Invalid operands to binary -

[Spiros Bousbouras]

#include <stdlib.h>

int main(void) {
char **p1 ;
const char **p2 ;

p1 = malloc(5 * sizeof(char *)) ;
if (p1 == 0) return EXIT_FAILURE ;
p2 = p1 + 1 ;
p2 - p1 ;
return 0 ;
}

When I try to compile the above on gcc 4.1.2 I get
t.c: 10: error: invalid operands to binary -

Is it a compiler bug ? It seems legal to me.

[Eric Sosman]

Spiros Bousbouras wrote:

#include <stdlib.h>

int main(void) {
char **p1 ;
const char **p2 ;

p1 = malloc(5 * sizeof(char *)) ;
if (p1 == 0) return EXIT_FAILURE ;
p2 = p1 + 1 ;
p2 - p1 ;
return 0 ;
}

When I try to compile the above on gcc 4.1.2 I get
t.c: 10: error: invalid operands to binary -

Is it a compiler bug ? It seems legal to me.

I see nothing wrong with it. Try a gcc forum
to see whether it's a known bug?

--
Eric Sosman
es*****@ieee-dot-org.invalid

[Robert Gamble]

On Aug 8, 7:42 am, Spiros Bousbouras <spi...@gmail.c omwrote:

#include <stdlib.h>

int main(void) {
char **p1 ;
const char **p2 ;

p1 = malloc(5 * sizeof(char *)) ;
if (p1 == 0) return EXIT_FAILURE ;
p2 = p1 + 1 ;
p2 - p1 ;
return 0 ;

}

When I try to compile the above on gcc 4.1.2 I get
t.c: 10: error: invalid operands to binary -

Is it a compiler bug ?

No, it's almost never a compiler bug.

Pointer subtraction occurs between compatible pointers, p1 and p2 are
incompatible types and cannot participate in pointer subtraction. You
should have received a warning on line 9 as well as you need a cast to
make the assignment work.

Robert Gamble

[Spoon]

Spiros Bousbouras wrote:

#include <stdlib.h>

int main(void) {
char **p1 ;
const char **p2 ;

p1 = malloc(5 * sizeof(char *)) ;
if (p1 == 0) return EXIT_FAILURE ;
p2 = p1 + 1 ;
p2 - p1 ;
return 0 ;
}

When I try to compile the above on gcc 4.1.2 I get
t.c: 10: error: invalid operands to binary -

gcc 3.4.4 said:

foo.c: In function `main':
foo.c:9: warning: assignment from incompatible pointer type
foo.c:10: error: invalid operands to binary -

[Spiros Bousbouras]

On Aug 8, 12:05 pm, Eric Sosman <[e-mail] e:

Spiros Bousbouras wrote:

#include <stdlib.h>

int main(void) {
char **p1 ;
const char **p2 ;

p1 = malloc(5 * sizeof(char *)) ;
if (p1 == 0) return EXIT_FAILURE ;
p2 = p1 + 1 ;
p2 - p1 ;
return 0 ;
}

When I try to compile the above on gcc 4.1.2 I get
t.c: 10: error: invalid operands to binary -

Is it a compiler bug ? It seems legal to me.

I see nothing wrong with it. Try a gcc forum
to see whether it's a known bug?

I did try Googling for "invalid operands to binary -"
in several gcc forums but I didn't find anything. I also
went to the gcc bugs page (gcc.gnu.org/bugs.html)
but once again nothing turned up.

[Spiros Bousbouras]

On Aug 8, 12:07 pm, Robert Gamble <rgambl...@gmai l.comwrote:

On Aug 8, 7:42 am, Spiros Bousbouras <spi...@gmail.c omwrote:

#include <stdlib.h>

int main(void) {
char **p1 ;
const char **p2 ;

p1 = malloc(5 * sizeof(char *)) ;
if (p1 == 0) return EXIT_FAILURE ;
p2 = p1 + 1 ;
p2 - p1 ;
return 0 ;

}

When I try to compile the above on gcc 4.1.2 I get
t.c: 10: error: invalid operands to binary -

Is it a compiler bug ?

No, it's almost never a compiler bug.

Pointer subtraction occurs between compatible pointers, p1 and p2 are
incompatible types and cannot participate in pointer subtraction. You
should have received a warning on line 9 as well as you need a cast to
make the assignment work.

I don't believe they are incompatible but rather p2 is
a qualified (with const) version of the type of p1. In
any case I don't see why the standard should forbid such
a thing ; it seems an unreasonable rstriction.

[Robert Gamble]

On Aug 8, 8:49 am, Spiros Bousbouras <spi...@gmail.c omwrote:

On Aug 8, 12:07 pm, Robert Gamble <rgambl...@gmai l.comwrote:

On Aug 8, 7:42 am, Spiros Bousbouras <spi...@gmail.c omwrote:

#include <stdlib.h>

int main(void) {
char **p1 ;
const char **p2 ;

p1 = malloc(5 * sizeof(char *)) ;
if (p1 == 0) return EXIT_FAILURE ;
p2 = p1 + 1 ;
p2 - p1 ;
return 0 ;

}

When I try to compile the above on gcc 4.1.2 I get
t.c: 10: error: invalid operands to binary -

Is it a compiler bug ?

No, it's almost never a compiler bug.

Pointer subtraction occurs between compatible pointers, p1 and p2 are
incompatible types and cannot participate in pointer subtraction. You
should have received a warning on line 9 as well as you need a cast to
make the assignment work.

I don't believe they are incompatible but rather p2 is
a qualified (with const) version of the type of p1.

You are confused, p2 is not const qualified, it is a "pointer to
pointer to const char", which is a completely distinct and
incompatible type from "pointer to pointer to char". If the pointers
were defined as:

char ** p1;
char ** const p2;

Then you would be correct as p2 would now be a "const pointer to
pointer to char" (the const qualifier being applied to the pointer
itself) thus being a "qualified version of a compatible type".

Check out questions 11.9 and 11.10 on the FAQ <http://www.c-faq.com/>
for more details.

Robert Gamble

[Eric Sosman]

Eric Sosman wrote On 08/08/07 08:05,:

Spiros Bousbouras wrote:

>>#include <stdlib.h>

int main(void) {
char **p1 ;
const char **p2 ;

p1 = malloc(5 * sizeof(char *)) ;
if (p1 == 0) return EXIT_FAILURE ;
p2 = p1 + 1 ;
p2 - p1 ;
return 0 ;
}

When I try to compile the above on gcc 4.1.2 I get
t.c: 10: error: invalid operands to binary -

Is it a compiler bug ? It seems legal to me.

I see nothing wrong with it. Try a gcc forum
to see whether it's a known bug?

Seems I didn't see clearly enough ... Thanks
to Robert Gamble for the correction.

--
Er*********@sun .com

[CBFalconer]

Spiros Bousbouras wrote:

>
#include <stdlib.h>

int main(void) {
char **p1 ;
const char **p2 ;

p1 = malloc(5 * sizeof(char *)) ;
if (p1 == 0) return EXIT_FAILURE ;
p2 = p1 + 1 ;
p2 - p1 ;
return 0 ;
}

When I try to compile the above on gcc 4.1.2 I get
t.c: 10: error: invalid operands to binary -

Is it a compiler bug ? It seems legal to me.

IMO it is a misapplied bug. The initialization of p2 cannot
succeed, because it is a const, and can only be set at declaration
time. Gcc apparently remembers that it has invalid content, and
thus rejects the arithmetic statement.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

--
Posted via a free Usenet account from http://www.teranews.com

[CBFalconer]

Robert Gamble wrote:

On Aug 8, 8:49 am, Spiros Bousbouras <spi...@gmail.c omwrote:

.... snip ...

>

>I don't believe they are incompatible but rather p2 is
a qualified (with const) version of the type of p1.

You are confused, p2 is not const qualified, it is a "pointer to
pointer to const char", which is a completely distinct and
incompatible type from "pointer to pointer to char". If the

Me too. Scrub my earlier answer.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

--
Posted via a free Usenet account from http://www.teranews.com