Re: gdb printing of dynamically allocated matrix

[Jim Blandy <jimb at codesourcery dot com>]

Peter Toft <pto@linuxbog.dk> writes:
> On Fri, 20 Jul 2007, Jim Blandy wrote:
>
>> 
>> Peter Toft <pto@linuxbog.dk> writes:
>> >> (gdb) matprint c 2 3
>> 
>> Could you use sizeof to compute the dimensions automatically?  Or at
>> least one of them?
>
> Dear Jim
>
> I think I understand which direction you are point me to, but
> "sizeof" in GDB does not ring a bell here. In C yes, but in GDB no.
>
> Can you give me an example e.g. for 1D arrays? We might target this 
> snippet befor we move to 2D matrices;
>
>   int *a;
>   a=(int *)malloc(3*sizeof(int));
>   a[0] = 1;
>   a[1] = 6;
>   a[2] = 8;

Ah, if you're allocating dynamically, then there's no way to do it
using sizeof.

GDB supports the C 'sizeof' operator; in your example above, 'print
sizeof (a)' will give you '4' or '8' or whatever is appropriate for a
pointer on the program being debugged.  There are no facilities (in C
or in GDB) for finding the size of a malloc block; since the library
may round up the size of the block, it might be impossible to do
without changing the heap data structure.