malloc and realloc

This is a discussion on malloc and realloc within the C Programming forums, part of the General Programming Boards category; Hi All, Could anyone tell me the difference between memory allocation scheme used by malloc and realloc. I would like ...

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    malloc and realloc

    Hi All,

    Could anyone tell me the difference between memory allocation scheme used by malloc and realloc. I would like to know how realloc reasssigns the memory to a pointer variable. In other words, what will happen if the availabe memory is not contiguous?

    What will be the situation with malloc if the available memory is not contiguous?

    Finally, is the memory allocated by calloc is contiguous always?


    Please answer to this.

    Thanks in advance.

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    Just Lurking Dave_Sinkula's Avatar
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    malloc will assign the amount of memory that you need like void *malloc(size_t n) and returns NULL if there is not enought mem for the req. void * malloc is a great way to have a pointer to the array when you dont know what type the pointer should be. Where realloc (which I think is still just used in boreal C and not in ANSI) will in real time re reallocate memory(increasing and decreasing), thus as the program is running you can increase the size of the memory set aside but you could run into issues if you went to increase the memory to a point where memory is allready in use and thus you could cause A. Program to crash, B. delete values to other arrays, C. Unpredictable results. From the lectures I have sat in, it would be better to use pointers to set aside memory.
    Last edited by KoG Metalgod; 01-05-2007 at 07:13 AM.
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    ATH0 quzah's Avatar
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    Quote Originally Posted by KoG Metalgod
    Where realloc (which I think is still just used in boreal C and not in ANSI) will in real time re reallocate memory(increasing and decreasing), thus as the program is running you can increase the size of the memory set aside.
    realloc is an ANSI C function, and as best I recall, always has been.


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    CSharpener vart's Avatar
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    Quote Originally Posted by KoG Metalgod
    thus as the program is running you can increase the size of the memory set aside but you could run into issues if you went to increase the memory to a point where memory is allready in use and thus you could cause A. Program to crash, B. delete values to other arrays, C. Unpredictable results. From the lectures I have sat in, it would be better to use pointers to set aside memory.
    __________________
    if the current regeon cannot be increased according to request, realloca will allocate the required amount of the memory in another plaace, copy the source data there and free the source.
    if the new regeon cannot be allocated the realloc returns NULL exactly as malloc does.
    So the program can crash only if it does not check the return value of the realloc (but the same issue is with malloc)

    It is impossible to get the memory overrun due to inproper reallocation. Only due to not checking bouds of the allocated memory. But it also the same issue as by using malloc. So I'm not really understand what you intended to say here
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