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| From | jt@toerring.de (Jens Thoms Toerring) |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: counting bytes |
| Date | 2015-06-06 22:31 +0000 |
| Organization | Freie Universitaet Berlin |
| Message-ID | <cthalvFnjb7U1@mid.uni-berlin.de> (permalink) |
| References | <mkun7o$pc2$1@dont-email.me> <mkunkk$r2n$1@dont-email.me> <mkuon6$vf9$1@dont-email.me> <YqDcx.12392$OH7.4462@fx28.iad> |
Lew Pitcher <lew.pitcher@digitalfreehold.ca> wrote:
> On Saturday June 6 2015 08:24, in comp.lang.c, "bilsch01"
> <king621@comcast.net> wrote:
> > On 06/06/2015 05:05 AM, Bartc wrote:
> >> On 06/06/2015 12:58, bilsch01 wrote:
> >>> The code below writes a file to disk that has 52 bytes. There are 4
> >>> float and 8 int. Isn't that 4x4 + 8x2 = 32 bytes? I don't get it.
> >>
> >> Isn't int usually 4 bytes too?
> >>
> >> But anyway, those are the binary sizes. You're writing out these values
> >> as text. It's completely different. That 65535 2-byte int value will
> >> take 5 bytes as text: "65535", while a 2-byte zero will take 1 byte or 1
> >> character: "0".
> >>
> >> You need to decide on your file format, packed binary or human-readable
> >> text, first.
> >>
> >>> fp = fopen("gdata", "w");
> >>>
> fprintf(fp,"%f%f%f%f%i%i%i%i%i%i%i%i",w1,w2,w3,k1,xf,yf,xof,yof,xm,ym,delay,glength);
> >>>
> >>
> >
> > I didn't know %i and %f meant to save them as text. OK. How do I save
> > them as packed binary.
> One way would be to store your data in a structure, and read/write the
> structure to file
> As in (off the top of my head/uncompiled/untested):
> #include <stdio.h>
> #include <stdlib.h>
> int main(void)
> {
> struct
> {
> int xf,
> yf,
> xof,
> yof,
> xm,
> ym,
> delay,
> glength;
> float w1,
> w2,
> w3,
> k1;
> } gdata = {65535,
> 0,
> 65535,
> 0,
> 65535,
> 0,
> 0,
> 65535,
> 0.5,
> 2.0,
> 0.14159,
> 0.01};
> FILE *fp;
> if ((fp = fopen("gdata", "wb")) != NULL)
> {
> fwrite(&gdata,sizeof(gdata),1,fp);
> fclose(fp);
> }
> else return EXIT_FAILURE;
> return EXIT_SUCCESS;
> }
> > I read that the 'b' in "rb" and "wb" only
> > counted in Windows. I'm using Ubuntu.
> Open the file as "rb" or "wb"; in Linux, it doesn't make a difference, but on
> other platforms it will. You /may/ have need to transport your data file to a
> different platform one day.
If you do so please be aware that this DOES NOT store the date in
a format that you're guaranteed to be able to read in on a dif-
ferent computer. If you e.g. write them out on a 32-bit machine,
transfer the resulting file to a 64-bit machine (even when run-
ning the exact same operating system) and try to read the file
in you could be in for a surprise. The number of bytes of a e.g.
an int are not something that's the same on all computers. And
neither is the way binary data are stored.
So already your assumption that an "int" is always 4 bytes is
wrong. It may be 2 bytes, 4 bytes or even more - it depends on
the CPU. And neither is it a given that the data stored for an
int start with the least or the most siginificant byte. So, un-
less you know very well what you're doing refrain from trying
to store data in binary a format. If you do you must understand
what differences exist and how to deal with the resulting issues.
Otherwise you may end up with data files that you can read in on
one type of machine but not on a different one.
I'm a bit sensitive on this point because, some years back, I
was bitten by exactly these issues rather badly. I had written
a program that my collegues liked and started to use. Being
young and naive (or incompetent, to be truthful) I had fi-
gured that writing out structures in binary would be a nice
thing, resulting in shorter data files and making reading the
data back in so much simpler (just a single line of fread()
instead of lots of code to parse ASCII data). But then we got
a different machine (long-endian 64-bits instead of the lit-
tle-endian 32-bit machines I was used to) and none of the
files, which were the result of quite a bit of work, could
be read in on the new machine. A bit of red-face time for me
- but it made me learn and, hopefully, become a bit less in-
competent;-)
So, before you decide to store data in binary form make sure
that you understand that there are different binary repre-
sentations for even the most common types in C:
* with fwrite() you "dump" memory contents, exactly as they
are, into a file - with no "translation" at all - and
fread() does exactly the reverse, i.e., it pull data from
a file and writes them into memory with no modifications,
* the size of even an int is not necessarily the same on dif-
ferent machines,
* some machines store the less important bytes first, others
do it the other way round (and, if something has more than
2 bytes, the number of theoretically possible ways to store
them grows with the factorial of the number of bytes),
* structures have the additinal draw-back that there can be
padding between the members of the structure, which can not
only be different from machine to machine, but also, in prin-
ciple, from compiler to compiler - or even change between
compiler versions.
If you want to store data in a binary format then you have to
either store also information about the binary representation
on the machine that wrote the data (and, for reading them back
in, functions that can extract them according to that informa-
tion) or make sure that they always get written in a common
format that can be interpreted on all machines that may have
to read them in. Or use ASCII. Or use some serialization li-
brary that does all the ugly, low-level work for you.
Regards, Jens
--
\ Jens Thoms Toerring ___ jt@toerring.de
\__________________________ http://toerring.de
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
counting bytes bilsch01 <king621@comcast.net> - 2015-06-06 04:58 -0700
Re: counting bytes Bartc <bc@freeuk.com> - 2015-06-06 13:05 +0100
Re: counting bytes bilsch01 <king621@comcast.net> - 2015-06-06 05:24 -0700
Re: counting bytes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-06-06 05:41 -0700
Re: counting bytes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-06-06 14:00 +0100
Re: counting bytes Bartc <bc@freeuk.com> - 2015-06-06 13:52 +0100
Re: counting bytes Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-06-06 10:27 -0400
Re: counting bytes jt@toerring.de (Jens Thoms Toerring) - 2015-06-06 22:31 +0000
THANKS EVERYBODY FOR THE INFO bilsch01 <king621@comcast.net> - 2015-06-06 23:35 -0700
Re: THANKS EVERYBODY FOR THE INFO Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-06-07 10:08 +0100
Re: THANKS EVERYBODY FOR THE INFO Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-06-07 09:22 -0700
Re: THANKS EVERYBODY FOR THE INFO Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-06-07 18:07 +0100
Re: THANKS EVERYBODY FOR THE INFO Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-06-08 04:03 -0700
Re: THANKS EVERYBODY FOR THE INFO Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-11 08:12 -0700
Re: THANKS EVERYBODY FOR THE INFO Keith Thompson <kst-u@mib.org> - 2015-06-07 08:01 -0700
Re: counting bytes gordonb.3gcai@burditt.org (Gordon Burditt) - 2015-06-07 05:02 -0500
Re: counting bytes David Brown <david.brown@hesbynett.no> - 2015-06-07 15:32 +0200
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-06-07 12:48 -0700
Re: counting bytes David Brown <david.brown@hesbynett.no> - 2015-06-07 23:05 +0200
Re: counting bytes Richard Heathfield <rjh@cpax.org.uk> - 2015-06-07 22:17 +0100
Re: counting bytes David Brown <david.brown@hesbynett.no> - 2015-06-07 23:49 +0200
Re: counting bytes Richard Heathfield <rjh@cpax.org.uk> - 2015-06-07 23:37 +0100
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-11 08:25 -0700
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-11 08:21 -0700
Re: counting bytes Martin Shobe <martin.shobe@yahoo.com> - 2015-07-11 13:54 -0500
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-20 06:33 -0700
Re: counting bytes Martin Shobe <martin.shobe@yahoo.com> - 2015-07-20 09:51 -0500
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-24 09:49 -0700
Re: counting bytes Martin Shobe <martin.shobe@yahoo.com> - 2015-07-24 12:26 -0500
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-30 11:16 -0700
Re: counting bytes Martin Shobe <martin.shobe@yahoo.com> - 2015-07-30 23:15 -0500
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-02 06:07 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-08-02 14:31 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-09 05:31 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-08-09 17:09 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-05 09:58 -0700
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-09-05 19:29 -0700
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-06 14:38 -0700
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-09-06 17:46 -0700
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-07 23:50 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-09-06 22:00 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-07 23:24 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-07-24 20:03 -0400
Re: counting bytes Martin Shobe <martin.shobe@yahoo.com> - 2015-07-24 22:13 -0500
Re: counting bytes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 02:22 -0700
Re: counting bytes Martin Shobe <martin.shobe@yahoo.com> - 2015-07-25 11:39 -0500
Re: counting bytes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 09:48 -0700
Re: counting bytes Martin Shobe <martin.shobe@yahoo.com> - 2015-07-25 14:04 -0500
Re: counting bytes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 14:22 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-07-25 11:07 -0400
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-25 12:43 -0400
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-07-25 13:32 -0400
Re: counting bytes glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-25 20:14 +0000
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-25 17:24 -0400
Re: counting bytes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 14:42 -0700
Re: counting bytes glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-25 23:02 +0000
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-25 19:17 -0400
Re: counting bytes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-25 16:37 -0700
Re: counting bytes glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-26 06:10 +0000
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-26 08:31 -0400
Re: counting bytes Bartc <bc@freeuk.com> - 2015-07-30 21:32 +0100
Re: counting bytes raltbos@xs4all.nl (Richard Bos) - 2015-07-31 13:25 +0000
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-31 11:31 -0400
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-07-31 11:30 -0700
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-31 14:29 -0400
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-07-31 12:14 -0700
Re: counting bytes David Kleinecke <dkleinecke@gmail.com> - 2015-07-31 17:39 -0700
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-31 23:32 -0400
Re: counting bytes David Kleinecke <dkleinecke@gmail.com> - 2015-08-01 16:41 -0700
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-08-01 20:23 -0400
Re: counting bytes Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-07-31 11:01 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-07-25 19:42 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-30 12:16 -0700
Re: counting bytes Bartc <bc@freeuk.com> - 2015-07-30 22:04 +0100
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-02 06:10 -0700
Re: counting bytes David Kleinecke <dkleinecke@gmail.com> - 2015-07-25 19:10 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-07-25 22:16 -0400
Re: counting bytes David Kleinecke <dkleinecke@gmail.com> - 2015-07-25 19:46 -0700
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-30 11:52 -0700
Re: counting bytes raltbos@xs4all.nl (Richard Bos) - 2015-07-31 08:51 +0000
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-02 06:19 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-08-01 09:39 -0400
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-08-01 13:06 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-08-01 20:43 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-02 06:52 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-08-02 15:14 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-09 07:54 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-08-09 16:43 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-09-05 10:48 -0700
Re: counting bytes Richard Damon <Richard@Damon-Family.org> - 2015-07-11 17:50 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-20 06:36 -0700
Re: counting bytes raltbos@xs4all.nl (Richard Bos) - 2015-07-20 15:14 +0000
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-24 09:38 -0700
Re: counting bytes raltbos@xs4all.nl (Richard Bos) - 2015-07-25 19:12 +0000
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-30 12:18 -0700
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-07-11 17:03 -0700
Re: counting bytes glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-12 00:15 +0000
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-07-11 17:42 -0700
Re: counting bytes glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-12 02:46 +0000
Re: counting bytes James Kuyper <jameskuyper@verizon.net> - 2015-07-12 12:33 -0400
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-20 06:48 -0700
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-07-20 11:18 -0700
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-24 09:58 -0700
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-07-24 11:14 -0700
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-07-30 11:02 -0700
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-07-30 13:10 -0700
Re: counting bytes Tim Rentsch <txr@alumni.caltech.edu> - 2015-08-01 10:53 -0700
Re: counting bytes Keith Thompson <kst-u@mib.org> - 2015-06-07 14:46 -0700
Re: counting bytes David Brown <david.brown@hesbynett.no> - 2015-06-08 09:05 +0200
Re: counting bytes Siri Cruz <chine.bleu@yahoo.com> - 2015-06-06 05:06 -0700
Re: counting bytes bartekltg <bartekltg@gmail.com> - 2015-07-26 00:58 +0200
Re: counting bytes bartekltg <bartekltg@gmail.com> - 2015-07-26 01:00 +0200
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