How to Embed Stabs in Sections
==============================

   The assembler creates two custom sections, a section named `.stab'
which contains an array of fixed length structures, one struct per stab,
and a section named `.stabstr' containing all the variable length
strings that are referenced by stabs in the `.stab' section.  The byte
order of the stabs binary data depends on the object file format.  For
ELF, it matches the byte order of the ELF file itself, as determined
from the `EI_DATA' field in the `e_ident' member of the ELF header.
For SOM, it is always big-endian (is this true??? FIXME).  For COFF, it
matches the byte order of the COFF headers.  The meaning of the fields
is the same as for a.out (see Symbol Table Format.), except that
the `n_strx' field is relative to the strings for the current
compilation unit (which can be found using the synthetic N_UNDF stab
described below), rather than the entire string table.

   The first stab in the `.stab' section for each compilation unit is
synthetic, generated entirely by the assembler, with no corresponding
`.stab' directive as input to the assembler.  This stab contains the
following fields:

`n_strx'
     Offset in the `.stabstr' section to the source filename.

`n_type'
     `N_UNDF'.

`n_other'
     Unused field, always zero.  This may eventually be used to hold
     overflows from the count in the `n_desc' field.

`n_desc'
     Count of upcoming symbols, i.e., the number of remaining stabs for
     this source file.

`n_value'
     Size of the string table fragment associated with this source
     file, in bytes.

   The `.stabstr' section always starts with a null byte (so that string
offsets of zero reference a null string), followed by random length
strings, each of which is null byte terminated.

   The ELF section header for the `.stab' section has its `sh_link'
member set to the section number of the `.stabstr' section, and the
`.stabstr' section has its ELF section header `sh_type' member set to
`SHT_STRTAB' to mark it as a string table.  SOM and COFF have no way of
linking the sections together or marking them as string tables.

   For COFF, the `.stab' and `.stabstr' sections are simply
concatenated by the linker.  GDB then uses the `n_desc' fields to
figure out the extent of the original sections.  Similarly, the
`n_value' fields of the header symbols are added together in order to
get the actual position of the strings in a desired `.stabstr' section.
Although this design obviates any need for the linker to relocate or
otherwise manipulate `.stab' and `.stabstr' sections, it also requires
some care to ensure that the offsets are calculated correctly.  For
instance, if the linker were to pad in between the `.stabstr' sections
before concatenating, then the offsets to strings in the middle of the
executable's `.stabstr' section would be wrong.