size of the void *

Hi,

I have tried following program and the output is 1. Does this mean by default void * can be treated as char *.

Code:

---

#include<stdio.h>
int main()
{
        void *ptr;
        int i;
        i = sizeof(*ptr);
        printf("%d",i);
        return 0;
}

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Refer to the C standard:

Quote:

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Originally Posted by C99 Clause 6.2.5 Paragraph 19

The void type comprises an empty set of values; it is an incomplete type that cannot be completed.

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Quote:

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Originally Posted by C99 Clause 6.5.3.4 Paragraph 1

The sizeof operator shall not be applied to an expression that has function type or an incomplete type, to the parenthesized name of such a type, or to an expression that designates a bit-field member.

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This is unlikely to have changed in C11, so I would conclude that you are observing the effects of undefined behaviour: the compiler could have rightfully refused to compile your program since the type of *ptr is void.

What platform/compiler are you using?

If it's undefined behavior than it doesn't matter what the compiler is being used.

Anything that is "Undefined behavior" should never be used in ANY code. Standard C code should be portable to any standards compliant compiler. Undefined behavior is NOT.

As Henny Youngman would say, "Patient: Doctor, Doctor! It hurts when I do this. Doctor: Don't do it!"

Quote:

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Originally Posted by rstanley

If it's undefined behavior than it doesn't matter what the compiler is being used.

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I was asking so that I could provide compiler command options to turn on higher warning/error levels. Don't make assumptions. They'll get you in trouble.

Quote:

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Originally Posted by Elkvis

I was asking so that I could provide compiler command options to turn on higher warning/error levels. Don't make assumptions. They'll get you in trouble.

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You didn't say that. I was following the full thread, and responding to the full thread, not just your question.

Quote:

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Originally Posted by rstanley

You didn't say that. I was following the full thread, and responding to the full thread, not just your question.

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To be fair, I didn't give any reason for my asking. You made an assumption, and you were wrong.

compiler is not even giving warning also regarding sizeof on a incomplete pointer(void *). I am using gcc version 4.6.3. Is there any issue with compiler installation?

Code:

---

#include<stdio.h>
int main()
{
        void *ptr;
        int i;
        i = sizeof(*ptr);
        printf("%d",i);
        return 0;
}

---

It is probably fairer to say it is a concern with quality of implementation of the compiler (i.e. what diagnostics it produces) rather than compiler installation.

sizeof is rather unique in C as it does not evaluate its operand (it only evaluates the size of the result, should the operand be evaluated). You are playing with one edge case which, in practice, few developers would bother to tickle. Such edge cases tend to be fertile ground for finding unexpected or invalid behaviour from compilers.