Re: Quick question about "print EXPR"

Andrew Cagney
2005-02-08T16:39:34Z

Brian Desany wrote:
[sorry, going backwards through my e-mail]

>>Brian Desany wrote:
>>
>>>>Did you include <math.h> in your program?
>>>
>>>
>>>Yep, I did (the program itself works properly).
>>>
>>>FWIW the only lines of the program that aren't listed in my
>>
>>original post are:
>>
>>>#include <stdio.h>
>>>#include <math.h>
>>>
>>>And just in case it matters, I compiled using the command
>>
>>"cc -lm -g test.c".
>>
>>>If I compile with "gcc -lm -g test.c", "p floor(whatever)"
>>
>>is always
>>
>>>16 (rather than 1 as in the original post).
>>
>>Wonder if GDB can see this. What does:
>>
>>(gdb) ptype floor
>>
>>print?
>
>
> type = int ()

Which hopefully explains why things go wrong - GDB doesn't know the
function's signature and hence invokes it incorrectly. Here GDB is
probably pulling the return value out of an integer register instead of
floating-point register or stack address (as you step through the code
that integer register's value would change and hence the value would
change).

There are two things from here:

- is the debug info stabs?
I believe that only dwarf>=2 debug info can describe the above.
=> Should GDB issue a warning when calling a function with no visible
prototype?

- is the debug info dwarf
Check the input file (you'll need to look at binutils's readelf program)
to see if the info is there. Beyond that we'd have to think out why GDB
can't see it.

Andrew

> I was wrong about "p floor(whatever)" always returning a constant value. It
> returns a different value depending apparently on what line of the program
> I'm currently stopped on:
>
> Breakpoint 1, main (argc=1, argv=0xbfffcf54) at test.c:9
> 9 yada = 2.5;
> (gdb) p floor(123.456)
> $1 = 0
> (gdb) n
> 10 haha = floor(yada);
> (gdb) p floor(123.456)
> $2 = 0
> (gdb) n
> 11 printf("%0.1f %0.1f\n",yada, haha);
> (gdb) p floor(123.456)
> $3 = 3199
> (gdb) n
> 2.5 -0.0
> 12 return 0;
> (gdb) p floor(123.456)
> $4 = 9
> (gdb)
>
>
>>Andrew
>>
>>
>>>[bdesany]$ cc -v
>>>Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/2.96/specs
>>>gcc version 2.96 20000731 (Red Hat Linux 7.1 2.96-81)
>>
>>[bdesany]$ gcc
>>
>>>-v Reading specs from
>>>/home/bdesany/sys/lib/gcc/i686-pc-linux-gnu/3.4.2/specs
>>>Configured with: ./configure --prefix=/home/bdesany/sys
>>
>>Thread model:
>>
>>>posix gcc version 3.4.2
>>>
>>>Thanks-
>>>-Brian.
>
>
>