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From: tps@chem.ucsd.edu (Tom Stockfisch)
Newsgroups: comp.sys.sgi
Subject: Re: compressed files
Keywords: a question for the compression gods
Message-ID: <653@chem.ucsd.EDU>
Date: 18 Jan 90 22:46:33 GMT
References: <3339@uceng.UC.EDU>
Reply-To: tps@chem.ucsd.edu (Tom Stockfisch)
Organization: Chemistry Dept, UC San Diego
Lines: 67

In article <3339@uceng.UC.EDU> trohling@uceng.UC.EDU (tom rohling) writes:
>     Can a compressed file be accessed through fortran (or C) much in the 
>same way that 'zcat' uncompresses the file to std out but leaves the file 
>in its compressed state?  i.e. can I read the contents of a compressed file
>from a program without having to uncompress it first?  Sort of like zcat 
>it into ram where my program can get at it without creating file out 
>of it and taking up all that space.
>     We have these rather large files (20 Meg uncompressed) we are using 
>... and alot of the time there isn't enough room on 
>the disk to uncompress all the files and run the program for a while and 
>still leave enough disk space for other users.

Use the following routine in place of fopen( "bigFile", "r" ):

/* zopen():
 *	open a compressed file for reading, filtering it thru zcat.
 */

# include <stdio.h>

static void	defaultErrHndlr();

void	(*zopenErrHndlr)() =	defaultErrHndlr;

FILE *
zopen(name)
	char	*name;
{
	FILE	*stream;
	int	piped[2];
#		define READ	0
#		define WRITE	1

	if ( pipe(piped) == -1 )
		(*zopenErrHndlr)( "pipe failure\n" );
	switch ( fork() )
	{
	case -1:
		(*zopenErrHndlr)( "fork failure\n" );
	case 0:	/* child */
		close( piped[READ] );
		close(1);
		if ( dup( piped[WRITE] ) != 1 )
			(*zopenErrHndlr)( "dup screwup\n" );
		close( piped[WRITE] );
		execlp( "zcat", "zcat", name, (char *)0 );
		(*zopenErrHndlr)( "cannot start zcat" );
	default:	/* parent */
		close( piped[WRITE] );
		stream =	fdopen( piped[READ], "r" );
		if (stream == NULL)
			(*zopenErrHndlr)( "cannot open pipe\n" );
		break;
	}
	return	stream;
}

static void
defaultErrHndlr(diagnostic)
	char	*diagnostic;
{
	fprintf( stderr, "zopen(): %s\n", diagnostic );
	exit(1);
}
-- 

|| Tom Stockfisch, UCSD Chemistry	tps@chem.ucsd.edu