questions about realloc()

This is a discussion on questions about realloc() within the C Programming forums, part of the General Programming Boards category; Code: char *ptr1, *ptr2; ptr1 = (char *)malloc(m); ptr2 = ptr1; ptr2 = (char *)realloc(ptr1, n); // n > m ...

  1. #1
    Run! Forrest Gump! TalosChen's Avatar
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    questions about realloc()

    Code:
    char *ptr1, *ptr2;
    ptr1 = (char *)malloc(m);
    ptr2 = ptr1;
    ptr2 = (char *)realloc(ptr1, n); // n > m
    ptr2 could be different from ptr1 after realloc(), in that case, should I free(ptr1)?

  2. #2
    ATH0 quzah's Avatar
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    Yes. No. Read the FAQ on why you shouldn't typecast functions that return void pointers.


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    Hope is the first step on the road to disappointment.

  3. #3
    Run! Forrest Gump! TalosChen's Avatar
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    -> Yes. No.

    Is Yes said that ptr2 != ptr1 maybe true, and No said that I need not free(ptr1) if it's no longer used?

  4. #4
    and the hat of int overfl Salem's Avatar
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    > ptr2 could be different from ptr1 after realloc(), in that case, should I free(ptr1)?
    Only if ptr2 is NULL.

    If ptr2 isn't NULL, ptr1 has already been freed and the memory copied to where ptr2 now points.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  5. #5
    Run! Forrest Gump! TalosChen's Avatar
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    thanks.

    BTW, is there any function can figure out that if a pointer is allocated any memory?

  6. #6
    The Richness... Richie T's Avatar
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    Do you mean to check if allocation was successfull? the malloc
    set of functions return NULL if they fail, if its not equal to NULL,
    you can take it that allocation was successfull and the pointer
    points to some form of memory.
    No No's:
    fflush (stdin); gets (); void main ();


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  7. #7
    Run! Forrest Gump! TalosChen's Avatar
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    No, what I mean is, giving you a pointer ptr, then can you know that if the memory pointed by ptr was once allocated by using malloc(), realloc(), etc. ? For example,
    Code:
    char *ptr1 = &'a';
    char *ptr2 = malloc(20);
    My question is, is there any function can tell me that ptr1 just points to a single char 'a', while ptr2 points to a block of memory?

  8. #8
    The Richness... Richie T's Avatar
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    Well i asked google and came up with this. Naturally i can't stand
    over this because it's not my code/idea but it might help. Maybe
    one of the more experienced members here can shed more light

    [edit]
    The MSDN link at the bottom of that page recommends a function
    in malloc.h - this is outdated by about 20 years so there's
    probably a more modern way.
    [/edit]
    Last edited by Richie T; 05-10-2006 at 08:11 AM.
    No No's:
    fflush (stdin); gets (); void main ();


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  9. #9
    and the hat of int overfl Salem's Avatar
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    > then can you know that if the memory pointed by ptr was once allocated by using malloc(), realloc(), etc. ?
    No, there is no portable way to know this.
    Nor is there any portable way to know how much was allocated simply by looking at the pointer.
    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.

  10. #10
    Run! Forrest Gump! TalosChen's Avatar
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    Thanks any way.

  11. #11
    ATH0 quzah's Avatar
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    Quote Originally Posted by TalosChen
    No, what I mean is, giving you a pointer ptr, then can you know that if the memory pointed by ptr was once allocated by using malloc(), realloc(), etc. ? For example,
    Code:
    char *ptr1 = &'a';
    char *ptr2 = malloc(20);
    My question is, is there any function can tell me that ptr1 just points to a single char 'a', while ptr2 points to a block of memory?
    Your code won't compile. You cannot get the address of a character constant. It would be like getting the address of a number. You just can't do it.


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  12. #12
    Run! Forrest Gump! TalosChen's Avatar
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    :-) I know that. It's just an example to explain my question.

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