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SubjectRe: NFS file locking?
>    Instead of creating a lock file, create a lock symlink.
> Have the content of the symlink be something recognisably unique.
> e.g. hostname.pid
> If the "symlink" syscall succeeds, you have got the lock.
> If it fails, issue a readlink and see if the content is what you
> tried to create (RPC packet loss and retransmit could have caused
> an incorrect failure return). If it is, you have the lock.
> If not, you don't.

OK, tried that too, here's the code. Doesn't work. Neither does the
link approach. Am I doing something wrong? It seems to me that I'm
completely at the mercy of the client NFS implementation - if it caches
stuff wrong, I'm hosed. There has to be some cute trick to get past this.

--lm


int
sccs_lockfile(char *lockfile, int seconds)
{
char *s;
char buf[300];
int n, uslp = 1000, waited = 0;

s = aprintf("%u %s", getpid(), sccs_gethost());
for ( ;; ) {
if (symlink(s, lockfile) == 0) return (0);
n = readlink(lockfile, buf, sizeof(buf));
if (n > 0) {
buf[n] = 0;
if (streq(s, buf)) return (0);
}
if (seconds && ((waited / 1000000) >= seconds)) {
fprintf(stderr, "timed out waiting for %s\n", lockfile);
free(s);
return (-1);
}
usleep(uslp);
waited += uslp;
if (uslp < 20000) uslp <<= 1;
}
/* NOTREACHED */
}

/*
* Usage: a.out iterations lockfile
*/
int
main(int ac, char **av)
{
int i, iter;
int me = getpid();

unless (ac == 3) return (1);
unless ((iter = atoi(av[1])) > 0) return (1);
printf("%d starts\n", me);
for (i = 1; i <= iter; ++i) {
sccs_lockfile(av[2], 0);
assert(mine(av[2]));
unlink(av[2]);
unless (i % 10) printf("%d locked %d times\n", me, i);
}
printf("%d done\n", me);
return (0);
}

int
mine(char *file)
{
char buf[300];
char *s;
int n;

n = readlink(file, buf, sizeof(buf));
if (n > 0) {
s = aprintf("%u %s", getpid(), sccs_gethost());
buf[n] = 0;
n = streq(s, buf);
unless (n) fprintf(stderr, "%s != %s\n", s, buf);
free(s);
return (n);
}
return (0);
}

/*
* This function works like sprintf(), except it return a
* malloc'ed buffer which caller should free when done
*/
char *
aprintf(char *fmt, ...)
{
va_list ptr;
int rc, size = strlen(fmt) + 64;
char *buf = malloc(size);

va_start(ptr, fmt);
rc = vsnprintf(buf, size, fmt, ptr);
va_end(ptr);
/*
* On IRIX, it truncates and returns size-1.
* We can't assume that that is OK, even though that might be
* a perfect fit. We always bump up the size and try again.
* This can rarely lead to an extra alloc that we didn't need,
* but that's tough.
*/
while ((rc < 0) || (rc >= (size-1))) {
size *= 2;
free(buf);
buf = malloc(size);
va_start(ptr, fmt);
rc = vsnprintf(buf, size, fmt, ptr);
va_end(ptr);
}
return (buf); /* caller should free */
}

char *
sccs_gethost()
{
static char host[256];

if (gethostname(host, sizeof(host)) == -1) return "?";
return (host);
}
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