utils: Add utility function to calculate padding length

src/libstrongswan/tests/test_utils.c

@@ -167,11 +167,18 @@ START_TEST(test_untoh)
 END_TEST
 
 /*******************************************************************************
- * round_up/down
+ * pad_len/round_up/down
  */
 
 START_TEST(test_round)
 {
+	ck_assert_int_eq(pad_len(0, 4), 0);
+	ck_assert_int_eq(pad_len(1, 4), 3);
+	ck_assert_int_eq(pad_len(2, 4), 2);
+	ck_assert_int_eq(pad_len(3, 4), 1);
+	ck_assert_int_eq(pad_len(4, 4), 0);
+	ck_assert_int_eq(pad_len(5, 4), 3);
+
 	ck_assert_int_eq(round_up(0, 4), 0);
 	ck_assert_int_eq(round_up(1, 4), 4);
 	ck_assert_int_eq(round_up(2, 4), 4);

src/libstrongswan/utils/utils.h

@@ -679,26 +679,30 @@ static inline u_int64_t untoh64(void *network)
 }
 
 /**
- * Round up size to be multiple of alignement
+ * Get the padding required to make size a multiple of alignment
  */
-static inline size_t round_up(size_t size, int alignement)
+static inline size_t pad_len(size_t size, size_t alignment)
 {
-	int remainder;
+	size_t remainder;
 
-	remainder = size % alignement;
-	if (remainder)
-	{
-		size += alignement - remainder;
-	}
-	return size;
+	remainder = size % alignment;
+	return remainder ? alignment - remainder : 0;
 }
 
 /**
- * Round down size to be a multiple of alignement
+ * Round up size to be multiple of alignment
  */
-static inline size_t round_down(size_t size, int alignement)
+static inline size_t round_up(size_t size, size_t alignment)
 {
-	return size - (size % alignement);
+	return size + pad_len(size, alignment);
+}
+
+/**
+ * Round down size to be a multiple of alignment
+ */
+static inline size_t round_down(size_t size, size_t alignment)
+{
+	return size - (size % alignment);
 }
 
 /**