urgent regarding malloc()

This is a discussion on urgent regarding malloc() within the C Programming forums, part of the General Programming Boards category; Code: #include<stdio.h> #include<conio.h> #include<malloc.h> struct list{ int a; struct list *ptr; }; void main(void) { typedef struct list *nn ; ...

  1. #1
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    urgent regarding malloc()

    Code:
    #include<stdio.h>
    #include<conio.h>
    #include<malloc.h>
    struct list{
    int a;
    struct list *ptr;
    };
    void main(void)
    {
    typedef struct list *nn ;
    nn node;
    printf("\t%d",sizeof(nn));
    node=(nn)malloc(sizeof(nn));
    printf("\tenter node\n");
    scanf("%d",node->a);
    printf("\t%d",node->a);
    node->ptr=NULL;
    free(node);
    getch();
    }
    variable a is input from the user
    but it gives a garbage value as output.
    Last edited by samirself; 01-12-2005 at 08:59 AM. Reason: mistake

  2. #2
    Gawking at stupidity
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    A good reason to argue that hiding pointers is a bad idea:
    Code:
    node=(nn)malloc(sizeof(nn));
    You're only allocating enough memory for a pointer.

    There's a zillion other things wrong with that surprisingly short program that I'm not even going to get into. I don't know if I could throw that many bad habits into such a short program even if I tried.
    If you understand what you're doing, you're not learning anything.

  3. #3
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    plz write it in a corect manner,if possible,
    thanks

  4. #4
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    Sure, why not?
    Code:
    int main(void)
    Take good note.
    Code:
    int main(void)
    {
    	typedef struct list* nn;
    	nn node;
    	printf("\t%d",sizeof(struct list));
    	node=(nn)malloc(sizeof(struct list));
    	printf("\tenter node\n");
    	scanf("%d",&node->a);
    	printf("\t%d",node->a);
    	node->ptr=NULL;
    	free(node);
    	getch();
    	return 0;
    }
    Indentation is nice. If you really want to keep the pointer hidden, you'll need the size of the struct, not the pointer to the struct. In addition, scanf will need an address of a variable, or else it'll crash.
    If I did your homework for you, then you might pass your class without learning how to write a program like this. Then you might graduate and get your degree without learning how to write a program like this. You might become a professional programmer without knowing how to write a program like this. Someday you might work on a project with me without knowing how to write a program like this. Then I would have to do you serious bodily harm. - Jack Klein

  5. #5
    and the hat of wrongness Salem's Avatar
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    > node=(nn)malloc(sizeof(struct list));
    I'd prefer removing the cast

    Code:
    node=malloc( sizeof *node );
    Plus I notice the OP used #include<malloc.h> instead of #include<stdlib.h> which makes me suspicious that they're using some brain-dead fossil compiler.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  6. #6
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    >I'd prefer removing the cast
    Good call. I'm too used to C++, which would complain about that. Also, I wasn't sure if it was safe to dereference node since it hadn't been allocated any memory.
    If I did your homework for you, then you might pass your class without learning how to write a program like this. Then you might graduate and get your degree without learning how to write a program like this. You might become a professional programmer without knowing how to write a program like this. Someday you might work on a project with me without knowing how to write a program like this. Then I would have to do you serious bodily harm. - Jack Klein

  7. #7
    and the hat of wrongness Salem's Avatar
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    Despite what it looks like, sizeof *node is NOT a dereference of the pointer.

    sizeof is compile-time, and the only "dereferencing" is done on a type basis. So what you get is the size of the type which a pointer points to. This can all be done without any reference to actual memory locations - it's just a traversal of the compiler's internal type tables.

    Oh, and one more thing - the return result of malloc needs to be checked (for being not NULL) before it is dereferenced.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  8. #8
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    Good to know. Thanks a lot, Salem. Useful as usual.
    If I did your homework for you, then you might pass your class without learning how to write a program like this. Then you might graduate and get your degree without learning how to write a program like this. You might become a professional programmer without knowing how to write a program like this. Someday you might work on a project with me without knowing how to write a program like this. Then I would have to do you serious bodily harm. - Jack Klein

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