Using realloc to make memory smaller?

This is a discussion on Using realloc to make memory smaller? within the C Programming forums, part of the General Programming Boards category; Suppose you've allocated a certain number of bytes but later find that you want to add values to it. You ...

  1. #1
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    Red face Using realloc to make memory smaller?

    Suppose you've allocated a certain number of bytes but later find that you want to add values to it. You could copy everything into a larger memory area, which is inefficient, or you can allocate more bytes using realloc, without losing your data. So far so good...

    But does this also means that I can make the memory area smaller? In my app I allocate a BIG area of memory (to be safe) because I don't know how much I need. After this area is used I want to release the memory I don't need. Is that possible with realloc?

    Example: My memory area is 10000 bytes. I only use for example 1000 bytes. I want to release 9000 bytes.
    You cantīt teach an old dog new tricks.

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    Guest Sebastiani's Avatar
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    Absolutely you can.

    A better idea though, perhaps is just to keep up with the larger chunk once it reaches that size. Just double the size to grow it.

    It is of course possible to do your own memory management off a single buffer which is nice since you can run things your own way. It's interesting how fun this actually can be. No doubt a good way to experience what it must be like to write a memory manager for an operating system or something. My program featured little self-destructive pointers too that when nulled
    otherwise altered would deallocate the memory automatically:

    void track(void** a)
    store address 'a'
    store pointer to data 'd' (*a)
    if(*a != d ) deallocate

    basically. if you call realloc a lot you might fragment your memory space so much that it wouldn't even be worth it. Just reuse large buffers if necessary.



    ITSA
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    But does this also means that I can make the memory area smaller? In my app I allocate a BIG area of memory (to be safe) because I don't know how much I need. After this area is used I want to release the memory I don't need. Is that possible with realloc?
    Yes ,Realloc can be used to make memory area smaller.

    But Realloc dosen't free the memory that was allocated earlier.
    Memory can only be freed using free.
    "There is no alternative to consitency..."

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    Originally posted by gotyatech
    Yes ,Realloc can be used to make memory area smaller.

    But Realloc dosen't free the memory that was allocated earlier.
    Memory can only be freed using free.
    But what's the point then to use realloc? Must I use both realloc and free?

    Maybe it's better to allocate a BIG area, and after it's used I move the bytes to a new area that is exactly the size I want or?
    You cantīt teach an old dog new tricks.

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    Ya ur point seems to be vaild.

    realloc also does free....i checked it.

    check the following link u can find source code for realloc

    http://www.lua.org/source/src_lmem.c.html
    "There is no alternative to consitency..."

  6. #6
    Me want cookie! Monster's Avatar
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    A common mistake using realloc is that people think realloc always free the old memory block. This is not always true:
    Code:
    char *t;
    
    if((t = malloc(1024)) == NULL)
    {
       /* error handling */
    }
    
    if((t = realloc(t, 1024 * 2)) == NULL)
    {
       /* damn, realloc failed but old memory chunk is still allocated */
       /* and I have nothing pointing to it -> memory leak */
    }
    Better to do it this way:
    Code:
    char *t;
    char *tmp;
    
    if((t = malloc(1024)) == NULL)
    {
       /* error handling */
    }
    
    if((tmp = realloc(t, 1024 * 2)) == NULL)
    {
       /* t is still pointing to 1024 bytes allocated memory */
    }else
    {
       /* t is now pointing to nothing and tmp to 2048 bytes allocated memory */
    }

  7. #7
    Registered User Cela's Avatar
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    >>Memory can only be freed using free.
    Not true, realloc() can be used in place of both malloc() and free(). If you pass a null pointer and a nonzero size then it acts like malloc, if you pass a nonnull pointer and a zero size then it frees, otherwise it resizes the block you pass it according to the new size and maybe relocates to a new block. :-)
    *Cela*

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