diff --git a/generator/errnostring.ml b/generator/errnostring.ml
index d16a07ecd..4a544f337 100644
--- a/generator/errnostring.ml
+++ b/generator/errnostring.ml
@@ -231,7 +231,7 @@ extern int guestfs_int_string_to_errno (const char *errnostr);
 
 /* Private structure and function used by the perfect hash implementation. */
 struct errnostring_entry { char *name; int errnum; };
-extern const struct errnostring_entry *guestfs_int_string_to_errno_lookup (register const char *str, register unsigned int len);
+extern const struct errnostring_entry *guestfs_int_string_to_errno_lookup (register const char *str, register GPERF_SIZE_T len);
 
 #endif /* GUESTFS_ERRNOSTRING_H_ */
 "
diff --git a/m4/guestfs_progs.m4 b/m4/guestfs_progs.m4
index c63e62f09..198598cae 100644
--- a/m4/guestfs_progs.m4
+++ b/m4/guestfs_progs.m4
@@ -50,6 +50,26 @@ AC_CHECK_PROG([GPERF],[gperf],[gperf],[no])
 test "x$GPERF" = "xno" &&
     AC_MSG_ERROR([gperf must be installed])
 
+dnl Determine type of gperf hash function (changed in gperf 3.1).
+AC_MSG_CHECKING([type of gperf hash len parameter])
+cat > conftest.gperf <<EOF
+%language=ANSI-C
+%%
+A, 1
+%%
+EOF
+GPERF_SIZE_T=$(
+    $GPERF conftest.gperf |
+    grep '^hash (.*len)$' |
+    $SED 's/.*, register \([[^,]]*\) len)$/\1/'
+)
+if test "x$GPERF_SIZE_T" = "x"; then
+    AC_MSG_ERROR([could not detect type of gperf hash len parameter])
+fi
+AC_MSG_RESULT([$GPERF_SIZE_T])
+AC_DEFINE_UNQUOTED([GPERF_SIZE_T],[$GPERF_SIZE_T],[Type of gperf hash len parameter.])
+rm conftest.gperf
+
 dnl Check for genisoimage/mkisofs
 AC_PATH_PROGS([GENISOIMAGE],[genisoimage mkisofs],[no],
     [$PATH$PATH_SEPARATOR/usr/sbin$PATH_SEPARATOR/sbin])