--- posix.c.~1.132.~	2004-07-14 10:19:36.000000000 +1000
+++ posix.c	2004-07-23 17:12:23.000000000 +1000
@@ -1426,20 +1426,46 @@
 #undef FUNC_NAME
 #endif /* HAVE_SYNC */
 
+
+/* crypt() returns a pointer to a static buffer, so we use scm_i_misc_mutex
+   to avoid another thread overwriting it.  A test program running crypt
+   continuously in two threads can be quickly seen tripping this problem.
+   crypt() is pretty slow normally, so a mutex shouldn't add much overhead.
+
+   glibc has a thread-safe crypt_r, but (in version 2.3.2) it runs a lot
+   slower (about 5x) than plain crypt if you pass an uninitialized data
+   block each time.  Presumably there's some one-time setups.  The best way
+   to use crypt_r to allow parallel execution in multiple threads would
+   probably be to maintain a little pool of initialized crypt_data
+   structures, take one and use it, then return it to the pool.  That pool
+   could be garbage collected so it didn't add permanently to memory use in
+   case only a few crypt calls are made.  But we expect crypt will be used
+   rarely, and even more rarely will there be any desire for lots of
+   parallel execution on multiple cpus.  So for now we don't bother with
+   anything fancy, just ensure it's thread-safe.  */
+
 #if HAVE_CRYPT
-SCM_DEFINE (scm_crypt, "crypt", 2, 0, 0, 
+SCM_DEFINE (scm_crypt, "crypt", 2, 0, 0,
             (SCM key, SCM salt),
 	    "Encrypt @var{key} using @var{salt} as the salt value to the\n"
 	    "crypt(3) library call.")
 #define FUNC_NAME s_scm_crypt
 {
-  char * p;
-
+  SCM ret;
   SCM_VALIDATE_STRING (1, key);
   SCM_VALIDATE_STRING (2, salt);
 
-  p = crypt (SCM_STRING_CHARS (key), SCM_STRING_CHARS (salt));
-  return scm_makfrom0str (p);
+  scm_frame_begin (0);
+  scm_frame_unwind_handler ((void(*)(void*)) scm_mutex_unlock,
+                            &scm_i_misc_mutex,
+                            SCM_F_WIND_EXPLICITLY);
+  scm_mutex_lock (&scm_i_misc_mutex);
+
+  ret = scm_makfrom0str (crypt (SCM_STRING_CHARS (key),
+                                SCM_STRING_CHARS (salt)));
+
+  scm_frame_end ();
+  return ret;
 }
 #undef FUNC_NAME
 #endif /* HAVE_CRYPT */