Wireshark · Wireshark-dev: Re: [Wireshark-dev] Problem with val_to_str inside BASE_CUSTOM callback function

[Guy Harris <guy@xxxxxxxxxxxx>]

On Oct 21, 2016, at 9:57 AM, Thomas Wiens <th.wiens@xxxxxx> wrote:

> This happens only when calling val_to_str inside the callback function.
> When outside, then all is Ok (getting formatted result string on failure).
> When I'm calling try_val_to_str_idx with a non existing value, then I
> get -1 as index and NULL as result as expected, so I guess the problem is:
> 
> wmem_strdup_printf(wmem_packet_scope(), fmt, val);
> 
> Is there a limitation when I'm using format callbacks,

Yes. You're limited to using wmem scopes that are active even if you're *not* in the middle of dissecting a packet; the callback is called when the result of a dissection is being displayed or used for a custom column, and that's not in the middle of dissecting a packet.

Unfortunately, I don't think there's a scope for this, so you'd either have to use NULL as the scope and explicitly free the string when you're done with it, or use try_val_to_str() and deal with a NULL return (nothing matches) yourself.