Why use calloc()?

This is a discussion on Why use calloc()? within the C Programming forums, part of the General Programming Boards category; I was wondering what calloc() is good for (besides setting everything to zero). As far as I can see, Code: ...

  1. #1
    Frequently Quite Prolix dwks's Avatar
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    Why use calloc()?

    I was wondering what calloc() is good for (besides setting everything to zero). As far as I can see,
    Code:
    calloc(x, y);
    is the same as
    Code:
    malloc(x*y);
    (Except for calloc setting the allocated memory to zero.)

    So what is calloc() good for?
    dwk

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  2. #2
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    Code:
    So what is calloc() good for?
    Like you said, for initializing the allocated memory to 0...

    If you wanted to you could live your entire life without using calloc() by just doing:
    Code:
    void *ptr = malloc(SIZE);
    memset(ptr, 0, SIZE);
    ...instead.
    If you understand what you're doing, you're not learning anything.

  3. #3
    Frequently Quite Prolix dwks's Avatar
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    That's what I thought . . . .
    dwk

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    On some systems the calloc call could be optimized better for getting arrays of zeroed bytes. The system can keep a list of zeroed out memory and non-zeroed memory so calloc can look up memory in the zero pool while malloc returns pointers to the other pool and the memset is extra overhead that was already done on an idle moment for the zero pool.

    Windows does that for instance. When the system is idle it starts the zero page thread. I don't know about calloc implementations that make use of it thuogh.

  5. #5
    and the hat of wrongness Salem's Avatar
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    Last edited by Salem; 07-20-2005 at 05:28 AM. Reason: typo
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  6. #6
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    So it initialises to zero not NULL?

  7. #7
    and the hat of wrongness Salem's Avatar
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    > So it initialises to zero not NULL?
    Yes, calloc is nothing more than malloc + memset - the result is a block of memory with all bits set to 0.

    But nothing in C insists that NULL (as a pointer) has to be represented by all-bits-zero
    http://www.eskimo.com/~scs/C-faq/s5.html

    void *p = NULL;
    if ( p == NULL )
    if ( p == 0 )
    These will always be true, whether your machine uses all-bits-zero for a NULL pointer or not.

    void *p; memset( &p, 0, sizeof(void*) ); // just what calloc would do
    if ( p == NULL )
    if ( p == 0 )
    These will work on some machines, and not on others.
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  8. #8
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    Quote Originally Posted by Salem
    void *p; memset( &p, 0, sizeof(void*) ); // just what calloc would do
    if ( p == NULL )
    if ( p == 0 )
    These will work on some machines, and not on others.
    Do you know of any system where this is true btw? I'm not questioning the portability issue, just curious.

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