diff --git a/doc/ref/api-modules.texi b/doc/ref/api-modules.texi
index b9befc00c..47b81601b 100644
--- a/doc/ref/api-modules.texi
+++ b/doc/ref/api-modules.texi
@@ -945,14 +945,18 @@ the @var{name} is not bound in the module, signals an error.  Returns a
 variable, always.
 
 @example
+static SCM eval_string_var;
+
+/* NOTE: It is important that the call to 'my_init'
+   happens-before all calls to 'my_eval_string'. */
+void my_init (void)
+@{
+  eval_string_var = scm_c_public_lookup ("ice-9 eval-string",
+                                         "eval-string");
+@}
+
 SCM my_eval_string (SCM str)
 @{
-  static SCM eval_string_var = SCM_BOOL_F;
-
-  if (scm_is_false (eval_string_var))
-    eval_string_var =
-      scm_c_public_lookup ("ice-9 eval-string", "eval-string");
-
   return scm_call_1 (scm_variable_ref (eval_string_var), str);
 @}
 @end example
diff --git a/libguile/debug.c b/libguile/debug.c
index b1a90d84d..9e6328b3a 100644
--- a/libguile/debug.c
+++ b/libguile/debug.c
@@ -211,10 +211,14 @@ SCM_DEFINE (scm_debug_hang, "debug-hang", 0, 1, 0,
 SCM
 scm_local_eval (SCM exp, SCM env)
 {
-  static SCM local_eval_var = SCM_BOOL_F;
+  static SCM local_eval_var = SCM_UNDEFINED;
+  static scm_i_pthread_mutex_t local_eval_var_mutex
+    = SCM_I_PTHREAD_MUTEX_INITIALIZER;
 
-  if (scm_is_false (local_eval_var))
+  scm_i_scm_pthread_mutex_lock (&local_eval_var_mutex);
+  if (SCM_UNBNDP (local_eval_var))
     local_eval_var = scm_c_public_variable ("ice-9 local-eval", "local-eval");
+  scm_i_pthread_mutex_unlock (&local_eval_var_mutex);
 
   return scm_call_2 (SCM_VARIABLE_REF (local_eval_var), exp, env);
 }
diff --git a/libguile/ports.c b/libguile/ports.c
index 55808e272..8737a7672 100644
--- a/libguile/ports.c
+++ b/libguile/ports.c
@@ -418,10 +418,14 @@ SCM_DEFINE (scm_current_error_port, "current-error-port", 0, 0, 0,
 SCM
 scm_current_warning_port (void)
 {
-  static SCM cwp_var = SCM_BOOL_F;
+  static SCM cwp_var = SCM_UNDEFINED;
+  static scm_i_pthread_mutex_t cwp_var_mutex
+    = SCM_I_PTHREAD_MUTEX_INITIALIZER;
 
-  if (scm_is_false (cwp_var))
-    cwp_var = scm_c_private_lookup ("guile", "current-warning-port");
+  scm_i_scm_pthread_mutex_lock (&cwp_var_mutex);
+  if (SCM_UNBNDP (cwp_var))
+    cwp_var = scm_c_private_variable ("guile", "current-warning-port");
+  scm_i_pthread_mutex_unlock (&cwp_var_mutex);
   
   return scm_call_0 (scm_variable_ref (cwp_var));
 }
diff --git a/libguile/strports.c b/libguile/strports.c
index 14cc93f81..d1b293c21 100644
--- a/libguile/strports.c
+++ b/libguile/strports.c
@@ -534,13 +534,16 @@ SCM_DEFINE (scm_eval_string_in_module, "eval-string", 1, 1, 0,
             "procedure returns.")
 #define FUNC_NAME s_scm_eval_string_in_module
 {
-  static SCM eval_string = SCM_BOOL_F, k_module = SCM_BOOL_F;
+  static SCM eval_string = SCM_UNDEFINED, k_module = SCM_UNDEFINED;
+  static scm_i_pthread_mutex_t init_mutex = SCM_I_PTHREAD_MUTEX_INITIALIZER;
 
-  if (scm_is_false (eval_string))
+  scm_i_scm_pthread_mutex_lock (&init_mutex);
+  if (SCM_UNBNDP (eval_string))
     {
-      eval_string = scm_c_public_lookup ("ice-9 eval-string", "eval-string");
+      eval_string = scm_c_public_variable ("ice-9 eval-string", "eval-string");
       k_module = scm_from_locale_keyword ("module");
     }
+  scm_i_pthread_mutex_unlock (&init_mutex);
   
   if (SCM_UNBNDP (module))
     module = scm_current_module ();