diff --git a/libguile/posix.c b/libguile/posix.c
index f4035e8a0..cba143bb0 100644
--- a/libguile/posix.c
+++ b/libguile/posix.c
@@ -768,6 +768,15 @@ SCM_DEFINE (scm_setsid, "setsid", 0, 0, 0,
 #undef FUNC_NAME
 #endif /* HAVE_SETSID */
 
+
+/* ttyname returns its result in a single static buffer, hence
+   scm_i_misc_mutex for thread safety.  In glibc 2.3.2 two threads
+   continuously calling ttyname will otherwise get an overwrite quite
+   easily.
+
+   ttyname_r (when available) could be used instead of scm_i_misc_mutex, but
+   there's probably little to be gained in either speed or parallelism.  */
+
 #ifdef HAVE_TTYNAME
 SCM_DEFINE (scm_ttyname, "ttyname", 1, 0, 0, 
             (SCM port),
@@ -776,22 +785,32 @@ SCM_DEFINE (scm_ttyname, "ttyname", 1, 0, 0,
 #define FUNC_NAME s_scm_ttyname
 {
   char *result;
-  int fd;
+  int fd, err;
+  SCM ret;
 
   port = SCM_COERCE_OUTPORT (port);
   SCM_VALIDATE_OPPORT (1, port);
   if (!SCM_FPORTP (port))
     return SCM_BOOL_F;
   fd = SCM_FPORT_FDES (port);
+
+  scm_mutex_lock (&scm_i_misc_mutex);
   SCM_SYSCALL (result = ttyname (fd));
+  err = errno;
+  ret = scm_makfrom0str (result);
+  scm_mutex_unlock (&scm_i_misc_mutex);
+
   if (!result)
-    SCM_SYSERROR;
-  /* result could be overwritten by another call to ttyname */
-  return (scm_makfrom0str (result));
+    {
+      errno = err;
+      SCM_SYSERROR;
+    }
+  return ret;
 }
 #undef FUNC_NAME
 #endif /* HAVE_TTYNAME */
 
+
 /* For thread safety "buf" is used instead of NULL for the ctermid static
    buffer.  Actually it's unlikely the controlling terminal will change
    during program execution, and indeed on glibc (2.3.2) it's always just