diff --git a/zbar/decoder.c b/zbar/decoder.c
index 7f7336f..e9ffc45 100644
--- a/zbar/decoder.c
+++ b/zbar/decoder.c
@@ -25,6 +25,7 @@
 #include <stdlib.h>     /* malloc, calloc, free */
 #include <stdio.h>      /* snprintf */
 #include <string.h>     /* memset, strlen */
+#include <stddef.h>     /* offsetof */
 
 #include <zbar.h>
 
@@ -113,7 +114,7 @@ void zbar_decoder_destroy (zbar_decoder_t *dcode)
 
 void zbar_decoder_reset (zbar_decoder_t *dcode)
 {
-    memset(dcode, 0, (long)&dcode->buf_alloc - (long)dcode);
+    memset(dcode, 0, offsetof(zbar_decoder_t, buf_alloc));
 #if ENABLE_EAN == 1
     ean_reset(&dcode->ean);
 #endif