- On second thought, attempt to get the read-only lock, but don't

consider it a exit failure if it doesn't work.  This means that root
  processes can safely get the lock, but normal processes can still use
  the 'pw' utility to get information (which may change out from under
  them.)

usr.sbin/pw/pw.c

@@ -26,11 +26,12 @@
 
 #ifndef lint
 static const char rcsid[] =
-	"$Id: pw.c,v 1.7 1997/10/10 06:23:34 charnier Exp $";
+	"$Id: pw.c,v 1.8 1998/07/16 17:18:25 nate Exp $";
 #endif /* not lint */
 
 #include "pw.h"
 #include <err.h>
+#include <fcntl.h>
 #include <paths.h>
 #include <sys/wait.h>
 
@@ -49,6 +50,7 @@ static struct cargs arglist;
 
 static int      getindex(const char *words[], const char *word);
 static void     cmdhelp(int mode, int which);
+static void     filelock(const char *filename);
 
 
 int
@@ -147,6 +149,13 @@ main(int argc, char *argv[])
 	 * Now, let's do the common initialisation
 	 */
 	cnf = read_userconfig(getarg(&arglist, 'C') ? getarg(&arglist, 'C')->val : NULL);
+
+	/*
+	 * Try to lock the master passowrd and group files right away (we
+	 * don't care if it works, since this is just advisory locking.
+	 */
+	filelock(_PATH_GROUP);
+	filelock(_PATH_MASTERPASSWD);
 	ch = funcs[which] (cnf, mode, &arglist);
 
 	/*
@@ -177,6 +186,12 @@ main(int argc, char *argv[])
 	return ch;
 }
 
+static void
+filelock(const char *filename)
+{
+	open(filename, O_RDONLY | O_EXLOCK, 0);
+}
+
 static int
 getindex(const char *words[], const char *word)
 {