En Thu, 19 Feb 2009 16:51:39 -0200, harijay <hari...@gmail.com> escribió:
Hi I am very confused with the use of the struct module to read binary
data from a file.
( I have only worked with ascii files so far)
I have a file spec for a Data-logger (http://www.dataq.com/support/
techinfo/ff.htm)
That format is rather convoluted -- due to historical reasons, I imagine...
I am collecting some voltage , time traces on one channel and they are
written to the binary file on a 32 bit windows machine
The file spec says that the number of header bytes in the data file
header is stored as a 16 bit eye "I" at bits 6-7
If it says "at *byte* positions 6-7" you need a seek(6) to start reading
from there, not seek(5).
Now I want to get at that number. When I try format !h I get a
meaningful number
If f is my file handle opened with "rb" mode
f.seek(5)
(Integer,) = struct.unpack('!h',f.read(2))
(Integer,)
(9348,)
I am assuming that means that there are 9348 header bytes . Can
someone look at the format spec and tell me if I am on the right
track.
Not exactly. Why '!' (network byte order)? The spec doesn't say about byte
order, but since it's a Windows program we can assume little endian, '<'
or just '=' (native).
But instead of multiple seeks + micro-reads I'd read the whole header and
decode it at once (the fixed part is only 110 bytes long):
fixed_header_fmt = struct.Struct("<HHBBhLL...")
f = open(..., 'rb')
fixed_header = f.read(110)
elements = [None]
elements[1:] = fixed_header_fmt.unpack(fixed_header)
# just to keep the 1-based element numbering
Now, elements[4] is the fourth row in the table, "Number of bytes in each
channel info entry"
The format is built from the Type column: UI -> H, I -> h, B -> B, UL ->
L, L -> l, D -> d, F -> f.
--
Gabriel Genellina
--
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