Tuesday, July 28, 2009

Debugging perl programs/internals in gdb

When it comes to perl internals, print based debugging doesn't work that well. Compilation and installation are too slow and you can not place a print and quickly see output. At some point gbd should be used. In perl world we have Devel::Peek's Dump function to look behind curtain. In C world there is sv_dump.

    # threaded perl:
    (gdb) call Perl_sv_dump(my_perl, variable)
    # not threaded perl:
    (gdb) call Perl_sv_dump(variable)

Perl_ prefix is some magic, I don't care much why, but in most cases you need prefix things.

Using breakpoints is a must. Use pp_* functions to break, for example Perl_pp_entersub. Here is simple session where we stop before entering a sub and using dumper to figure out sub's name:

    > gdb ./perl
    GNU gdb 6.3.50-20050815 (Apple version gdb-962) (Sat Jul 26 08:14:40 UTC 2008)

    (gdb) break Perl_pp_entersub
    Breakpoint 1 at 0xe512c: file pp_hot.c, line 2663.

    (gdb) run -e 'sub foo { return $x } foo()'

    Breakpoint 1, Perl_pp_entersub (my_perl=0x800000) at pp_hot.c:2663
    2663        dVAR; dSP; dPOPss;
    (gdb) n
    2662    {
    2663        dVAR; dSP; dPOPss;
    2668        const bool hasargs = (PL_op->op_flags & OPf_STACKED) != 0;
    2670        if (!sv)

    (gdb) call Perl_sv_dump(my_perl, sv)
    SV = PVGV(0x8103fc) at 0x813ef0
      REFCNT = 2
      NAME = "foo"
      NAMELEN = 3
      GvSTASH = 0x8038f0    "main"
      GP = 0x3078f0
        SV = 0x0
        REFCNT = 1
        IO = 0x0
        FORM = 0x0  
        AV = 0x0
        HV = 0x0
        CV = 0x813eb0
        CVGEN = 0x0
        LINE = 1
        FILE = "-e"
        FLAGS = 0xa
        EGV = 0x813ef0      "foo"

Quite simple, but when you start investigating internals it's very helpful.

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