config: Align after saturating in flex_sizeof()

This ensures that it's legal to call aligned_alloc() with the result,
which requires a multiple of the alignment.

2 files changed, 11 insertions, 3 deletions

diff --git a/src/config.h b/src/config.h
index b2c58be..47aa529 100644
--- a/src/config.h
+++ b/src/config.h
@@ -141,10 +141,17 @@
 #define countof(array) (sizeof(array) / sizeof(0[array]))
 
 /**
+ * Round down to a multiple of an alignment.
+ */
+static inline size_t align_floor(size_t align, size_t size) {
+	return size & ~(align - 1);
+}
+
+/**
  * 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);
+	return align_floor(align, size + align - 1);
 }
 
 /**
@@ -171,8 +178,8 @@ static inline size_t flex_sizeof_impl(size_t align, size_t min, size_t offset, s
 	size_t mask = align - 1;
 	ret += mask;
 	overflow |= ret < mask;
-	ret &= ~mask;
 	ret |= -overflow;
+	ret &= ~mask;
 
 	// Make sure flex_sizeof(type, member, 0) >= sizeof(type), even if the
 	// type has more padding than necessary for alignment
diff --git a/tests/bfstd.c b/tests/bfstd.c
index 1917a53..a986a23 100644
--- a/tests/bfstd.c
+++ b/tests/bfstd.c
@@ -41,7 +41,8 @@ int main(void) {
 	};
 	assert(flex_sizeof(struct flexible, bar, 0) >= sizeof(struct flexible));
 	assert(flex_sizeof(struct flexible, bar, 16) % alignof(struct flexible) == 0);
-	assert(flex_sizeof(struct flexible, bar, SIZE_MAX / sizeof(int) + 1) == SIZE_MAX);
+	assert(flex_sizeof(struct flexible, bar, SIZE_MAX / sizeof(int) + 1)
+	       == align_floor(alignof(struct flexible), SIZE_MAX));
 	assert(flex_sizeof_impl(8, 16, 4, 4, 1) == 16);
 
 	// From man 3p basename