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* stabs.texinfo (Stabs In ELF, Statics): More on relocating stabs

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in ELF files.
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Jim Kingdon 1993-09-08 14:15:35 +00:00
parent c09bd4433e
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@ -1,3 +1,8 @@
Wed Sep 8 09:11:52 1993 Jim Kingdon (kingdon@lioth.cygnus.com)
* stabs.texinfo (Stabs In ELF, Statics): More on relocating stabs
in ELF files.
Tue Sep 7 13:45:02 1993 Jim Kingdon (kingdon@lioth.cygnus.com)
* stabs.texinfo (Stabs In ELF): Talk about N_FUN value.

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gdb/doc/stabs.texinfo
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@ -878,8 +878,10 @@ Initialized static variables are represented by the @samp{S} and
@c find the variables)
@findex N_STSYM
@findex N_LCSYM
In a.out files, @code{N_STSYM} means the data segment, @code{N_FUN}
means the text segment, and @code{N_LCSYM} means the bss segment.
In a.out files, @code{N_STSYM} means the data section, @code{N_FUN}
means the text section, and @code{N_LCSYM} means the bss section. For
those systems with a read-only data section separate from the text
section (Solaris), @code{N_ROSYM} means the read-only data section.
For example, the source lines:
@ -901,16 +903,17 @@ yield the following stabs:
@end example
In XCOFF files, each symbol has a section number, so the stab type
need not indicate the segment.
need not indicate the section.
In ECOFF files, the storage class is used to specify the section, so the
stab type need not indicate the segment.
stab type need not indicate the section.
@c In ELF files, it apparently is a big mess. See kludge in dbxread.c
@c in GDB. FIXME: Investigate where this kludge comes from.
@c
@c This is the place to mention N_ROSYM; I'd rather do so once I can
@c coherently explain how this stuff works for stabs-in-ELF.
In ELF files, for Solaris 2.1, symbol descriptor @samp{S} means that the
address is absolute (ld relocates it) and symbol descriptor @samp{V}
means that the address is relative to the start of the relevant section
for that compilation unit. I don't know what it does for @samp{S} stabs
on Solaris 2.3 (in which ld no longer relocates stabs). For more
information on ld stab relocation, @xref{Stabs In ELF}.
@node Parameters
@section Parameters
@ -2800,8 +2803,6 @@ BSS segment file-scope variable; see @ref{Statics}.
@item 0x2a N_MAIN
Name of main routine; see @ref{Main Program}.
@c FIXME: discuss this in the Statics node where we talk about
@c the fact that the n_type indicates the section.
@item 0x2c N_ROSYM
Variable in @code{.rodata} section; see @ref{Statics}.
@ -3600,30 +3601,37 @@ section, and the @code{.stabstr} section has its ELF section
header @code{sh_type} member set to @code{SHT_STRTAB} to mark it as a
string table.
It should not be necessary for the linker to process the @code{.stab}
section in any special way, so (except for Solaris 2.2 and earlier, see
below) none of the addresses in the @code{n_value} field of the stabs
are relocated by the linker. Instead they are relative to the source
file (or some entity smaller than a source file, like a function). To
find the address of each section corresponding to a given source file,
the (compiler? assembler?) puts out symbols giving the address of each
section for a given source file. Since these are ELF (not stab)
symbols, the linker can relocate them correctly. They are named
@code{Bbss.bss} for the bss section, @code{Ddata.data} for the data
section, and @code{Drodata.rodata} for the rodata section. For the text
section, there is no such symbol. The linker provided with Solaris 2.2
and earlier relocates stabs using relocation information from a
@code{.rela.stabs} section, which means that the value of an
@code{N_FUN} stab in an executable is the actual address. For Solaris
2.3 and later, the value of the @code{N_FUN} stab is zero and the
address of the function can be obtained from the ELF (non-stab) symbols.
Sun, in reference to bug 1142109, has verified that this is intentional.
To keep linking fast, it is a bad idea to have the linker relocating
stabs, so (except for Solaris 2.2 and earlier, see below) none of the
addresses in the @code{n_value} field of the stabs are relocated by the
linker. Instead they are relative to the source file (or some entity
smaller than a source file, like a function). To find the address of
each section corresponding to a given source file, the compiler puts out
symbols giving the address of each section for a given source file.
Since these are ELF (not stab) symbols, the linker can relocate them
correctly. They are named @code{Bbss.bss} for the bss section,
@code{Ddata.data} for the data section, and @code{Drodata.rodata} for
the rodata section. For the text section, there is no such symbol. GCC
does not provide these symbols; it instead relies on the stabs getting
relocated, which loses for Solaris 2.3 (see below). Thus address which
would normally be relative to @code{Bbss.bss}, etc., are absolute. The
linker provided with Solaris 2.2 and earlier relocates stabs using
relocation information from a @code{.rela.stab} section, which means
that the value of an @code{N_FUN} stab in an executable is the actual
address. I think this is pretty much just standard ELF relocations, as
it would do for any section, rather than a special-purpose stabs hack.
For Solaris 2.3 and later, the linker ignores relocations for the stabs
section. The value of a @code{N_FUN} stab is zero and the address of a
function can be obtained from the ELF (non-stab) symbols. Sun, in
reference to bug 1142109, has verified that this is intentional.
Because looking things up in the ELF symbols is slow and GDB currently
only has code to do this for functions (which is enough for Solaris
since read-only variables go in the @code{.rodata} section), it would
probably be better to use a @code{Ttext.text} symbol for stabs-in-elf on
non-Solaris machines, and make the address in the N_FUN relative to the
@code{Ttext.text} symbol.
non-Solaris machines, and make the address in the @code{N_FUN} relative
to the @code{Ttext.text} symbol. In addition to @code{N_FUN} symbols,
whether the linker relocates stabs also affects some @code{N_ROSYM},
@code{N_STSYM}, and @code{N_LCSYM} symbols; see @ref{Statics}.
@node Symbol Types Index
@unnumbered Symbol Types Index