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DATE 2016-12-01

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MESSAGE
DATE 2016-12-11
FROM ruben safir
SUBJECT Subject: [Learn] Fwd: Re: png data format
From learn-bounces-at-nylxs.com Sun Dec 11 00:40:00 2016
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-------- Forwarded Message --------
Subject: Re: png data format
Date: 7 Dec 2016 19:49:12 GMT
From: Jens Thoms Toerring
Organization: Freie Universitaet Berlin
Newsgroups: comp.unix.programmer
References:

ruben safir wrote:
> I'm having trouble with this imput of data from a PNG image. The
> specification says that "chunks" have a 4 byte field that is the length of
> the attached data segment. I tried to read the length in for a chunk that
> has a length of 13, which was confirmed in a hexdump

> 0000000 211 120 116 107 015 012 032 012 -->>000 000 000 015<<-- 111 110 104 122
> 0000010 000 000 041 215 000 000 007 165 010 006 000 000 001 206 055 074
> 0000020 336 000 000 000 004 147 101 115 101 000 000 261 217 013 374 141

> I am storing the data in a uint32_t variable using the following code, but
> the value keeps showing up with a huge number 218103808 which happens to be
> the number that is evaluated by iostream for the value of the whole chunk


> done reading header

> Sizeof Chunk 4
> Raw Chunk Number 0: 218103808
> ***LENGTH****
> Length value => 218103808

The data are stored in big-endian order, i.e. with the most
significant byte coming first, but you seem to have read it
in as a 32-bit integer on a machine using little-endian byte
order, e.g. with a machine with an Intel processor, where
the least-significant byte comes first in memory. That's
also what they expect when reading in binary data from a
file. So simply read in the four bytes separately into e.
g. an array

uint8_t bytes[4]

with the first byte you read from the file going into
the first array element etc. Then do

uint32_t len = ((bytes[0] * 256 + bytes[1]) * 256 + bytes[2]) * 256 +
bytes[3];

and you should get the correct value. The same applies, of
course, also for other fields containing values that occupy
multiple bytes! This then will work correctly on both big-
and little-endian machines.
Regards, Jens
--
\ Jens Thoms Toerring ___ jt-at-toerring.de
\__________________________ http://toerring.de
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