Android logcat binary reader example

#!/usr/bin/env python
# Quick example of a binary logcat reader which fails when it encounters one of
# the unexpected extra bytes occouring in some logcat entries.
#
# Run as:
# $ adb logcat -d -B | ./readlog.py -
#
# - Hal

from struct import unpack
from collections import namedtuple
from sys import argv, stdin, stderr, stdout
import re

# Define a log entry
LogEntry = namedtuple("LogEntry", "payloadlen pid tid time nanotime priority \
tag message")
# hackish soloution to finding the two null terminated strings in our payload
payloadpattern = re.compile("^(.*)\x00(.*)\x00$")

def readEntry(logstream):
    rawheader = logstream.read(20)
    if len(rawheader) < 20: # check for EOF
        return None

    # Unpack the header into a 2 byte length and 4 4 byte ints (unsigned)
    header = unpack("HxxIIII", rawheader)
    payload = logstream.read(header[0]) # first element of header is msg len
    if len(payload) < header[0]:
        raise Exception("Insufficient bytes read for payload. Expected: "\
                + "{0}, got: {1}".format(1, 2))
    priority, = unpack("B", payload[0])

    # Find the two null terminated strings with our regex
    match = payloadpattern.match(payload[1:])
    if not match:
        raise Exception("Invalid payload")
    tag, message = match.groups()

    return LogEntry(header[0], header[1], header[2], header[3], header[4], \
        priority, tag, message)

if __name__ == "__main__":
    if len(argv) != 2:
        print >> stderr, "Usage: {0} binarylogfile\n(or '-' to read stdin)"\
            .format(argv[0])
        exit(1)
    if argv[1] == "-":
        file = stdin
    else:
        file = open(argv[1])

    # Read all the entries
    while True:
        entry = readEntry(file)
        if entry:
            print entry
        else:
            break