sizeof always returns 4.

This is a discussion on sizeof always returns 4. within the C Programming forums, part of the General Programming Boards category; I'm trying to get to grips with malloc however the sizeof function - no matter what I do, always returns ...

  1. #1
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    Unhappy sizeof always returns 4.

    I'm trying to get to grips with malloc however the sizeof function - no matter what I do, always returns 4. Can someone explain to me what is up?

    Code:
    #include <stdlib.h>
    #include <stdio.h>
    
    int main()
    {
        int *ptr = malloc(sizeof(int) * 10);
        int len = sizeof(ptr);
        printf("Len: %i\n", len);
    }

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    You are getting the size of the pointer, most likely.

  3. #3
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    Why don't you check out a char, a char array with 20 elements, a double, a long int, etc. ?

    Two quantities being the same, does not an "always" make.

    And as noted above, you're only showing the size of the pointer.

  4. #4
    Woof, woof! zacs7's Avatar
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    Quote Originally Posted by Ploe View Post
    I'm trying to get to grips with malloc however the sizeof function - no matter what I do, always returns 4. Can someone explain to me what is up?
    sizeof is an operator, not a function. Secondly, C does not automatically maintain any size information at runtime for you. sizeof is done at compile time, it's not going to de-reference your pointer and work out how many bytes you asked to be allocated.

    All that it has told you is, sizeof(int *) is 4 for your machine.

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    So at runtime I'd need some form of variable as a reference for the size of the array? I didn't realise that sizeof was an operator, however that would explain why Code::Blocks was all neat and changed its colour.

    Thanks.

  6. #6
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by Ploe
    So at runtime I'd need some form of variable as a reference for the size of the array?
    Yes. Alternatively, you designate some special value to serve as the "last" value, e.g., how the null character in a string is used. Passing the size tends to be less error prone though.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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