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| Re: Reading Binary [message #49248 is a reply to message #4812] |
Wed, 12 July 2006 08:28  |
Peter Clinch
Messages: 98
Registered: April 1996
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Dirk1106@googlemail.com wrote:
> for 12 bits
>
> fread(&t0, 3, 1, fp); byteswap(&t0,3);
> fread(&tof, 1, 1, fp);
> fread(&pos1, 2, 1, fp); byteswap(&pos1,2);
> fread(&pos2, 2, 1, fp); byteswap(&pos2,2);
> fread(&ang, 2, 1, fp); byteswap(&ang,2);
> fread(&e1, 1, 1, fp);
> fread(&e2, 1, 1, fp);
As others have pointed out, C's fread works in bytes, not bits.
From man fread on my linux box:
*************
NAME
fread, binary stream input/output
SYNOPSIS
#include <stdio.h>
size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);
DESCRIPTION
The function fread reads nmemb elements of data, each size bytes
long, from the stream pointed to by stream, storing them at the
location given by ptr.
*************
Note that the location given by ptr is cast to void* above but in
practice will be a holding space for a valid C data type, the smallest
of which is an 8 bit byte (char or unsigned char).
Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net p.j.clinch@dundee.ac.uk http://www.dundee.ac.uk/~pjclinch/
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| Re: Reading Binary [message #49249 is a reply to message #4812] |
Wed, 12 July 2006 07:25 |
greg michael
Messages: 163
Registered: January 2006
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Senior Member |
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I haven't used C in many years, but those look like bytes and not bits
to me. You can't store positions and angles in 2 bits. And byteswapping
bits?
If so, you can use normal data structures, defined for either of your
two cases.
regards,
Greg
Dirk1106@googlemail.com wrote:
> for 12 bits
>
> fread(&t0, 3, 1, fp); byteswap(&t0,3);
> fread(&tof, 1, 1, fp);
> fread(&pos1, 2, 1, fp); byteswap(&pos1,2);
> fread(&pos2, 2, 1, fp); byteswap(&pos2,2);
> fread(&ang, 2, 1, fp); byteswap(&ang,2);
> fread(&e1, 1, 1, fp);
> fread(&e2, 1, 1, fp);
>
>
>
> for 13 bits.
>
> fread(&t, 8, 1, fp); byteswap(&t,8);
> fread(&pos, 2, 1, fp); byteswap(&pos,2);
> fread(&ang, 2, 1, fp); byteswap(&ang,2);
> fread(&e, 1, 1, fp);
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| Re: Reading Binary [message #49250 is a reply to message #4812] |
Wed, 12 July 2006 07:21 |
Paul Van Delst[1]
Messages: 1157
Registered: April 2002
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Senior Member |
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Dirk1106@googlemail.com wrote:
> for 12 bits
>
> fread(&t0, 3, 1, fp); byteswap(&t0,3);
> fread(&tof, 1, 1, fp);
> fread(&pos1, 2, 1, fp); byteswap(&pos1,2);
> fread(&pos2, 2, 1, fp); byteswap(&pos2,2);
> fread(&ang, 2, 1, fp); byteswap(&ang,2);
> fread(&e1, 1, 1, fp);
> fread(&e2, 1, 1, fp);
>
>
>
> for 13 bits.
>
> fread(&t, 8, 1, fp); byteswap(&t,8);
> fread(&pos, 2, 1, fp); byteswap(&pos,2);
> fread(&ang, 2, 1, fp); byteswap(&ang,2);
> fread(&e, 1, 1, fp);
>
I thought fread worked in units of (8-bit) bytes? My C reference double check confirmed
that also (from http://www.elook.org/programming/c/fread.html)
-------------
fread()
Syntax:
#include <stdio.h>
int fread( void *buffer, size_t size, size_t num, FILE *stream );
Description:
The function fread() reads num number of objects (where each object is size bytes) and
places them into the array pointed to by buffer. The data comes from the given input
stream. The return value of the function is the number of things read...use |feof()| or
|ferror()| to figure out if an error occurs.
-------------
paulv
--
Paul van Delst Ride lots.
CIMSS @ NOAA/NCEP/EMC Eddy Merckx
Ph: (301)763-8000 x7748
Fax:(301)763-8545
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| Re: Reading Binary [message #49251 is a reply to message #4812] |
Wed, 12 July 2006 07:13 |
Dirk1106@googlemail.c
Messages: 4
Registered: July 2006
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Junior Member |
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for 12 bits
fread(&t0, 3, 1, fp); byteswap(&t0,3);
fread(&tof, 1, 1, fp);
fread(&pos1, 2, 1, fp); byteswap(&pos1,2);
fread(&pos2, 2, 1, fp); byteswap(&pos2,2);
fread(&ang, 2, 1, fp); byteswap(&ang,2);
fread(&e1, 1, 1, fp);
fread(&e2, 1, 1, fp);
for 13 bits.
fread(&t, 8, 1, fp); byteswap(&t,8);
fread(&pos, 2, 1, fp); byteswap(&pos,2);
fread(&ang, 2, 1, fp); byteswap(&ang,2);
fread(&e, 1, 1, fp);
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| Re: Reading Binary [message #49252 is a reply to message #4812] |
Wed, 12 July 2006 06:30 |
Peter Clinch
Messages: 98
Registered: April 1996
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Member |
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Dirk1106@googlemail.com wrote:
> He reads it with C, but I want to work on it with IDL.
> Now I have the C-Code and he always reads 12 or 13 bits.
How does he do that in C?
It's been a while since I wrote any C and I'm more of a sysadmin these
days so I'm wary of talking Rubbish here, but I've written a fair bit in
my time and I can't recall anything in the stdio library that reads
anything less than a single byte at a time.
Furthermore, a [unsigned] char is (or was, IIRC) a single byte and the
smallest data type available in any flavour of C I've worked with, so
even if you /can/ read 12 or 13 bits you'll have to *put* it in a short
int at the very least. Although C has fairly comprehensive bitwaise
operation support at the language level, I've never seen it read or
store anything other than whole bytes.
Can you post the C fragment where he's reading 12/13 bits please?
Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net p.j.clinch@dundee.ac.uk http://www.dundee.ac.uk/~pjclinch/
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| Re: Reading Binary [message #49253 is a reply to message #4812] |
Wed, 12 July 2006 06:01 |
Dirk1106@googlemail.c
Messages: 4
Registered: July 2006
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Junior Member |
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He reads it with C, but I want to work on it with IDL.
Now I have the C-Code and he always reads 12 or 13 bits.
So I also have to use C to read it and import it in IDL with
Call_External.
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| Re: Reading Binary [message #49254 is a reply to message #4812] |
Wed, 12 July 2006 04:12 |
greg michael
Messages: 163
Registered: January 2006
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Senior Member |
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I don't know what an LMF file is. Are you certain it's stored the way
you think? Even if they are 12/13-bit values, it's likely they'd be
stored in 16-bit chunks with the rest empty. But if your colleague made
it, he'll know how to read it...
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| Re: Reading Binary [message #49256 is a reply to message #4812] |
Wed, 12 July 2006 02:14 |
Peter Clinch
Messages: 98
Registered: April 1996
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Member |
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Dirk1106@googlemail.com wrote:
> I get the Data from a collegue.
> It is an LMF-File. I don`t know, why they made such un anusual format.
>
> So there ist no chance, of getting the data directly right in IDL?
If there is it'll certainly be very awkward: even in C you're limited to
reading in byte sized chunks.
I imagine the best way is to read /everything/ that you might need into
memory in one go in a language that does bit operations fairly well
(like C) and then work through the data stream looking at fractions of
the bytes. And export from that into something readable by Normal
Computer Programs!
Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net p.j.clinch@dundee.ac.uk http://www.dundee.ac.uk/~pjclinch/
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| Re: Reading Binary [message #49262 is a reply to message #4812] |
Tue, 11 July 2006 05:43 |
greg michael
Messages: 163
Registered: January 2006
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Senior Member |
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You can't do that. You can only read the file in units of a byte. If
your data is really in 12/13-bit units (which seems highly unusual) -
you'll have to extract them afterwards (paying close attention to
putting them back in the right order!). Where did you get this data?
regards,
Greg
Dirk1106@googlemail.com wrote:
> Hi everyone,
>
> I have a problem reading a binary file.
> The file begins with a Header of 6 unsingned Int.
> After this I have to dicive, if I want to read 12 bits oder 13 bits
> (not byte) as one record.
>
> Has soneone an idea, how to read the 12 or 13 bits best.
> After i read the bits I have so split them in the units, they stand
> for.
>
> Thanks
> .
> Greetz Dirk
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