comment potentially-confusing use of struct crypt_data type

src/crypt/crypt.c

@@ -5,7 +5,12 @@ char *__crypt_r(const char *, const char *, struct crypt_data *);
 
 char *crypt(const char *key, const char *salt)
 {
-	/* Note: update this size when we add more hash types */
+	/* This buffer is sufficiently large for all
+	 * currently-supported hash types. It needs to be updated if
+	 * longer hashes are added. The cast to struct crypt_data * is
+	 * purely to meet the public API requirements of the crypt_r
+	 * function; the implementation of crypt_r uses the object
+	 * purely as a char buffer. */
 	static char buf[128];
 	return __crypt_r(key, salt, (struct crypt_data *)buf);
 }

src/crypt/crypt_r.c

@@ -11,6 +11,10 @@ char *__crypt_sha512(const char *, const char *, char *);
 
 char *__crypt_r(const char *key, const char *salt, struct crypt_data *data)
 {
+	/* Per the crypt_r API, the caller has provided a pointer to
+	 * struct crypt_data; however, this implementation does not
+	 * use the structure to store any internal state, and treats
+	 * it purely as a char buffer for storing the result. */
 	char *output = (char *)data;
 	if (salt[0] == '$' && salt[1] && salt[2]) {
 		if (salt[1] == '1' && salt[2] == '$')