--- a/solvers/dtoa1.c
+++ b/solvers/dtoa1.c
@@ -16,6 +16,8 @@
 of or in connection with the use or performance of this software.
 *******************************************************************/
 
+#include <stddef.h>
+
 #ifndef No_dtoa /*{{*/
 #ifdef __cplusplus
 #include "memory.h"