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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 (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_strxOffset in the .stabstr section to the source filename.
n_typeN_UNDF.
n_otherUnused field, always zero.
This may eventually be used to hold overflows from the count in
the n_desc field.
n_descCount of upcoming symbols, i.e., the number of remaining stabs for this source file.
n_valueSize 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
(see Names of 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.