Pointers type casting

Question

The output of the below code is 0.0000. What is happening in the code and why the output is 0.0000 or to ask precisely what happens when we type cast the int* to float* and print the de-referenced value in the below code?

int main(){
   int i = 10;
   int *p = &i;
   printf("%f\n", *(float*)p);
   return 0;
}

If you wants to correct your code read Digital_Reality's answer. If you were expecting that your code should print `10.0..` then no it is undefined behaviour here is one more link to answer you [In C, if I cast & dereference a pointer, does it matter which one I do first?](http://stackoverflow.com/questions/4634252/in-c-if-i-cast-dereference-a-pointer-does-it-matter-which-one-i-do-first) — Grijesh Chauhan, Jan 01 '14 at 07:36
Also see http://floating-point-gui.de/formats/fp/ for an introduction to how floating point numbers are typically stored. (Actually, pretty much universally, but the C language does not mandate this.) — Christopher Creutzig, Jan 01 '14 at 08:48
This violates strict aliasing and invokes undefined behavior. — Siyuan Ren, Jan 01 '14 at 09:06

score 4 · Accepted Answer · edited Jan 01 '14 at 09:26

4

Did you mean?

printf("%f\n", (float)*p);

Edit: After you clarified your question.

There will be undefined behaviour if you try to convert Int* to float*. Pointer is a address, and int float are stored in memory in different format. so you can not cast it.

edited Jan 01 '14 at 09:26

Grijesh Chauhan

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answered Jan 01 '14 at 07:26

Digital_Reality

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I think yes OP should cast value instead of address that is undefined behaviour. – Grijesh Chauhan Jan 01 '14 at 07:29
I guess he does intend to ask what he has pasted. He has made it clear in his question too - `when we type cast the int* to float* and print the de-referenced value` – Suvarna Pattayil Jan 01 '14 at 07:30
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@GrijeshChauhan yes at least %f says so :) – Digital_Reality Jan 01 '14 at 07:30
I want to know what happens if we type cast the int* to float* ie typecasting the memory address and dereferencing the same and also is this kind of type casting common or where it will be used (use case if any) – Venkat Jan 01 '14 at 07:38
This question was asked to me in a interview:) – Venkat Jan 01 '14 at 07:39
@user3148581 read the answers I linked with your question. – Grijesh Chauhan Jan 01 '14 at 07:40
@GrijeshChauhan for float* to char* type conversion truncation may happen but int* to float* there is no truncation since they are of same sizes. so why iam not getting 10.0000 but 1.0000. My doubt may be so silly but bear with me:P – Venkat Jan 01 '14 at 07:45
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@user3148581 NO size may be different, it depends on platform thats why it is Undefined behavior – Grijesh Chauhan Jan 01 '14 at 07:47
@user3148581 Updated my answer after you clarified the question.. – Digital_Reality Jan 01 '14 at 08:38
and you can read link posted by @GrijeshChauhan for detailed answer! – Digital_Reality Jan 01 '14 at 08:46
thank you so much @Grijesh Chauhan Digital_Reality – Venkat Jan 01 '14 at 09:23

score 1 · Answer 2 · answered Jan 01 '14 at 07:29

1

The pointer always holds the memory address, which is either 16 bits or 32 bits. You are basically type casting the Memory address which is unusual behavior. And solution to your problem depends upon what are you trying to achieve.

answered Jan 01 '14 at 07:29

irshad.ahmad

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How is it unusual to cast an `A*` to a `B*`? – Christopher Creutzig Jan 01 '14 at 08:43

Pointers type casting

2 Answers2