Memory allocation for structures

This is a discussion on Memory allocation for structures within the C Programming forums, part of the General Programming Boards category; Why do i get a Seg Fault if I run: Code: #include <stdio.h> struct rec { int i; float f; ...

  1. #1
    Comment your source code! Lynux-Penguin's Avatar
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    Memory allocation for structures

    Why do i get a Seg Fault if I run:
    Code:
    #include <stdio.h>
    
    struct rec
    {
        int i;
        float f;
        char c;
    };
    
    int main()
    {
        struct rec *p;
        
        (*p).i=10;
        (*p).f=3.14;
        (*p).c='a';
        printf("%d %f %c\n",(*p).i,(*p).f,(*p).c);
        free(p);
        return 0;
    }
    but not
    Code:
    #include <stdio.h>
    
    struct rec
    {
        int i;
        float f;
        char c;
    };
    
    int main()
    {
        struct rec *p;
        p=(struct rec *) malloc (sizeof(struct rec));
        (*p).i=10;
        (*p).f=3.14;
        (*p).c='a';
        printf("%d %f %c\n",(*p).i,(*p).f,(*p).c);
        free(p);
        return 0;
    }
    can someone explain why exactly.
    and do I always have to do p=(struct rec *) malloc(sizeof(struct rec)); ?
    Asking the right question is sometimes more important than knowing the answer.
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  2. #2
    and the hat of wrongness Salem's Avatar
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    struct rec *p;
    (*p).i=10;
    You have absolutely no idea where you just tried to write that integer - usually somewhere unpleasant, which is why the OS steps in with a seg fault.
    If you're unfortunate, it will work, thus suckering you into thinking your program is correct, when in fact it isn't.


    struct rec *p;
    p=(struct rec *) malloc (sizeof(struct rec));
    (*p).i=10;
    This on the other hand is better - you know exactly where that int is going - into the memory returned by malloc

    > and do I always have to do p=(struct rec *) malloc(sizeof(struct rec)); ?
    You've always got to initialise your pointers, and malloc is a good way to do it

    A less useful way is
    struct rec tmp;
    struct rec *p = &tmp;

  3. #3
    End Of Line Hammer's Avatar
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    Originally posted by Salem
    struct rec *p;
    p=(struct rec *) malloc (sizeof(struct rec));
    (*p).i=10;
    This on the other hand is better - you know exactly where that int is going - into the memory returned by malloc
    Not necessarily better tho if malloc fails. Best do some error checking before using that pointer!

    Code:
    if ((p = (struct rec *) malloc (sizeof(struct rec))) == NULL)
    {
    /* malloc has failed, bail out or somthing */
    }
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

  4. #4
    Comment your source code! Lynux-Penguin's Avatar
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    thanks
    yeah, I usually have ERROR checks in my programs
    I am going to write a new function
    void bail_out(char* error[]);
    Asking the right question is sometimes more important than knowing the answer.
    Please read the FAQ
    C Reference Card (A MUST!)
    Pointers and Memory
    The Essentials
    CString lib

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