🚀 go-pugleaf

In article <FRIEDMAN.92Jul24055302@nutrimat.gnu.ai.mit.edu> friedman@gnu.ai.mit.edu (Noah Friedman) writes:
>In article <1992Jul23.124327.20843@cv.ruu.nl> rvloon@cv.ruu.nl (Ronald van Loon) writes:
>>Somewhere on our system is a log-file. This logfile is used to gather
>>statistical information. However, it is possible that people write to the file
>>at the same time, thus resulting in either the merge of the input-streams or
>>total garbage in the file.
>>
>>How can I - within a shell script - lock the file exclusive for writing ? If
>>that's not possible, what do I need to use to do so from C, for example ?
>   If you can design the locking protocol yourself, and make sure that
>every interface people will use to write the log file will follow that
>protocol, you can specify when a file is locked by creating a separate
>lockfile.  Other programs seeing this file will know someone else has a
>lock on the log file (many mail programs use this "advisory" locking
>method).  You can easily create a lock file in a C program using
>
>            open (lockfile_path, O_CREAT|O_EXCL|..., mode);
>
>(appropriate error checking omitted, since that part is obvious).  This
>means that open will only create the file if it does not already exist.
>This is an atomic operation, so you shouldn't have to worry about any race
>conditions (locally, anyway.. I don't know if anything is guaranteed over
>NFS, including other locking methods!)

Nothing is guaranteed over NFS, apart from link(2)[*].  There was a
thread about a week ago which covered this exact subject.  You could use
ln(1) for this purpose, but Sys V has broken ln(1), so you have to write
your own little locking program, or use the lock program provided below
(which is 'newslock' from the c-news distribution.) You have to have
something like this in your script:

LOCKFILE=LOCKFILE
TMP=L.$$

echo $$ > $TMP
if newslock $TMP $LOCKFILE
then
	# you have the lock
else
	# you dont have the lock
fi

And this is the program:

/*
 * newslock - simple, unbroken version of ln(1) for shell-program locking
 *
 * (System V has broken ln(1) itself.)
 */
#include <stdio.h>

main(argc, argv)
int argc;
char *argv[];
{
	if (argc != 3) {
		fprintf(stderr, "Usage: %s tempname lockname\n", argv[0]);
		exit(2);
	}

	if (link(argv[1], argv[2]) < 0)
		exit(1);
	else
		exit(0);
	/* NOTREACHED */
}

Can someone please add this to the FAQ?

Dylan.
------
[*] With NFS, the locking is atomic, but you can find yourself with
stale locks hanging around, if you're unlucky.
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