diff --git a/configure.ac b/configure.ac
index 400c33bf0..0cfe52484 100644
--- a/configure.ac
+++ b/configure.ac
@@ -378,6 +378,21 @@ AC_PATH_PROG([GPERF], [gperf], [], [$PATH:/bin:/usr/bin:/usr/local/bin])
 AC_MSG_CHECKING([gperf version >= 3.0.0])
 if test -x "$GPERF"; then
 	if test "`$GPERF --version | $AWK -F' ' '/^GNU gperf/ { print $3 }' | $AWK -F. '{ print $1 }'`" -ge "3"; then
+		GPERF_OUTPUT="`echo foo | ${GPERF}`"
+		AC_COMPILE_IFELSE(
+			[AC_LANG_PROGRAM(
+				[[#include <string.h>
+				  const char *in_word_set(const char*, size_t); $GPERF_OUTPUT]])],
+			[GPERF_LEN_TYPE=size_t],
+			[AC_COMPILE_IFELSE(
+				[AC_LANG_PROGRAM(
+					[[#include <string.h>
+					  const char *in_word_set(const char*, unsigned); $GPERF_OUTPUT]])],
+				[GPERF_LEN_TYPE=unsigned],
+				[AC_MSG_ERROR([unable to determine gperf len type])]
+			)]
+		)
+		AC_DEFINE_UNQUOTED([GPERF_LEN_TYPE], [$GPERF_LEN_TYPE], [gperf len type])
 		AC_MSG_RESULT([yes])
 	else
 		AC_MSG_RESULT([no])
diff --git a/src/libstrongswan/crypto/proposal/proposal_keywords_static.h b/src/libstrongswan/crypto/proposal/proposal_keywords_static.h
index bc421dcc5..c046fab92 100644
--- a/src/libstrongswan/crypto/proposal/proposal_keywords_static.h
+++ b/src/libstrongswan/crypto/proposal/proposal_keywords_static.h
@@ -19,7 +19,7 @@
 #include "proposal_keywords.h"
 
 const proposal_token_t* proposal_get_token_static(register const char *str,
-												  register unsigned int len);
+												  register GPERF_LEN_TYPE len);
 
 #endif /* PROPOSAL_KEYWORDS_STATIC_H_ */
 
diff --git a/src/starter/confread.c b/src/starter/confread.c
index f98fc8936..e78f2a70b 100644
--- a/src/starter/confread.c
+++ b/src/starter/confread.c
@@ -48,7 +48,7 @@ static const char firewall_defaults[] = IPSEC_SCRIPT " _updown iptables";
 /**
  * Provided by GPERF
  */
-extern kw_entry_t *in_word_set (char *str, unsigned int len);
+extern const kw_entry_t *in_word_set(register const char*, register GPERF_LEN_TYPE);
 
 /**
  * Process deprecated keywords
@@ -95,7 +95,7 @@ static void load_setup(starter_config_t *cfg, conf_parser_t *parser)
 {
 	enumerator_t *enumerator;
 	dictionary_t *dict;
-	kw_entry_t *entry;
+	const kw_entry_t *entry;
 	char *key, *value;
 
 	DBG2(DBG_APP, "Loading config setup");
@@ -146,7 +146,7 @@ static void load_ca(starter_ca_t *ca, starter_config_t *cfg,
 {
 	enumerator_t *enumerator;
 	dictionary_t *dict;
-	kw_entry_t *entry;
+	const kw_entry_t *entry;
 	kw_token_t token;
 	char *key, *value;
 
@@ -559,7 +559,7 @@ static void load_conn(starter_conn_t *conn, starter_config_t *cfg,
 {
 	enumerator_t *enumerator;
 	dictionary_t *dict;
-	kw_entry_t *entry;
+	const kw_entry_t *entry;
 	kw_token_t token;
 	char *key, *value;
 
diff --git a/src/stroke/stroke_keywords.h b/src/stroke/stroke_keywords.h
index 4a1016277..51caba68b 100644
--- a/src/stroke/stroke_keywords.h
+++ b/src/stroke/stroke_keywords.h
@@ -73,6 +73,6 @@ typedef enum {
 
 typedef struct stroke_token stroke_token_t;
 
-extern const stroke_token_t* in_word_set(register const char *str, register unsigned int len);
+extern const stroke_token_t* in_word_set(register const char *str, register GPERF_LEN_TYPE len);
 
 #endif /* _STROKE_KEYWORDS_H_ */