A bug?

[Michel Van den Bergh]

Hi,

The following program segfaults when compiled with gcc
but runs fine when compiled with g++ or icc (the intel C compiler)

#include 
struct Hello {
   char world[20];
};
struct Hello s(){
   struct Hello r;
   r.world[0]='H';
   r.world[1]='\0';
   return r;
}

int main(){
   printf("%s\n",s().world);
}

Assigning s() to a variable and then using the variable avoids the 
segfault.


gcc --version returns  (GCC) 4.2.4 (Ubuntu 4.2.4-1ubuntu3).

Regards,
Michel

Re: A bug?

[Michel Van den Bergh]

That's strange. When I try to compile this with gcc 4.3.2 on Ubuntu 8.10 
(Intel core2 duo)

I get

stest.c: In function ‘main’:
stest.c:13: warning: format ‘%s’ expects type ‘char *’, but argument 2 
has type ‘char[20]’


The resulting binary does not segfault but prints garbage (probably 
uninitialized data).


g++ still works fine and does not give any warning, even when compiling 
with -Wall.


Michel





Dennis Clarke wrote:

Hi,

The following program segfaults when compiled with gcc
but runs fine when compiled with g++ or icc (the intel C compiler)

#include 
struct Hello {
char world[20];
};
struct Hello s(){
struct Hello r;
r.world[0]='H';
r.world[1]='\0';
return r;
}

int main(){
printf("%s\n",s().world);
}

Assigning s() to a variable and then using the variable avoids the
segfault.



compiles and works fine with GCC 4.3.2 on Solaris 8/9/10 sun4m/sun4u/i386

$ /opt/csw/gcc4/bin/gcc -v -o foo.o -c foo.c
Using built-in specs.
Target: sparc-sun-solaris2.8
Configured with: ../gcc-4.3.2/configure --prefix=/opt/csw/gcc4
--with-local-prefix=/opt/csw --with-as=/usr/ccs/bin/as --without-gnu-ld
--with-ld=/usr/ccs/bin/ld --with-cpu=v7 --enable-threads=posix
--enable-nls --enable-shared --enable-languages=c,c++,fortran,objc
--with-gmp=/opt/csw --with-mpfr=/opt/csw --enable-multilib
--with-included-gettext --with-libiconv-prefix=/opt/csw --with-x
--enable-java-awt=xlib --with-system-zlib --enable-bootstrap
Thread model: posix
gcc version 4.3.2 (GCC)
COLLECT_GCC_OPTIONS='-v' '-o' 'foo.o' '-c' '-mcpu=v7'
 /opt/csw/gcc4/libexec/gcc/sparc-sun-solaris2.8/4.3.2/cc1 -quiet -v foo.c
-quiet -dumpbase foo.c -mcpu=v7 -auxbase-strip foo.o -version -o
/var/tmp//ccAHrz2q.s
ignoring nonexistent directory
"/opt/csw/gcc4/lib/gcc/sparc-sun-solaris2.8/4.3.2/../../../../sparc-sun-solaris2.8/include"
#include "..." search starts here:
#include <...> search starts here:
 /opt/csw/include
 /opt/csw/gcc4/include
 /opt/csw/gcc4/lib/gcc/sparc-sun-solaris2.8/4.3.2/include
 /opt/csw/gcc4/lib/gcc/sparc-sun-solaris2.8/4.3.2/include-fixed
 /usr/include
End of search list.
GNU C (GCC) version 4.3.2 (sparc-sun-solaris2.8)
compiled by GNU C version 4.3.2, GMP version 4.2.2, MPFR version
2.3.1.
warning: GMP header version 4.2.2 differs from library version 4.2.4.
GGC heuristics: --param ggc-min-expand=47 --param ggc-min-heapsize=32768
Compiler executable checksum: 1ac791ab3c2b7cc8775dc74d45095fef
COLLECT_GCC_OPTIONS='-v' '-o' 'foo.o' '-c' '-mcpu=v7'
 /usr/ccs/bin/as -V -Qy -s -xarch=v8 -o foo.o /var/tmp//ccAHrz2q.s
/usr/ccs/bin/as: Sun WorkShop 6 2003/12/18 Compiler Common 6.0 Patch
114802-02
COMPILER_PATH=/opt/csw/gcc4/libexec/gcc/sparc-sun-solaris2.8/4.3.2/:/opt/csw/gcc4/libexec/gcc/sparc-sun-solaris2.8/4.3.2/:/opt/csw/gcc4/libexec/gcc/sparc-sun-solaris2.8/:/opt/csw/gcc4/lib/gcc/sparc-sun-solaris2.8/4.3.2/:/opt/csw/gcc4/lib/gcc/sparc-sun-solaris2.8/:/usr/ccs/bin/
LIBRARY_PATH=/opt/csw/gcc4/lib/gcc/sparc-sun-solaris2.8/4.3.2/:/usr/ccs/lib/:/opt/csw/gcc4/lib/gcc/sparc-sun-solaris2.8/4.3.2/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-o' 'foo.o' '-c' '-mcpu=v7'

$ /opt/csw/gcc4/bin/gcc -v -o foo.s -S -c foo.c
Using built-in specs.
Target: sparc-sun-solaris2.8
Configured with: ../gcc-4.3.2/configure --prefix=/opt/csw/gcc4
--with-local-prefix=/opt/csw --with-as=/usr/ccs/bin/as --without-gnu-ld
--with-ld=/usr/ccs/bin/ld --with-cpu=v7 --enable-threads=posix
--enable-nls --enable-shared --enable-languages=c,c++,fortran,objc
--with-gmp=/opt/csw --with-mpfr=/opt/csw --enable-multilib
--with-included-gettext --with-libiconv-prefix=/opt/csw --with-x
--enable-java-awt=xlib --with-system-zlib --enable-bootstrap
Thread model: posix
gcc version 4.3.2 (GCC)
COLLECT_GCC_OPTIONS='-v' '-o' 'foo.s' '-S' '-c' '-mcpu=v7'
 /opt/csw/gcc4/libexec/gcc/sparc-sun-solaris2.8/4.3.2/cc1 -quiet -v foo.c
-quiet -dumpbase foo.c -mcpu=v7 -auxbase-strip foo.s -version -o foo.s
ignoring nonexistent directory
"/opt/csw/gcc4/lib/gcc/sparc-sun-solaris2.8/4.3.2/../../../../sparc-sun-solaris2.8/include"
#include "..." search starts here:
#include <...> search starts here:
 /opt/csw/include
 /opt/csw/gcc4/include
 /opt/csw/gcc4/lib/gcc/sparc-sun-solaris2.8/4.3.2/include
 /opt/csw/gcc4/lib/gcc/sparc-sun-solaris2.8/4.3.2/include-fixed
 /usr/include
End of search list.
GNU C (GCC) version 4.3.2 (sparc-sun-solaris2.8)
compiled by GNU C version 4.3.2, GMP version 4.2.2, MPFR version
2.3.1.
warning: GMP header version 4.2.2 differs from library version 4.2.4.
GGC heuristics: --param ggc-min-expand=47 --param ggc-min-heapsize=32768
Compiler executable checksum: 1ac791ab3c2b7cc8775dc74d45095fef
COMPILER_PATH=/opt/csw/gcc4/libexec/gcc/sparc-sun-solaris2.8/4.3.2/:/opt/csw/gcc4/libexec/gcc/sparc-sun-solaris2.8/4.3.2/:/opt/csw/gcc4/libexec/gcc/sparc-sun-solaris2.8/:/opt/csw/gcc4/lib/gcc/sparc-sun-solaris2.8/4.3.2/:/opt/csw/gcc4/lib/gcc/sparc-sun-solaris2.8/:/usr/ccs/bin/
LIBRARY_PATH=/opt/csw/gcc4/lib/gcc/sparc-sun-sola

Re: no conversion from char[] to char* on function calls under circumstances [was: A bug?]

[Michel Van den Bergh]

Ok thanks for the clear explanation!

Not being able to threat char[] as a string is rather shocking to me though.

Regards,
Michel

Re: A bug

[Michel Van den Bergh]

The C standard says no such thing; only integer promotions are 
performed. (See 6.5.2.2 of the C99 final draft.)



Ok one more question. Why does this not give a warning then (and runs fine)?


#include 
struct Hello {
   char world[20];
};
struct Hello s(){
   struct Hello r;
   r.world[0]='H';
   r.world[1]='\0';
   return r;
}

int main(){
   struct Hello a;
   a=s();
   printf("%s\n",a.world);
   return 0;
}

In this case an implicit conversion of char[] to (char *) is happening 
as well as far as I can see.


Regards,
Michel

Re: no conversion from char[] to char* on function calls under circumstances [was: A bug?]

[Michel Van den Bergh]

Andrew Haley wrote:

Andrew Thomas Pinski wrote:
  

C++98 is not C99 :) there is no rvalue to lvalue conversion for rvalue
arrays in C++98. Also this code is still undefined C99 but will most
likely become valid C1x.



Ah, it's an rvalue array.  Good point.
  
Ok now I understand. I assume this behaviour is not triggered often as 
in C it is

not so common to have an array which is an rvalue.

Michel


  

Sent from my iPhone



Advertising on gcc list.  Dear me... ;-)

Andrew.