bfd_error_handler extension

[Alan Modra <amodra at bigpond dot net dot au>]

Seeing HJ's patch to report unrecognized i386 relocations reminded me
of an idea I had about extending _bfd_default_error_handler to
handle the common cases of printing a bfd filename (which might be
an archive) and a section name (which might need group info).

The main difficulty in extending a printf style function is that
vfprintf doesn't return any info on how far its va_list arg has
been incremented.  You can't call vfprintf passing part of the format
string, do some processing on a special % specifier, then call vprintf
again.  That is, not without duplicating a lot of the work vfprintf does
in parsing the format string.

So my idea was to always pass extension args first.  A call might look
like

  (*_bfd_error_handler)
    (_("%B: bad reloc symbol index (0x%lx >= 0x%lx) for offset 0x%lx in section `%S'"),
     abfd, sec,
     (unsigned long) r_symndx, (unsigned long) nsyms, irela->r_offset);

instead of the existing code (from elf_link_read_relocs_from_section)

  char *sec_name = bfd_get_section_ident (sec);
  (*_bfd_error_handler)
    (_("%s: bad reloc symbol index (0x%lx >= 0x%lx) for offset 0x%lx in section `%s'"),
     bfd_archive_filename (abfd), (unsigned long) r_symndx,
     (unsigned long) nsyms, irela->r_offset,
     sec_name ? sec_name : sec->name);
  if (sec_name)
    free (sec_name);

_bfd_default_error_handler then would process any %B or %S in the format
string, and pass a substituted format string on to vfprint.  There is
some nastiness with this approach, since you need to expand "%" in any
substitutions to "%%".

Comments?

-- 
Alan Modra
IBM OzLabs - Linux Technology Centre