copy struct contents

This is a discussion on copy struct contents within the C Programming forums, part of the General Programming Boards category; hi, How can I write a function to copy the contents of struct first to second ? Code: typedef struct ...

  1. #1
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    copy struct contents

    hi,
    How can I write a function to copy the contents of struct first to second ?
    Code:
    typedef struct copyone{ char key[32];char value[32];}copyone,copytwo;
    typedef struct first{ char data[83];copyone *ptrone; } first;
    
    typedef struct second{ char name[83];copytwo *ptrtwo; } second;

  2. #2
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    Eh? Why are you typedefing the same type twice? (copyone and copytwo). and then defining the same type twice (first, second).

    What have you tried so far?

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    hi,
    So far I am doing this to copy the contents(which is working just fine), each time I need the values of the "first" renewed.

    Code:
    n = malloc(sizeof(second));
    p = malloc(sizeof(first));
     for(i=0; i < count ; i++) {
            strcpy(n,p);
            strcpy(n->other[i].key,p->main[i].key);
            strcpy(n->other[i].value,p->main[i].value);
         }
    But I want to put the above code in a function that accepts any struct and copies the contents into "first" struct .

  4. #4
    C++ Witch laserlight's Avatar
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    The use of strcpy() does not look correct here. However, I think that you are making some more fundamental mistake.

    Instead of using meta-syntactic names like first and second, give the real names that you are using in the program, and tell us what you are trying to do. (As in the goal of this part of the program, not the implementation detail like "copy the contents of struct first to second".)
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    Well...
    I meant to say copy the contents of the second struct to the first.
    There was a hurried typo.

  6. #6
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by bandal27
    I meant to say copy the contents of the second struct to the first.
    Well, it really depends. Consider this example program:
    Code:
    #include <stdio.h>
    
    typedef struct MappedType
    {
        char key[32];
        char value[32];
    } MappedType;
    
    int main()
    {
        MappedType x = {"ball", "red"};
        MappedType y;
        y = x; /* Copy the contents of x to y. */
        printf("%s %s\n", y.key, y.value);
        return 0;
    }
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  7. #7
    cus
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    this isn't in a funtion

    This isn't in a function - but the result is that the contents of one struct are copied into a second.

    How to copy one struct in C to another?

  8. #8
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by cus
    This isn't in a function - but the result is that the contents of one struct are copied into a second.
    That would be using an assignment operator, in particular, copy assignment (also known as simple assignment). I demonstrated it in my previous post.
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  9. #9
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    Quote Originally Posted by laserlight View Post
    Well, it really depends. Consider this example program:
    Code:
    #include <stdio.h>
    
    typedef struct MappedType
    {
        char key[32];
        char value[32];
    } MappedType;
    
    int main()
    {
        MappedType x = {"ball", "red"};
        MappedType y;
        y = x; /* Copy the contents of x to y. */
        printf("%s %s\n", y.key, y.value);
        return 0;
    }
    Laserlight,
    Can this be used for my kind of structs too...?

  10. #10
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by bandal27
    Can this be used for my kind of structs too...?
    What exactly are your kind of structs? If you are talking about a struct that has a pointer to another struct or an element of an array that should be deep copied, then no, simple assignment is not enough since it would assign only the address stored in the pointer without assigning the contents pointed to. For that you may wish to use memcpy().
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