Re: Breaking on C labels?

Michael Snyder
2007-01-26T00:05:45Z

On Thu, 2007-01-25 at 13:19 -0800, Jim Blandy wrote:
> Joel Brobecker <brobecker@adacore.com> writes:
> > A customer asked us a question about the ability of breaking on labels
> > inside C code. Consider for instance:
> >
> > void
> > procedure foo (void)
> > {
> > [...]
> > error_handler:
> > [...]
> > }
> >
> > The customer asked if it was possible to break at the error_handler
> > label. We are helping them with finding alternative solutions, but
> > I still did a bit of researching...
> >
> > I discovered without much surprise that DWARF does indeed have provision
> > for labels (DW_TAG_label). However, I also noticed that GCC already
> > generates the associated DIEs. We actually even process them. However,
> > all my attempts in trying to reference them from the debugger failed.
> > I tried "break error_handler", or more simply "p error_handler", etc.
> >
> > Did we ever make any attempt in implementing this sort of functionality
> > in the past?
> >
> > Looking at the source code, I found in new_symbol ():
> >
> > case DW_TAG_label:
> > attr = dwarf2_attr (die, DW_AT_low_pc, cu);
> > if (attr)
> > {
> > SYMBOL_VALUE_ADDRESS (sym) = DW_ADDR (attr) + baseaddr;
> > }
> > SYMBOL_CLASS (sym) = LOC_LABEL;
> > break;
> >
> > So we setup the symbol, but we don't add to any of our symbol lists...
> > I am left wondering what it is that we'll be doing with this symbol.
>
> You're right, it looks to me as if we just throw the symbol away, too.
>
> The logical thing would be to put it in the block of the function it's
> in, since goto labels are block-scoped. It would go in LABEL_DOMAIN,
> since that's a separate namespace. And then you'd need to teach
> linespec.[ch] about them.

There's a risk that some symbol-lookup function would then select that
label instead of the function entry label when you tried to look up the
nearest label preceeding a given address.