. Why does this cause problems?

    Thanks again,

    rotis23

  2. #2
    & the hat of GPL slaying Thantos's Avatar
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    What problem are you getting? It compiled and ran fine for me.

  3. #3
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    Segmentation fault on Linux.
    Last edited by rotis23; 09-06-2005 at 09:04 AM.

  4. #4
    Supermassive black hole cboard_member's Avatar
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    Did you allocate space for the char* in base?
    Good class architecture is not like a Swiss Army Knife; it should be more like a well balanced throwing knife.

    - Mike McShaffry

  5. #5
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    No - because it's pointing to an existing char* at the time of the printf. Maybe this is where I'm going wrong?

  6. #6
    Supermassive black hole cboard_member's Avatar
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    Hmm you never know. Thantos has a point though, some things - especially char* based I find - are a PITA under Linux. Not sure why.

    I'd just try calloc'ing some space first.
    Good class architecture is not like a Swiss Army Knife; it should be more like a well balanced throwing knife.

    - Mike McShaffry

  7. #7
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    OK - this works OK:

    Code:
    struct base
    {
            char *string;
    };
    
    struct parent
    {
            struct base my_base;
    };
    
    int main()
    {
            struct parent *my_parent;
            char *temp_string;
    
            temp_string = (char*)malloc(21 * sizeof(char));
            //strcpy(temp_string,"temp string for test");
    
            my_parent->my_base.string = temp_string;
            strcpy(temp_string,"temp string for test");
    
            printf("String = %s\n",my_parent->my_base.string);
    
            return 0;
    }
    I just want to know if the struct initialisation and the use of char pointers is OK like this.

  8. #8
    Just Lurking Dave_Sinkula's Avatar
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    Allocate space for the parent object.
    Code:
    struct parent *my_parent;
    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.*

  9. #9
    Supermassive black hole cboard_member's Avatar
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    Well I'm no guru but I know you shouldn't cast - it's evil and a shooting offense.

    Do this instead:

    Code:
    #include <stdlib.h>  /* include this along with others */
    
    int main ()
    {
    	char* string = calloc (sizeof(char), 20 * sizeof *string);
    	/* use */
    	free (string);
    	return 0;
    }
    Don't forget stdlib.h or it won't compile.
    Good class architecture is not like a Swiss Army Knife; it should be more like a well balanced throwing knife.

    - Mike McShaffry

  10. #10
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    Two issues;

    1) The primary cause of your crash is that my_parent in main() is not initialised to point at anything.

    2) You might also want to initialise the string member of base so it actually points at an array of char, and then use strcpy(my_parent->my_base.string, temp_string); instead of my_parent->my_base.string = temp_string; In this case, it's not strictly incorrect, but probably doesn't do everything you intend (in particular, the assignment does NOT create a copy of temp_string).

  11. #11
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    Yes - of course the problem was allocating space for the parent pointer - thanks Dave.

  12. #12
    Supermassive black hole cboard_member's Avatar
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    Damn I knew that.
    ...
    Thanks dave.
    Good class architecture is not like a Swiss Army Knife; it should be more like a well balanced throwing knife.

    - Mike McShaffry

  13. #13
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    And the correct way to allocate mem for a pointer to a struct is?

    Code:
    parent = (struct parent*)malloc(sizeof(struct parent*));
    Or should I be using calloc ahluka? I don't want to start the thread of death but is there any instance where a cast with malloc is considered correct/essential?

  14. #14
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    thanks Grumpy - Dave beat you to it slightly. I don't want to copy the string just point to it.

  15. #15
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    I can see that it needs space allocated but why does it compile fine for me as it is?
    Strange isn't it?

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