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(stabs.info)Stab Section Basics

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How to Embed Stabs in Sections
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   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 (Note: 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 may be 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.

   The GNU linker is able to optimize stabs information by merging
duplicate strings and removing duplicate header file information (Note:
Include Files).  When some versions of the GNU linker optimize stabs
in sections, they remove the leading `N_UNDF' symbol and arranges for
all the `n_strx' fields to be relative to the start of the `.stabstr'
section.