From:	MERC::"uunet!CRVAX.SRI.COM!RELAY-INFO-VAX" 23-JAN-1993 22:36:33.89
To:	info-vax@kl.sri.com
CC:	
Subj:	Re: Flushing file buffers to disk in VAX C

In article <5946@osc.COM>, vivekr@osc.COM (Vivek Rau) writes:
=The problem:
=------------
=I am running VMS 5.5-1 on a VAXstation 4000 model 60. I find that
=the C library call fsync() does not write the file buffers to disk
=as it is supposed to. Could someone else run the program below on 
=this or earlier versions of VMS and check it? Run the program on 
=one terminal, and when it stops to wait for input, kill the process
=using the STOP command from another terminal. If I understand the
=functionality of fsync() correctly, the buffers should have been
=flushed to disk - but when you look at the file after stopping the
=process, it is still empty. Is this a bug? 
=
=#include <stdio.h>
=#include <file.h>
=
=main()
={
=	int fd;

	FILE *fp;

=	char buf[] = "This is a test record\n";
=	char testFileName[] = "chks.out";
=
=	if ( (fd = open(testFileName, O_RDWR | O_CREAT)) < 0) {
=		fprintf(stderr, "Error opening file. Exiting... \n");
=		exit(0);
=	}
=
=	fprintf(stderr, "The file being used is %s\n\n", testFileName);
=	fprintf(stderr, "%s%s%s%s%s",
=	"A record has been written to the file and sync'ed. If the fsync()\n",
=	"call works correctly the data should be in the file even if the\n",
=	"process is stopped. You can now halt the process from another\n",
=	"terminal or window using the STOP command, or hit any key to let\n",
=	"it run to completion...\n"
=		);
=	fflush(stderr);
=
=	write(fd, buf, sizeof(buf));

	fp = fdopen(fd, "w");
	fflush(fp);

=	fsync(fd);
=
=	/**
=		You can now stop the process from another terminal or
=		window while it waits, or let it complete.
=	**/
=	getchar();
=
=	close(fd);
=	fprintf(stderr, "Wrote the record and closed the file\n");
=}

Hope that helps.  As we discussed in gory detail quite recently, you need to
perform an fflush() before the fsync().
--------------------------------------------------------------------------------
Carl J Lydick | INTERnet: CARL@SOL1.GPS.CALTECH.EDU | NSI/HEPnet: SOL1::CARL

Disclaimer:  Hey, I understand VAXen and VMS.  That's what I get paid for.  My
understanding of astronomy is purely at the amateur level (or below).  So
unless what I'm saying is directly related to VAX/VMS, don't hold me or my
organization responsible for it.  If it IS related to VAX/VMS, you can try to
hold me responsible for it, but my organization had nothing to do with it.