config: Properly align flex_sizeof()

2 files changed, 34 insertions, 10 deletions

diff --git a/src/config.h b/src/config.h
index 4408feb..c6dcc1e 100644
--- a/src/config.h
+++ b/src/config.h
@@ -140,11 +140,12 @@
  */
 #define countof(array) (sizeof(array) / sizeof(0[array]))
 
-// Lower bound on flex_sizeof()
-#define BFS_FLEX_LB(type, member, length) (offsetof(type, member) + sizeof(((type *)NULL)->member[0]) * (length))
-
-// Maximum macro for BFS_FLEX_SIZE()
-#define BFS_FLEX_MAX(a, b) ((a) > (b) ? (a) : (b))
+/**
+ * Round up to a multiple of an alignment.
+ */
+static inline size_t align_ceil(size_t align, size_t size) {
+	return (size + align - 1) & ~(align - 1);
+}
 
 /**
  * Computes the size of a struct containing a flexible array member of the given
@@ -154,13 +155,23 @@
  *         The type of the struct containing the flexible array.
  * @param member
  *         The name of the flexible array member.
- * @param length
+ * @param count
  *         The length of the flexible array.
  */
-#define flex_sizeof(type, member, length) \
-	(sizeof(type) <= BFS_FLEX_LB(type, member, 0) \
-		? BFS_FLEX_LB(type, member, length) \
-		: BFS_FLEX_MAX(sizeof(type), BFS_FLEX_LB(type, member, length)))
+#define flex_sizeof(type, member, count) \
+	flex_sizeof_impl(alignof(type), sizeof(type), offsetof(type, member), sizeof(((type *)NULL)->member[0]), count)
+
+static inline size_t flex_sizeof_impl(size_t align, size_t min, size_t offset, size_t size, size_t count) {
+	size_t ret = align_ceil(align, offset + size * count);
+
+	// Make sure flex_sizeof(type, member, 0) >= sizeof(type), even if the
+	// type has more padding than necessary for alignment
+	if (min > align_ceil(align, offset) && ret < min) {
+		ret = min;
+	}
+
+	return ret;
+}
 
 /**
  * Initialize a variable, unless sanitizers would detect uninitialized uses.
diff --git a/tests/bfstd.c b/tests/bfstd.c
index 8c61072..c6a9e9f 100644
--- a/tests/bfstd.c
+++ b/tests/bfstd.c
@@ -1,8 +1,12 @@
 // Copyright © Tavian Barnes <tavianator@tavianator.com>
 // SPDX-License-Identifier: 0BSD
 
+#undef NDEBUG
 #include "../src/bfstd.h"
+#include "../src/config.h"
+#include <assert.h>
 #include <stdio.h>
+#include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -30,6 +34,15 @@ static void check_base_dir(const char *path, const char *dir, const char *base)
 }
 
 int main(void) {
+	// Check flex_sizeof()
+	struct flexible {
+		alignas(64) int foo;
+		int bar[];
+	};
+	assert(flex_sizeof(struct flexible, bar, 0) >= sizeof(struct flexible));
+	assert(flex_sizeof(struct flexible, bar, 16) % alignof(struct flexible) == 0);
+	assert(flex_sizeof_impl(8, 16, 4, 4, 1) == 16);
+
 	// From man 3p basename
 	check_base_dir("usr", ".", "usr");
 	check_base_dir("usr/", ".", "usr");