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From: pwb@newt.phys.unsw.OZ.AU (Paul W. Brooks)
Newsgroups: comp.os.msdos.programmer
Subject: Re: Fastest dump to disk?
Summary: write(), not fwrite()
Message-ID: <1091@usage.csd.unsw.oz.au>
Date: 18 Feb 91 02:33:19 GMT
References: <91043.105618JJLUCSY@MTUS5.BITNET> <2184@umriscc.isc.umr.edu>
Sender: news@usage.csd.unsw.oz.au
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In article <2184@umriscc.isc.umr.edu>, mcastle@mcs213f.cs.umr.edu (Mike Castle {Nexus}) writes:
> In article <91043.105618JJLUCSY@MTUS5.BITNET> JJLUCSY@MTUS5.BITNET writes:
> >Could someone please tell me what is the fastest way to dump a chunk of memory
> >to disk using either Turbo C v.2 or Turbo C++. I'm not to particular on the
> >method, but it should be real quick. Thanks in advance.
> >
> 
> Try fwrite.  The prototype is:
> 
> size_t fwrite(const void *ptr, size_t size, size_t n, FILE *stream);
> 
> It starts at *ptr and writes n records of size size to file pointed to by 
> steam.

Sorry - incorrect. The fastest way is using low-level file access -
write(). Using fwrite() requires the disk library internals to keep up
to date with internal buffers, etc., and is quite slow. In my systems I
usually get at least 3 times the speed when using write() instaed of
fwrite()!
	There are a couple of things to watch, though. Preferably the
file handle is obtained through the low-level call to open() or creat(). If you
need to write to a file opened using the stream method (fopen()), and
get to the underlying file handle using fileno(stream), then beware that
the buffers probably will not be correct, and any later streams-based
calls (fwrite(), fread(), fgetc(), etc etc) may fail unpredictably.
There are techniques to bring the low-level and higher-level routines
back in line, e-mail me if you need some hints. In my software I have to
manipulate 500K astronomical images, and use only the low-level file
operations:
( in MSC, with some rigorous bits omitted 8-) )

		int	file,row,bytesout;
	
		( bits missed here )
		file = creat("filename",S_IREAD|S_IWRITE);
		if (file==-1)
			bomb_out();
		bytesout =write(file,bufptr,buflen);
		if (bytesout!=buflen)
			bomb_out();

		( more writes here! )

		close(file);

Anyone know of any faster way of accessing large amounts of disk data?
I'd really appreciate hearing it. :-)
Regards,

Paul Brooks        |Internet: pwb@newt.phys.unsw.edu.au
Uni. of N.S.W.     |If you have trouble sleeping, try lying on the end of
Kensington NSW 2033|   your bed. With a little luck you'll drop off. 
AUSTRALIA          |                              - Mark Twain.