Does anybody have gdb pretty-printer code for an fd_set on linux?

Question

Modern versions of gdb allow integration of python code to "pretty print" complex data structures. There are some great pretty printer implementations for C++'s STL classes as well as some of the more common boost.org types.

In network programming, one commonly encounters select/poll calls. While poll() uses an array of data structures, select() uses fd_set.

Has anyone run across a pretty printer implementation for fd_set, preferably portable, but even platform-specific would be okay. Ideally, it'd be linux/x86, but I'd take anything and hope to be able to adapt.

Answer 1

Alrighty, here's something I wrote which seems to work for me under Linux. Let me know if it works for you:

anonprint.py

import gdb

class fd_set_printer:
  """
  Prints an fd_set, which is normally an opaque
  array of ints, each bit representing one file descriptor
  """

  def __init__(self, val, val_array):
    self.val = val
    self.val_array = val_array

  @staticmethod
  def find_set_bits(bit_array):
    """
    Finds set bits in a long bit list.
    Expects a gdb.Value which contains a C array,
      such as int[10], and treats it as a bitlist
      of int_size * 10 bits long.  Returns an array of
      bit positions, starting with 0, for which the bits
      are on.
    e.g. for int foo[] = [1, 6], it will return [ 0, 33, 34 ]
    The array should be given as a gdb.Value
    """
    set_bits = []
    bits_length = bit_array[0].type.sizeof * 8
    current_bit = 0

    # Can not use 'for current_byte in gdb.Value:' even if
    #  gdb.Value.type.code == gdb.TYPE_CODE_ARRAY
    # So iteration happens this ugly C-style way
    for current_byte_pos in xrange(*bit_array.type.range()):
      current_byte = bit_array[current_byte_pos]
      for bit in xrange(0, bits_length):
        bit_mask = 1 << bit
        if bit_mask & current_byte == bit_mask:
          set_bits.append(current_bit)
        current_bit += 1

    return set_bits

  def to_string(self):
    fd_list = self.find_set_bits(self.val_array)
    if len(fd_list) == 0:
      output = "Empty file descriptor set."
    else:
      output = "File descriptor set: "
      output += ', '.join(map(str, fd_list))
    return output


def anon_struct_lookup_function(val):
  """
  Checks if the given value looks like an fd_set.
  If it does, delegates printing to the printer
  """
  lookup_tag = val.type.tag
  if lookup_tag == None:
    return None
  if lookup_tag != "<anonymous struct>":
    return None

  fields = val.type.fields()
  val_array = None

  if len(fields) == 1 and fields[0].name == 'fds_bits':
    val_array = val['fds_bits']
  elif len(fields) == 1 and fields[0].name == '__fds_bits':
    val_array = val['__fds_bits']

  if not val_array is None:
    return fd_set_printer(val, val_array)

  return None


def add_fd_set_printer(obj = gdb):
  "Adds the fd_set pretty printer to the given object"
  obj.pretty_printers.append(anon_struct_lookup_function)

And then make your ~/.gdbinit:

python
import sys
sys.path.insert(0, '/home/user/anonprint_py_directory_here')
from anonprint import add_fd_set_printer
add_fd_set_printer()
end

This is the first time I tried to interact with gdb internals via Python, so commentary and suggestions are welcome.