[Dwarf-Discuss] DW_AT_decl_file etc understood now. Question answered.

Philip Craig philipjcraig@gmail.com
Tue Feb 18 03:01:32 GMT 2020

On Tue, 18 Feb 2020 at 10:40, David Anderson via Dwarf-Discuss
<dwarf-discuss at lists.dwarfstd.org> wrote:
> A bit of thought, and it becomes clear. Since 0 is a reserved
> (and always has been)
> value for DW_AT_decl_file the numbers used by the compiler
> must start at 1.
> So to index into the array of names from the line table header
> make sure your value is greater than zero and
> subtract 1 from the DW_AT_decL_file.
> With DWARF5 the file numbered 1 has to be the file of the compilation unit
> (same as DW_AT_name of the compilation unit).

That seems wrong to me. That would mean that the index used for
DW_AT_decl_file is different from the index used for DW_LNS_set_file,
which uses values beginning with 0. From Section 6.2.4:

"The line number program references file names in this sequence beginning
with 0, and uses those numbers instead of file names in the line number
program that follows."

It goes on to say:

"Prior to DWARF Version 5, the current compilation file name was not
represented in
the file_names field. In DWARF Version 5, the current compilation file name is
explicitly present and has index 0. This is needed to support the
common practice of
stripping all but the line number sections ( .debug_line and .debug_line_str )
from an executable."

but I think that statement is wrong. Prior to DWARF Version 5, the
currently compilation file name was an entry in the file_names field,
just like any other entry.

Note that this is different from the handling for the compilation
directory, where for DWARF4 an index of 0 is valid and refers to the
implicit entry that is the DW_AT_comp_dir attribute.

Additionally, clang-10 emits DW_AT_decl_file values of 0 that seem to
be meant to refer to the compilation file, even though the standard
says 0 means no source file has been specified.

For my DWARF5 producer, I avoid the issue by emitting an entry at
index 0 but never referencing it.

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