how to navigate in a file ?

[Boris \BXS\ Schulz]

Hi,

I would like to open a binary file and read its contents.
The opening is no problem, but if I wanted to jump to a specific byte,
how would I do that ? Also, how would I read this specific byte ? I have
some code reading the byte, but it quite ugly. Is there something like
DATA.readbyte(position) ?

greetings, BXS

 

[Ara.T.Howard]

Date: Fri, 05 Dec 2003 01:58:22 +0100
From: Boris "BXS" Schulz <[email protected]>
Newsgroups: comp.lang.ruby
Subject: how to navigate in a file ?

Hi,

I would like to open a binary file and read its contents.
The opening is no problem, but if I wanted to jump to a specific byte,
how would I do that ? Also, how would I read this specific byte ? I have
some code reading the byte, but it quite ugly. Is there something like
DATA.readbyte(position) ?

greetings, BXS

~/eg/ruby > cat byte.rb

require 'mmap' # guy's package from the RAA - get it!

binary = 'bin.dat'

# create a binary file
open(binary, 'wb') do |f|
bytes = [1 << 2, 1 << 1].pack 'cc'
f.write bytes
end

# memory map it (man mmap)
mmap = Mmap.new binary, 'r'
byte_a, byte_b = mmap[0], mmap[1]

print byte_a
print byte_b
puts

~/eg/ruby > ruby byte.rb
42


in addition to being really easy to use, this package is _really_ fast.

-a
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[Robert Klemme]

What you're looking for is File#seek(offset). For instance:

foo.rb(main):001:0> # Create a binary file where the byte at position N = N
foo.rb(main):002:0* File.open("binfile", "wb") do
foo.rb(main):003:1* |f|
foo.rb(main):004:1* 256.times do
foo.rb(main):005:2* |b|
foo.rb(main):006:2* f.write(b.chr)
foo.rb(main):007:2> end
foo.rb(main):008:1> end
=> 256
foo.rb(main):009:0>
foo.rb(main):010:0* # Seek to position 128 and read the byte there
foo.rb(main):011:0* File.open("binfile", "rb") do
foo.rb(main):012:1* |f|
foo.rb(main):013:1* f.seek(128)
foo.rb(main):014:1> b = f.getc
foo.rb(main):015:1> puts b
foo.rb(main):016:1> end
128
=> nil
foo.rb(main):017:0>

Note that for reading and writing "r+b" is the appropriate open mode.

File.open("io-test.txt", "r+b") do |io|
puts "read"
io.seek(0)

while ( b = io.read(1) )
printf "%4d 0x%02x %s\n", io.tell, b[0], b.inspect
end

puts "write"
io.seek(0)

io.write( "xxx" )

puts "read again"
io.seek(0)

while ( b = io.read(1) )
printf "%4d 0x%02x %s\n", io.tell, b[0], b.inspect
end
end

Regards

robert

 

[Boris \BXS\ Schulz]

[mmap]

sounds like a good idea, except, I am a WindowsXP user, and it says
something about beeing only for unix on
http://raa.ruby-lang.org/list.rhtml?name=mmap
So i'll have to stick with the other guys advice.

Thanks to all of you, BXS

 

[Bill Kelly]

Hi,

[mmap]

sounds like a good idea, except, I am a WindowsXP user, and it says
something about beeing only for unix on
http://raa.ruby-lang.org/list.rhtml?name=mmap
So i'll have to stick with the other guys advice.

By the way, if you like, you can define [] operators for
File class, like:

class File
def [](pos)
seek(pos)
read(1)
end
def []=(pos, str)
seek(pos)
write(str)
end
end

Then you can:
=> #<File:zzz.wav>

# wav files start out with 'RIFF'

f[0] => "R"
f[1] => "I"
f[2] => "F"
f[3]

=> "F"

# let's write a 'Z' in place of the 'R'

f[0] = "Z"

=> "Z"

f[0] => "Z"
f[1] => "I"
f[2]

=> "F"

# let's write the string 'ZOOM' in place of the 'RIFF'

f[0] = "ZOOM"

=> "ZOOM"

f[0] => "Z"
f[1] => "O"
f[2] => "O"
f[3]

=> "M"


Hope this helps,

Bill

 

[Boris \BXS\ Schulz]

Hi,

thanks, I was trying to handle MP3s, and not WAVs, but that should
really help
I will try all of this after dinner.

greetings, BXS