Patch against NetHack 3.4.2 from ftp://alt.org/pub/nethack/nh342-fcntl-1.diff
updated for Slash'EM 7E6F3 (line numbers only) by Ali 2004-12-03.

diff -Naurd ../slashem-0.0.7E6F3/include/unixconf.h ./include/unixconf.h
--- ../slashem-0.0.7E6F3/include/unixconf.h	2004-11-13 08:58:52.000000000 +0000
+++ ./include/unixconf.h	2004-12-03 10:29:25.000000000 +0000
@@ -272,6 +272,12 @@
 
 #define FCMASK	0660	/* file creation mask */
 
+/* fcntl(2) is a POSIX-portable call for manipulating file descriptors.
+ * Comment out the USE_FCNTL if for some reason you have a strange
+ * os/filesystem combination for which fcntl(2) does not work. */
+#ifdef POSIX_TYPES
+# define USE_FCNTL
+#endif
 
 /*
  * The remainder of the file should not need to be changed.
diff -Naurd ../slashem-0.0.7E6F3/src/files.c ./src/files.c
--- ../slashem-0.0.7E6F3/src/files.c	2004-11-13 08:58:52.000000000 +0000
+++ ./src/files.c	2004-12-03 10:29:25.000000000 +0000
@@ -23,7 +23,7 @@
 
 #include <ctype.h>
 
-#if !defined(MAC) && !defined(O_WRONLY) && !defined(AZTEC_C)
+#if (!defined(MAC) && !defined(O_WRONLY) && !defined(AZTEC_C)) || defined(USE_FCNTL)
 #include <fcntl.h>
 #endif
 
@@ -1427,9 +1427,12 @@
 
 static int nesting = 0;
 
-#ifdef NO_FILE_LINKS	/* implies UNIX */
+#if defined(NO_FILE_LINKS) || defined(USE_FCNTL) 	/* implies UNIX */
 static int lockfd;	/* for lock_file() to pass to unlock_file() */
 #endif
+#ifdef USE_FCNTL
+struct flock sflock; /* for unlocking, same as above */
+#endif
 
 #define HUP	if (!program_state.done_hup)
 
@@ -1467,7 +1470,6 @@
 #endif
 }
 
-
 /* lock a file */
 boolean
 lock_file(filename, whichprefix, retryct)
@@ -1487,18 +1489,51 @@
 	    return TRUE;
 	}
 
+#ifndef USE_FCNTL
 	lockname = make_lockname(filename, locknambuf);
-	filename = fqname(filename, whichprefix, 0);
-#ifndef NO_FILE_LINKS	/* LOCKDIR should be subsumed by LOCKPREFIX */
+# ifndef NO_FILE_LINKS	/* LOCKDIR should be subsumed by LOCKPREFIX */
 	lockname = fqname(lockname, LOCKPREFIX, 2);
+# endif
+#endif
+	filename = fqname(filename, whichprefix, 0);
+
+#ifdef USE_FCNTL
+	lockfd = open(filename,O_RDWR);
+	if (lockfd == -1) {
+		HUP raw_printf("Cannot open file %s. This is a program bug.",
+			filename);
+	}
+	sflock.l_type = F_WRLCK;
+	sflock.l_whence = SEEK_SET;
+	sflock.l_start = 0;
+	sflock.l_len = 0;
 #endif
 
 #if defined(UNIX) || defined(VMS)
-# ifdef NO_FILE_LINKS
+# ifdef USE_FCNTL
+	while (fcntl(lockfd,F_SETLK,&sflock) == -1) {
+# else 
+#  ifdef NO_FILE_LINKS
 	while ((lockfd = open(lockname, O_RDWR|O_CREAT|O_EXCL, 0666)) == -1) {
-# else
+#  else
 	while (link(filename, lockname) == -1) {
-# endif
+#  endif
+# endif 
+
+#ifdef USE_FCNTL
+		if (retryct--) {
+			HUP raw_printf(
+				"Waiting for release of fcntl lock on %s. (%d retries left).",
+				filename, retryct);
+			sleep(1);
+		} else {
+		    HUP (void) raw_print("I give up.  Sorry.");
+		    HUP raw_printf("Some other process has an unnatural grip on %s.",
+					filename);
+		    nesting--;
+		    return FALSE;
+		}
+#else
 	    register int errnosv = errno;
 
 	    switch (errnosv) {	/* George Barbanis */
@@ -1544,11 +1579,11 @@
 		nesting--;
 		return FALSE;
 	    }
-
+#endif /* USE_FCNTL */
 	}
 #endif  /* UNIX || VMS */
 
-#if defined(AMIGA) || defined(WIN32) || defined(MSDOS)
+#if (defined(AMIGA) || defined(WIN32) || defined(MSDOS)) && !defined(USE_FCNTL)
 # ifdef AMIGA
 #define OPENFAILURE(fd) (!fd)
     lockptr = 0;
@@ -1599,25 +1634,33 @@
 	const char *lockname;
 
 	if (nesting == 1) {
+#ifdef USE_FCNTL
+		sflock.l_type = F_UNLCK;
+		if (fcntl(lockfd,F_SETLK,&sflock) == -1) {
+			HUP raw_printf("Can't remove fcntl lock on %s.", filename);
+			(void) close(lockfd);
+		}
+# else
 		lockname = make_lockname(filename, locknambuf);
-#ifndef NO_FILE_LINKS	/* LOCKDIR should be subsumed by LOCKPREFIX */
+# ifndef NO_FILE_LINKS	/* LOCKDIR should be subsumed by LOCKPREFIX */
 		lockname = fqname(lockname, LOCKPREFIX, 2);
-#endif
+# endif
 
-#if defined(UNIX) || defined(VMS)
+# if defined(UNIX) || defined(VMS)
 		if (unlink(lockname) < 0)
 			HUP raw_printf("Can't unlink %s.", lockname);
-# ifdef NO_FILE_LINKS
+#  ifdef NO_FILE_LINKS
 		(void) close(lockfd);
-# endif
+#  endif
 
-#endif  /* UNIX || VMS */
+# endif  /* UNIX || VMS */
 
-#if defined(AMIGA) || defined(WIN32) || defined(MSDOS)
+# if defined(AMIGA) || defined(WIN32) || defined(MSDOS)
 		if (lockptr) Close(lockptr);
 		DeleteFile(lockname);
 		lockptr = 0;
-#endif /* AMIGA || WIN32 || MSDOS */
+# endif /* AMIGA || WIN32 || MSDOS */
+#endif /* USE_FCNTL */
 	}
 
 	nesting--;