Programming

we know that sizeof never returns zero when used with structure

then why in this case it is returning zero


struct foo
{
char c[];
};

void main()
{
struct foo f;
cout<<sizeof(f);
}

i am working on solaris 5.8
isn't the above function should return the size of empty structure

How is your program even compiling? I tried to complie the following program on HP-UX 11.11 and SunOS 5.8:

Code:

#include<stdio.h>
#include<unistd.h>

struct s {
        char sarr[];
};

int main() {
        int a;
        char c;
        struct s b;

        fprintf(stdout,"size of int: %d\n",sizeof(a));
        fprintf(stdout,"size of char: %d\n",sizeof(c));
        fprintf(stdout,"size of char arr: %d\n",sizeof(b));
}

And I am getting the following errors:
On HP:
cc: "t3.c", line 5: error 1578: Size of struct or union member is unknown.
cc: "t3.c", line 4: error 1613: Zero-sized struct.
cc: "t3.c", line 15: warning 504: The sizeof operator applied to a zero-sized object.
Note the errors in lines 4 and 5 above.

On Sun:
t3.c:5: error: flexible array member in otherwise empty struct

but i am getting the result as zero

my program is like this


struct foo
{
char c[];

};


void main()
{
struct foo s;
cout<<sizeof(s);
}



when i compile this and run on solaris 5.8 it gives the output as 0
i dont know why

there is no error in my compilation

I think this has to do with the fact that you are using C++. Cant test that now. Sorry.

All C and C++ compilers reject this code. Even g++ would not compile it until I added:
#include <iostream>
at that point I had to run it with:
./bogus ; sleep 10
to see the output. To fix that, I made this change:
cout << sizeof(s) << "\n" ;
Now that I can easily view the output, yes it is zero. I am a liitle surprised that it compiles at all. Using:
g++ -ansi -pedantic bogus.C -o bogus
I get a warning:
bogus.C:4 warning: ANSI C++ forbids zero-size array `c'

How many characters did you expect c[] to be sized for? 5? 17? 80? Is zero really that unexpected? I really cannot imagine what result you were hoping for...

man u havent under stand my problem

as you said that it return zero
the same result i too am getting

but i have read that sizeof can never ever return zero even if the class or structure is empty

if you remove the char from the structure the value of sizeof is 1
but when include the char[] then its value is zero
why?

thats was my question

My guess is that sizeof cannot work out the size of the struture and is passing back 0, representing a null or undetermined value or an error condition.

What does your documentation for sizeof say about error conditions/values from this function?