diff --git a/reactos/lib/inflib/infcore.c b/reactos/lib/inflib/infcore.c
index 18ad68dba98..e58383685c0 100644
--- a/reactos/lib/inflib/infcore.c
+++ b/reactos/lib/inflib/infcore.c
@@ -181,7 +181,8 @@ InfpAddSection(PINFCACHE Cache,
     }
 
   /* Allocate and initialize the new section */
-  Size = sizeof(INFCACHESECTION) + (_tcslen (Name) * sizeof(TCHAR));
+  Size = FIELD_OFFSET(INFCACHESECTION,
+                      Name[_tcslen (Name) + 1]);
   Section = (PINFCACHESECTION)MALLOC (Size);
   if (Section == NULL)
     {
@@ -285,7 +286,8 @@ InfpAddFieldToLine(PINFCACHELINE Line,
   PINFCACHEFIELD Field;
   ULONG Size;
 
-  Size = sizeof(INFCACHEFIELD) + (_tcslen(Data) * sizeof(TCHAR));
+  Size = FIELD_OFFSET(INFCACHEFIELD,
+                      Data[_tcslen(Data) + 1]);
   Field = (PINFCACHEFIELD)MALLOC(Size);
   if (Field == NULL)
     {
diff --git a/reactos/lib/inflib/infpriv.h b/reactos/lib/inflib/infpriv.h
index df13c04b02c..2619b1ca77a 100644
--- a/reactos/lib/inflib/infpriv.h
+++ b/reactos/lib/inflib/infpriv.h
@@ -9,6 +9,9 @@
 #ifndef INFPRIV_H_INCLUDED
 #define INFPRIV_H_INCLUDED
 
+#ifndef FIELD_OFFSET
+#define FIELD_OFFSET(t,f) ((ptrdiff_t)&(((t*)0)->f))
+#endif
 
 #define INF_STATUS_INSUFFICIENT_RESOURCES  (0xC000009A)
 #define INF_STATUS_BAD_SECTION_NAME_LINE   (0xC0700001)