Re: sizeof evaluates expression to compute size?

Senthil Kumar Selvaraj
2015-09-17T14:10:26Z

On Wed, Sep 16, 2015 at 07:56:32AM -0700, Joel Brobecker wrote:
> > Is it true that gdb evaluates the expression in a "print
> > sizeof(expression)" to figure out the size?
>
> Yes; it actually evaluates the entire expression (including the use
> of the sizeof operator).
>
> > For one expression on the ARM target
> >
> > print sizeof((char*)(s->m)
>
> The expression is incomplete; but with the expression you gave and
> assuming we're just missing a closing parenthesis, I don't see why
> it would need to allocate memory in the inferior. Perhaps you've found
> a bug.

Here's a more complete testcase.

typedef struct tagFoo
{
int x;
char arr[10];
} Foo;

Foo *pFoo;
int main()
{
return 1;
}

Compiling with
$ gcc test.c -O0 -g3

and then starting gdb (native x86_64, v7.10) gives
$ gdb a.out
>>> print sizeof((char*)pFoo->arr)
evaluation of this expression requires the target program to be active

Starting the program, putting a breakpoint on malloc, and then executing
the above command causes the breakpoint to be hit.

Evaluation works fine without the cast to char*
>>> print sizeof(pFoo->arr)
$2 = 10

Should I file a bug?

Regards
Senthil