Re: [PATCH 13/13] Document "no debug info debugging" improvements

Pedro Alves
2017-07-13T11:09:40Z

On 07/13/2017 03:19 AM, Pedro Alves wrote:

> +Sometimes, a function you wish to call is missing debug information.
> +In such case, @value{GDBN} does not know the type of the function,
> +including the types of the function's parameters. To avoid calling
> +the inferior function incorrectly, which could result in the called
> +function functioning erroneously and even crash, @value{GDBN} refuses
> +to call the function unless you tell it at least the return type of
> +the function, with a cast. For example:
> +
> +@smallexample
> + (@value{GDBP}) p getenv ("PATH")
> + 'getenv' has unknown return type; cast the call to its declared return type
> + (@value{GDBP}) p (char *) getenv ("PATH")
> + $1 = 0x7fffffffe7ba "/usr/local/bin:/"...
> +@end smallexample
> +
> +Casting the return type of the function is equivalent to casting the
> +function to a pointer to an unprototyped function, and calling that:
> +
> +@smallexample
> + (@value{GDBP}) p ((char * (*) ()) getenv) ("PATH")
> + $2 = 0x7fffffffe7ba "/usr/local/bin:/"...
> +@end smallexample

I woke up thinking that mapping to unprototyped is the wrong
equivalence -- that it'd be better to assume the function is
prototyped, since that's how most C functions are written as
nowadays. Also, there's no such thing as an unprototyped
function in C++.

Assuming prototyped would allow this, for example:

float mult (float v1, float v2) { return v1 * v2; }

(gdb) p (float) mult (2.0f, 3.0f)
$1 = 6
(gdb) p (float) mult ((float) 2, (float) 3)
$2 = 6
(gdb) p ((float (*) (float, float)) mult) (2, 3)
$3 = 6

(gdb) ptype 2.0f
type = float
(gdb) ptype 2.0
type = double

If the function really is unprototyped, then you'd still be
able to call it correctly via the function pointer cast syntax:

float mult_noproto (v1, v2)
float v1, v2;
{ return v1 * v2; }

(gdb) p ((float (*) ()) mult_noproto) (2.0f, 3.0f)
$1 = 6
(gdb) p ((float (*) ()) mult_noproto) (2.0, 3.0)
$2 = 6
(gdb) p ((float (*) ()) mult_noproto) ((float) 2, (float) 3)
$3 = 6

I'll give this a try, and add those as tests to gdb.base/nodebug.exp.

> +
> +If the function you wish to call is declared as prototyped and has
> +floating point parameters or integer parameters narrower than int, you
> +may need to cast the function to a function pointer of the same type
> +as the function and call that, to avoid @value{GDBN} coercing
> +arguments to integer/double, as would be required if calling an
> +unprototyped function. @xref{ABI, float promotion}. For example,
> +given this prototyped (i.e.@: ANSI/ISO style) function:

This paragraph will need adjustment.

Thanks,
Pedro Alves