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c++/8242: Invalid class size evaluation with long long and inheritance
- From: David Decotigny <David dot Decotigny at irisa dot fr>
- To: gcc-gnats at gcc dot gnu dot org
- Cc: ddecotig at irisa dot fr
- Date: Wed, 16 Oct 2002 17:56:24 +0200
- Subject: c++/8242: Invalid class size evaluation with long long and inheritance
>Number: 8242
>Category: c++
>Synopsis: Invalid struct size evaluation with long long and inheritance (g++-3.x)
>Confidential: no
>Severity: critical
>Priority: medium
>Responsible: unassigned
>State: open
>Class: wrong-code
>Submitter-Id: net
>Arrival-Date: Wed Oct 16 09:06:03 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator: David.Decotigny@irisa.fr
>Release: 3.1
>Organization:
IRISA, Campus de Beaulieu, 35042 Rennes Cedex, FRANCE
>Environment:
System: SunOS blutch 5.7 Generic_106541-19 sun4u sparc SUNW,Ultra-5_10
Architecture: sun4
host: sparc-sun-solaris2.7
build: sparc-sun-solaris2.7
target: sparc-sun-solaris2.7
configured with: ../gcc/gcc-3.1/configure --prefix /usr/local/gcc-3.1 --enable-shared --enable-languages=c++,f77
>Description:
When you have a class Ancestor (struct in the example below, but it
doesn't matter) that contains at least a 64 bits attribute (even in a
much larger union, or with any __attribute__ ((aligned(xxx))) ), then
a class that inherits it and defines additional fields will have its
sizeof() wrong as soon as the size of the data /after/ the long long
is not multiple of 64 bits...
>How-To-Repeat:
Compile the following source (any option) with g++-3.0, g++-3.0.1,
g++-3.0.3, g++-3.0.4, g++-3.1 (maybe others) on a Solaris 2.7 host
(g++ configured *without* the --enable-long-long option), and run
it. You will notice that Child and Ancestor have the same sizeof(),
and hence the memset in zero_ancestor(), which normally does not touch
the Child::foo field, will overwrite it :
=====> With g++-3.1 (Wrong) :
[blutch] ~ >make clean all && ./a
rm -f *.o
g++-3.1 -Wall -c cpp_main.cc -o cpp_main.o
g++-3.1 -Wall -o a cpp_main.o
sizeof(struct Ancestor) = 16
sizeof(struct Child) = 16
#1 c.foo = 42
#2 c.foo = 0
=====> With g++-2.95.3 (Ok) :
[blutch] ~ >make clean all && ./a 1060
rm -f *.o
g++-2.95.3 -Wall -c cpp_main.cc -o cpp_main.o
g++-2.95.3 -Wall -o a cpp_main.o
sizeof(struct Ancestor) = 16
sizeof(struct Child) = 24
#1 c.foo = 42
#2 c.foo = 42
Here is the source:
#include <string.h> // only for memset
#include <iostream> // only for std::cout
struct Ancestor { // Or class if you prefer
long long i64; // Or any array of 64bits integers, or even an union
// with at least a long long member !!!
int not64; // Or char, short..., or whatever non multiple
// of 64 bits (int not64[3] will fail for example...)
};
struct Child : public Ancestor { // Or class if you prefer
int foo; // or anything else
};
void zero_ancestor(struct Ancestor * a)
{
// Both will fail...
// *((int*)(((int)a)+sizeof(struct Ancestor)-sizeof(int))) = 0;
memset(a, 0x0, sizeof(struct Ancestor));
}
int main()
{
Child c;
std::cout << "sizeof(struct Ancestor) = " << sizeof(struct Ancestor)
<< std::endl;
std::cout << "sizeof(struct Child) = " << sizeof(struct Child)
<< std::endl;
c.foo = 42;
std::cout << "#1 c.foo = " << c.foo << std::endl;
zero_ancestor(& c);
std::cout << "#2 c.foo = " << c.foo << std::endl;
return 0;
}
>Fix:
Non-gcc fix : make sure that what is /after/ the long long fields in
Ancestor are of size multiple of 64bits... This is ugly and painful to
check.
>Release-Note:
>Audit-Trail:
>Unformatted: