Possible invalid code - GCC 4.8/5.1.

[Krzysztof Hałasa]

Hi,

I wonder if the following is a bug:

#include 

int main(void)
{
struct str {
struct a {
int a1, a2;
} a;
};

struct str src = {.a = {.a1 = 1, .a2 = 2}};

struct str dest = {.a = src.a, .a.a2 = 3};

printf("src: %u %u, dest: %u %u\n", src.a.a1, src.a.a2, dest.a.a1, 
dest.a.a2);
return 0;
}

$ gcc -W -Wall test.c -o test
$ ./test
src: 1 2, dest: 0 3

GCC 4.8.4 (ARM), Red Hat 4.9.2-6, Red Hat 5.1.1-1 (x86-64 Fedora 21 and 22).

Should I file a bz report?



BTW changing the initializer from ".a = src.a" to ".a = {1, 2}" produces
the expected result:

$ gcc -W -Wall test.c -o test
test.c: In function ‘main’:
test.c:13:9: warning: initialized field overwritten [-Woverride-init]
  struct str dest = {.a = {1, 2}, .a.a2 = 3};
 ^
test.c:13:9: warning: (near initialization for ‘dest.a.a2’) [-Woverride-init]

$ ./test
src: 1 2, dest: 1 3

-- 
Krzysztof Halasa

Industrial Research Institute for Automation and Measurements PIAP
Al. Jerozolimskie 202, 02-486 Warsaw, Poland

GCC 4.9.3 Release Candidate available from gcc.gnu.org

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Re: Possible invalid code - GCC 4.8/5.1.

[Jonathan Wakely]

On 17 June 2015 at 09:16, Krzysztof Hałasa  wrote:
> Hi,
>
> I wonder if the following is a bug:
>
> #include 
>
> int main(void)
> {
> struct str {
> struct a {
> int a1, a2;
> } a;
> };
>
> struct str src = {.a = {.a1 = 1, .a2 = 2}};
>
> struct str dest = {.a = src.a, .a.a2 = 3};
>
> printf("src: %u %u, dest: %u %u\n", src.a.a1, src.a.a2, dest.a.a1, 
> dest.a.a2);
> return 0;
> }
>
> $ gcc -W -Wall test.c -o test
> $ ./test
> src: 1 2, dest: 0 3
>
> GCC 4.8.4 (ARM), Red Hat 4.9.2-6, Red Hat 5.1.1-1 (x86-64 Fedora 21 and 22).
>
> Should I file a bz report?

N.B. mails about bug are almost never appropriate on this mailing
list, either ask on the gcc-help mailing list or file a bugzilla,
don't ask "should I file a bugzilla" on this list.

This is the correct behaviour.

When the initializer .a.a2 = 3 is processed the current object is the
whole of dest, so all the other members of the current object are set
to zero, which overrides the values set by the earlier .a = src.a
initializer.

It would be nice if the compiler warned you about this.

> BTW changing the initializer from ".a = src.a" to ".a = {1, 2}" produces
> the expected result:
>
> $ gcc -W -Wall test.c -o test
> test.c: In function ‘main’:
> test.c:13:9: warning: initialized field overwritten [-Woverride-init]
>   struct str dest = {.a = {1, 2}, .a.a2 = 3};
>  ^
> test.c:13:9: warning: (near initialization for ‘dest.a.a2’) [-Woverride-init]
>
> $ ./test
> src: 1 2, dest: 1 3

I believe this is also correct behaviour.

The brace-enclosed initializer list {1, 2} makes .a the "current
object", then the elements make .a.a1 and .a.a2 the current object in
turn. When the .a.a2 = 3 initializer is processed .a.a2 is the current
object, and that object has no other members, so nothing else is set
to zero.

Re: Possible invalid code - GCC 4.8/5.1.

[Joseph Myers]

See bug 63944 and DR#413 regarding such cases.

-- 
Joseph S. Myers
jos...@codesourcery.com

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