While running a debugging perl under gdb, you can access the same dumping facility (Perl_do_sv_dump, in perl's dump.c) that Devel::Peek uses. The following is a short example, using a threaded perl. On non-threaded non-multiplicity perls, the Perl_get_context() arguments should be omitted.

I'll set up a nested structure to feed to perl's not operator, and then dump it out from within the not op. 

$ gdb perl5.8.7
GNU gdb 6.3.50_2004-12-28-cvs (cygwin-special)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and y
+ou are
welcome to change it and/or distribute copies of it under certain cond
+itions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for det
+ails.
This GDB was configured as "i686-pc-cygwin"...
(gdb) set args -we'![a=>{b=>0,c=>1},sub{"foo"}]'
(gdb) b Perl_pp_not
Function "Perl_pp_not" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y

Breakpoint 1 (Perl_pp_not) pending.
(gdb) r
Starting program: /usr/local/bin/perl5.8.7.exe -we'![a=>{b=>0,c=>1},su
+b{"foo"}]'


Breakpoint 2 at 0x100bbe8f: file pp.c, line 2396.
Pending breakpoint "Perl_pp_not" resolved
Useless use of not in void context at -e line 1.

Breakpoint 2, Perl_pp_not (my_perl=0x103c1f58) at pp.c:2396
2396        dSP; tryAMAGICunSET(not);
(gdb) s
2397        *PL_stack_sp = boolSV(!SvTRUE(*PL_stack_sp));
(gdb) call Perl_do_sv_dump(Perl_get_context(),0,Perl_PerlIO_stderr(Per
+l_get_con
text()),*sp,0,4,0,0)
SV = RV(0x103e9824) at 0x103e7588
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x103c323c
    SV = PVAV(0x103e9c40) at 0x103c323c
      REFCNT = 2
      FLAGS = ()
      IV = 0
      NV = 0
      ARRAY = 0x103c9ba8
      FILL = 2
      MAX = 2
      ARYLEN = 0x0
      FLAGS = (REAL)
        Elt No. 0
        SV = PV(0x103c3468) at 0x103e7564
          REFCNT = 1
          FLAGS = (POK,pPOK)
          PV = 0x103c4898 "a"\0
          CUR = 1
          LEN = 2
        Elt No. 1
        SV = RV(0x103e981c) at 0x103e7570
          REFCNT = 1
          FLAGS = (ROK)
          RV = 0x103c3188
            SV = PVHV(0x103e2990) at 0x103c3188
              REFCNT = 3
              FLAGS = (SHAREKEYS)
              IV = 2
              NV = 0
              ARRAY = 0x103e8e48  (0:6, 1:2)
              hash quality = 125.0%
              KEYS = 2
              FILL = 2
              MAX = 7
              RITER = -1
              EITER = 0x0
                Elt "c" HASH = 0xeeba5d59
                SV = IV(0x103e9428) at 0x103e74a4
                  REFCNT = 1
                  FLAGS = (IOK,pIOK)
                  IV = 1
        Elt No. 2
        SV = RV(0x103e9820) at 0x103e757c
          REFCNT = 1
          FLAGS = (ROK)
          RV = 0x103e7504
            SV = PVCV(0x103e0f08) at 0x103e7504
              REFCNT = 3
              FLAGS = (PADBUSY,PADMY,ANON,WEAKOUTSIDE)
              IV = 0
              NV = 0
              COMP_STASH = 0x103c30b0   "main"
              START = 0x103e5608 ===> 1
              ROOT = 0x103eaa48
              XSUB = 0x0
              XSUBANY = 0
              GVGV::GV = 0x103e7534     "main" :: "__ANON__"
              FILE = "-e"
              DEPTH = 0
              FLAGS = 0x404
              OUTSIDE_SEQ = 0
              PADLIST = 0x103e7510
                PADNAME = 0x103e751c(0x103e7ce8) PAD = 0x103e7528(0x10
+3e7c68)
              OUTSIDE = 0x103c32b4 (MAIN)
(gdb)

[download]

Perl_do_sv_dump is prototyped: 
Perl_do_sv_dump(pTHX_ I32 level, PerlIO *file, SV *sv, I32 nest, I32 m
+axnest, bool dumpops, STRLEN pvlim)

[download]
Most of the parameters can be left as zero; file needs to be a PerlIO* to where the output should go, sv is the sv to dump, and maxnest is how deep to go in complex data structures.

Replies are listed 'Best First'.
Re: How to dump perl data structures from gdb
by salva (Canon) on Apr 19, 2005 at 15:31 UTC
    then, the next step is to include in your C code a small wrapper function around Perl_sv_dump that you can call from gdb just as 
    call my_dump_sv(any_sv)

    [download]
    and forget about contexts, threads, PerlIO streams and long argument lists!
[reply]
[d/l]
      the next step is to include in your C code a small wrapper function around Perl_sv_dump that you can call from gdb

      What C. code? He's debugging the perl binary directly, and snooping as it runs on a mini-program specified using the -command line argument.

      Unless he wants to customize the perl binary itself, he doesn't have any C. code to add functions to.
      --
      AC

[reply]
        no problem, just put my_dump_sv in a dummy XS module.
[reply]
Re: How to dump perl data structures from gdb
by RMGir (Prior) on Apr 19, 2005 at 14:44 UTC
    My initial reaction was "You know you're in trouble if you need to know this" :)

    But I've been in situations before where it would have been great to be able to do this, but it didn't even occur to me that it was possible.

    So thanks!!! (and ++, of course)


    Mike
[reply]

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