Add UP_CAST macro, for safer casting from contained data to its container.

[pspp] / src / libpspp / cast.h

diff --git a/src/libpspp/cast.h b/src/libpspp/cast.h
new file mode 100644 (file)
index 0000000..f74442c
--- /dev/null
+++ b/src/libpspp/cast.h
@@ -0,0 +1,34 @@
+/* PSPP - a program for statistical analysis.
+   Copyright (C) 2009 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>. */
+
+#ifndef LIBPSPP_CAST_H
+#define LIBPSPP_CAST_H 1
+
+#include <stddef.h>
+
+/* Given expressions A and B, both of which have pointer type,
+   expands to a void expression that causes a compiler warning if
+   A and B are not pointers to qualified or unqualified versions
+   of compatible types. */
+#define CHECK_POINTER_COMPATIBILITY(A, B) ((void) sizeof ((A) == (B)))
+
+/* Given POINTER, a pointer to the given MEMBER within structure
+   STRUCT, returns the address of the STRUCT. */
+#define UP_CAST(POINTER, STRUCT, MEMBER)                                \
+        (CHECK_POINTER_COMPATIBILITY (&((STRUCT *) 0)->MEMBER, POINTER), \
+         (STRUCT *) ((char *) (POINTER) - offsetof (STRUCT, MEMBER)))
+
+#endif /* libpspp/cast.h */