On Wed, Feb 27, 2019 at 9:41 AM Gargi Sharma <gs051...@gmail.com> wrote:
>
> On Tue, Feb 26, 2019 at 11:34 PM Ian Lance Taylor <i...@golang.org> wrote:
> >
> > On Tue, Feb 26, 2019 at 6:08 PM Gargi Sharma <gs051...@gmail.com> wrote:
> > >
> > > I meant the Section and SectionByType calls.
> >
> > Those calls do different things. They do more than just get the
> > symbol table. They aren't equivalent.
> >
> >
> > > [ 6] .gosymtab PROGBITS 00000000004d7758 000d7758
> > > 0000000000000000 0000000000000000 A 0 0 1
> > > [22] .symtab SYMTAB 0000000000000000 002aa000
> > > 0000000000012c30 0000000000000018 23 116 8
> > >
> > > From readelf, I see that the binary has both symtab and gosymtab. Is
> > > gosymtab a superset of information of the .symtab? If yes, why do we
> > > need the .symtab?
> >
> > I explained .gosymtab in my last message. It is not a superset of
> > .symtab. It is different.
> >
> > In any case we need .symtab because that is what ELF tools expect.
> Got it, I was really confused why we needed two different symbol tables.
>
> I am trying to get the program counter if I know the line number. For
> go binaries, I did this:
>
> func getPCFromLine(line int, binary string) {
> symbolTableSection := executable.Section(".gosymtab")
> symbolTableData, _ := symbolTableSection.Data()
>
> lineTableForText := gosym.NewLineTable(pcToLineData,
> executable.Section(".text").Addr)
>
> newSymbolTable, _ := gosym.NewTable(symbolTableData, lineTableForText)
>
> pc, fn, _ := newSymbolTable.LineToPC(binary, line)
> }
>
> The above function was where I was trying to use
> executable.Section(".symtab") and got this error:
> panic: bad symbol type '0' at byte 0x4
There is code that does this in cmd/internal/objfile/goobj.go, in the
method PCToLine. You can probably do something similar.
> I am assuming if I want to do something similar for non-go binaries, I
> will have to look inside .symtab to get this information.
No, the ELF .symtab only contains the information displayed by
"readelf -s". To get file/line information you have to parse the
debug information, normally DWARF. The libbacktrace library can do
that for you; it has a backtrace_pcinfo function.
Ian
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