What you have to look for on 64 bit

[marco atzeri]

On 3/19/2013 6:12 PM, Ken Brown wrote:
> On 3/19/2013 6:48 AM, Corinna Vinschen wrote:
>> Hi guys,
>>
>>
>> it just occured to me that it might be helpful to report a typical 64
>> bit porting bug we observed yesterday, while testing the sigdelayed
>> patch.
>>
>> Kai was going to run the gas testsuite which requires dejagnu and
>> expect, but expect simply crashed.  After a bit of debugging it
>> turned out that expect was calling the openpty function, like so:
>>
>>    char *master, *slave,*name;
>>    openpty (master, slave, name, 0, 0);
>>
>> The last two parameters to openpty are pointers, just like the first
>> three.  However, the constant 0 is int by default.  int is 32 bit, but
>> pointers are 64 bit on x86_64.  This would have been no problem, if
>> expect had included the pty.h header which provides a prototype for
>> openpty.  GCC would have known to extend the 0 constants to 64 bit in
>> this case.  Alas, expect did not include pty.h and so the 0 were not 64
>> bit extended, but given as 32 bit parameter to openpty.  Since all
>> parameters take 64 bit slots, the upper 32 bit of the parameters were
>> undefined.  Instead of NULL pointers, openpty got pointers with just
>> the lower 32 bit guaranteed to be 0.
>>
>> Another simple example:  Try `printf ("%s\n", strerror (EINVAL));'
>> without including string.h.  Same thing as above.  The missing prototype
>> for strerror leads gcc to think it's a function returning int.  No
>> problem on 32 bit where sizeof(int)==sizeof(char*), but on 64 bit...
>>
>> <annoying lecturing>
>>
>> Bottom line is, when building packages for 64 bit, make sure to
>> switch on warnings for missing prototypes and if you get these
>> warnings, make sure to include the right headers to get the prototype.
>> Otherwise, SEGV ensues.
>>
>> </annoying lecturing>
>
> I tried your printf example, and as you said, it compiled but crashed.
> But gcc didn't report the missing prototype, even though I compiled with
> `gcc -Wmissing-prototypes'.  Did I do something wrong?  Here's the
> source file:
>
> #include <stdio.h>
> #include <errno.h>
> int
> main ()
> {
>    printf ("%s\n", strerror (EINVAL));
> }
>
> Ken

with -Wall

prova.c: In function ‘main’:
prova.c:6:3: warning: implicit declaration of function ‘strerror’ 
[-Wimplicit-function-declaration]
    printf ("%s\n", strerror (EINVAL));
    ^
prova.c:6:3: warning: format ‘%s’ expects argument of type ‘char *’, but 
argument 2 has type ‘int’ [-Wformat=]
prova.c:6:3: warning: format ‘%s’ expects argument of type ‘char *’, but 
argument 2 has type ‘int’ [-Wformat=]
prova.c:7:1: warning: control reaches end of non-void function 
[-Wreturn-type]
  }
  ^