[lldb-dev] breakpoint command

Michael Sartain mikesart at valvesoftware.com
Tue Jun 18 18:46:17 PDT 2013

On Mon, Jun 17, 2013 at 2:33 PM, Greg Clayton <gclayton at apple.com> wrote:

> The "symbol.st_info" is in bits 7:0 of the flags, and the
> "symbol.st_other" is in bits 15:8. So the flags are:
> 0x00000002 __printf     STB_LOCAL  | STT_FUNC
> 0x00000012 _IO_printf   STB_GLOBAL | STT_FUNC
> 0x00000012 printf       STB_GLOBAL | STT_FUNC
> So there isn't much to be able to do with these symbols. Does anyone know
> why we have 3 symbols? Are these just aliases? Is there anyway to know that
> we should use "printf" by inspecting any other sections?

The printf libc source code looks like this:

  24 #undef printf
  28 int
  29 __printf (const char *format, ...)
  41 #undef _IO_printf
  42 ldbl_strong_alias (__printf, printf);
  43 /* This is for libg++.  */
  44 ldbl_strong_alias (__printf, _IO_printf);

So yes, there are two strong aliases and one local symbol. gdb seems to
prefer the local one when running stacktraces and doing "disassemble
printf" or "disassemble _IO_printf". I'll investigate this further though.

> I checked out the line table for printf and found that there is indeed no
> "prologue_end" marker in the line table state machine, so we are setting a
> breakpoint on the second line table entry in the sequence which maps to
> 0x0000000000053864 as we saw in your previous output (which was slid to
> 0x7f961e2a2864). So everything is working as expected here given the
> information we have.

If we have no prologue_end marker, would it make sense to not skip anything?

Thanks Greg.
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