Recursive Union

[Girish]

I have 2 differesnt defination of same Union as below and a piece of
code for printing size of Union and its members..

union U
{
union U
{
int i;
int j;
}a[10];
int b[10];
}u;

union M
{
union N
{
int i;
int j;
}a[10];
int b[10];
}m;

void unionTest ()
{
printf("Union u\n");
printf("%d\n", sizeof(u));
printf("%d\n", sizeof(u.a)); // Its the cause for my headache...
printf("%d\n", sizeof(u.a[4].i));
printf("Union M\n");
printf("%d\n", sizeof(m));
printf("%d\n", sizeof(m.a));
printf("%d\n", sizeof(m.a[4].i));
}

For my surprise.. the output is
Union u
40
400
4
Union M
40
40
4

It will be complied if its a c file and Its ( redefination of Union U)
an error with c++ file..

Please let me know the reason for the out put ( 3rd printf statement)
And I obeserved that the memory layour for Union U is very peculiar!!
I have tried this with Microsoft Compiler..

[Jonathan Adams]

In article <88************ **************@ posting.google. com>,
gi***********@r ediffmail.com (Girish) wrote:

I have 2 differesnt defination of same Union as below and a piece of
code for printing size of Union and its members..

union U
{
union U
{
int i;
int j;
}a[10];
int b[10];
}u;
.... void unionTest ()
{
printf("Union u\n");
printf("%d\n", sizeof(u));
printf("%d\n", sizeof(u.a)); // Its the cause for my headache...
printf("%d\n", sizeof(u.a[4].i));
printf("Union M\n");
printf("%d\n", sizeof(m));
printf("%d\n", sizeof(m.a));
printf("%d\n", sizeof(m.a[4].i));
}

For my surprise.. the output is
Union u
40
400
4
Union M
40
40
4

I'm pretty sure you've just had an encounter with the wonderful world of
undefined behavior! Passing your code to the Sun C Compiler v5.5
results in:

"tmpc.c", line 6: (union) tag redeclared: U

and refuses to compile it in any way, shape, or form. gcc complains:

tmpc.c: In function `unionTest':
tmpc.c:28: union has no member named `i'

and (if I fix that line to let it compile) it outputs "40" for sizeof
(u.a).

In short: don't do that, then.

Cheers,
- jonathan

[Girish]

Jonathan Adams <jw*****@gmail. com> wrote in message news:<jw******* *************** *****@news1nwk. sfbay.sun.com>. ..

In article <88************ **************@ posting.google. com>,
gi***********@r ediffmail.com (Girish) wrote:

I have 2 differesnt defination of same Union as below and a piece of
code for printing size of Union and its members..

union U
{
union U
{
int i;
int j;
}a[10];

int b[10];

}u;

...

void unionTest ()
{
printf("Union u\n");
printf("%d\n", sizeof(u));
printf("%d\n", sizeof(u.a)); // Its the cause for my headache...
printf("%d\n", sizeof(u.a[4].i));
printf("Union M\n");
printf("%d\n", sizeof(m));
printf("%d\n", sizeof(m.a));
printf("%d\n", sizeof(m.a[4].i));
}

For my surprise.. the output is
Union u
40
400
4
Union M
40
40
4

I'm pretty sure you've just had an encounter with the wonderful world of
undefined behavior! Passing your code to the Sun C Compiler v5.5
results in:

"tmpc.c", line 6: (union) tag redeclared: U

and refuses to compile it in any way, shape, or form. gcc complains:

tmpc.c: In function `unionTest':
tmpc.c:28: union has no member named `i'

and (if I fix that line to let it compile) it outputs "40" for sizeof
(u.a).

In short: don't do that, then.

Cheers,
- jonathan

Hi Jonathan,

I have tried the same in Linux with CC compiler and it does not give
me any error for the Union Defination. But in the function, unionTest
(), it gives me compiler error for this statement.

printf("%d\n", sizeof(u.a[4].i));

Error: union has no member named `i'

If I comment out that statement, my code gets comiped and I get proper
output.

So the conclusion is, its a BUG in Windows compiler right?

Thanks a lot
Girish

[Jonathan Adams]

In article <88************ **************@ posting.google. com>,
gi***********@r ediffmail.com (Girish) wrote:

Jonathan Adams <jw*****@gmail. com> wrote in message
news:<jw******* *************** *****@news1nwk. sfbay.sun.com>. ..

In article <88************ **************@ posting.google. com>,
gi***********@r ediffmail.com (Girish) wrote:

I have 2 differesnt defination of same Union as below and a piece of
code for printing size of Union and its members..

union U
{
union U
{
int i;
int j;
}a[10]; int b[10];

}u;

...

void unionTest ()
{
printf("Union u\n");
printf("%d\n", sizeof(u));
printf("%d\n", sizeof(u.a)); // Its the cause for my headache...
printf("%d\n", sizeof(u.a[4].i));
printf("Union M\n");
printf("%d\n", sizeof(m));
printf("%d\n", sizeof(m.a));
printf("%d\n", sizeof(m.a[4].i));
}

For my surprise.. the output is
Union u
40
400
4
Union M
40
40
4

I'm pretty sure you've just had an encounter with the wonderful world of
undefined behavior! Passing your code to the Sun C Compiler v5.5
results in:

"tmpc.c", line 6: (union) tag redeclared: U

and refuses to compile it in any way, shape, or form. gcc complains:

tmpc.c: In function `unionTest':
tmpc.c:28: union has no member named `i'

and (if I fix that line to let it compile) it outputs "40" for sizeof
(u.a).

In short: don't do that, then.

Cheers,
- jonathan

Hi Jonathan,

I have tried the same in Linux with CC compiler and it does not give
me any error for the Union Defination. But in the function, unionTest
(), it gives me compiler error for this statement.

printf("%d\n", sizeof(u.a[4].i));

Error: union has no member named `i'

Which is exactly what I said above, after "gcc complains:".
If I comment out that statement, my code gets comiped and I get proper
output.

So the conclusion is, its a BUG in Windows compiler right?

No. The bug is in the code which is redefining a union while it is being
defined. Why are you doing that?

- jonathan

[Girish]

Jonathan Adams <jw*****@gmail. com> wrote in message news:<jw******* *************** *****@news1nwk. sfbay.sun.com>. ..

In article <88************ **************@ posting.google. com>,
gi***********@r ediffmail.com (Girish) wrote:

Jonathan Adams <jw*****@gmail. com> wrote in message
news:<jw******* *************** *****@news1nwk. sfbay.sun.com>. ..

In article <88************ **************@ posting.google. com>,
gi***********@r ediffmail.com (Girish) wrote:

> I have 2 differesnt defination of same Union as below and a piece of
> code for printing size of Union and its members..
>
> union U
> {
> union U
> {
> int i;
> int j;
> }a[10]; int b[10]; > }u;
> ... > void unionTest ()
> {
> printf("Union u\n");
> printf("%d\n", sizeof(u));
> printf("%d\n", sizeof(u.a)); // Its the cause for my headache...
> printf("%d\n", sizeof(u.a[4].i));
> printf("Union M\n");
> printf("%d\n", sizeof(m));
> printf("%d\n", sizeof(m.a));
> printf("%d\n", sizeof(m.a[4].i));
> }
>
> For my surprise.. the output is
> Union u
> 40
> 400
> 4
> Union M
> 40
> 40
> 4

I'm pretty sure you've just had an encounter with the wonderful world of
undefined behavior! Passing your code to the Sun C Compiler v5.5
results in:

"tmpc.c", line 6: (union) tag redeclared: U

and refuses to compile it in any way, shape, or form. gcc complains:

tmpc.c: In function `unionTest':
tmpc.c:28: union has no member named `i'

and (if I fix that line to let it compile) it outputs "40" for sizeof
(u.a).

In short: don't do that, then.

Cheers,
- jonathan

Hi Jonathan,

I have tried the same in Linux with CC compiler and it does not give
me any error for the Union Defination. But in the function, unionTest
(), it gives me compiler error for this statement.

printf("%d\n", sizeof(u.a[4].i));

Error: union has no member named `i'

Which is exactly what I said above, after "gcc complains:".

If I comment out that statement, my code gets comiped and I get proper
output.

So the conclusion is, its a BUG in Windows compiler right?

No. The bug is in the code which is redefining a union while it is being
defined. Why are you doing that?

- jonathan

Hi Jonathan,

I was just doing some timepasss RnD while I was bit free and I thought
how different compilers will behave for such codes. (I got different
output for same code in different compiler) and later I started
thinking why it behaves so and could not get any answer :-)

Girish