Module Name: src
Committed By: rillig
Date: Sun Apr 23 11:52:43 UTC 2023
Modified Files:
src/tests/usr.bin/xlint/lint1: msg_135.c
Log Message:
tests/lint: add rationale for not warning about alignment problems
To generate a diff of this commit:
cvs rdiff -u -r1.14 -r1.15 src/tests/usr.bin/xlint/lint1/msg_135.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/tests/usr.bin/xlint/lint1/msg_135.c
diff -u src/tests/usr.bin/xlint/lint1/msg_135.c:1.14 src/tests/usr.bin/xlint/lint1/msg_135.c:1.15
--- src/tests/usr.bin/xlint/lint1/msg_135.c:1.14 Sat Apr 22 20:54:28 2023
+++ src/tests/usr.bin/xlint/lint1/msg_135.c Sun Apr 23 11:52:43 2023
@@ -1,4 +1,4 @@
-/* $NetBSD: msg_135.c,v 1.14 2023/04/22 20:54:28 rillig Exp $ */
+/* $NetBSD: msg_135.c,v 1.15 2023/04/23 11:52:43 rillig Exp $ */
# 3 "msg_135.c"
// Test for message: converting '%s' to '%s' increases alignment from %u to %u [135]
@@ -77,13 +77,21 @@ plain_char_to_unsigned_type(char *cp)
/*
* Converting a pointer with a low alignment requirement to a union that
- * includes other types with higher alignment requirements is safe. While
- * accessing any other member of the union might trigger an alignment
- * violation, such an access would invoke undefined behavior anyway.
+ * includes other types with higher alignment requirements is considered safe.
+ * While accessing any other member of the union might trigger an alignment
+ * violation, such an access is not likely from an application point of view,
+ * as it would access undefined memory and thus invoke undefined behavior.
*
* A practical case for this pattern are tagged unions, in which the first
* member of the struct determines how the remaining members are interpreted.
* See sbin/newfs_udf, function udf_validate_tag_and_crc_sums for an example.
+ *
+ * C99 6.2.5p26 defines the representation and alignment of types, stating
+ * that pointers to union types need not have the same representation and
+ * alignment as pointers to other types.
+ *
+ * C99 6.7.2.1p14 and C23 6.7.2.1p18 both state that a "pointer to a union
+ * object [...] points to each of its members [...], and vice versa".
*/
double
cast_to_union(void)
