write a 20GB file

[Jackie Lee]

Hello there,

I have a 22 GB binary file, a want to change values of specific
positions. Because of the volume of the file, I doubt my code a
efficient one:

#! /usr/bin/env python
#coding=utf-8
import sys
import struct

try:
        f=open(sys.argv[1],'rb+')
except (IOError,Exception):
    print '''usage:
        scriptname segyfilename
'''
    sys.exit(1)

#skip EBCDIC header
try:
    f.seek(3200)
except Exception:
    print 'Oops! your file is broken..'

#read binary header
binhead = f.read(400)
ns = struct.unpack('>h',binhead[20:22])[0]
if ns < 0:
    print 'file read error'
    sys.exit(1)

#read trace header
while True:
    f.seek(28,1)
    f.write(struct.pack('>h',1))
    f.seek(212,1)
    f.seek(ns*4,1)

f.close()