Avoid exceeding 80-column per line

The macros provided by this library would expand to an unguarded
conditional that could have potential unwanted consequences. Consider
the following example:

    dynstr_append_or_ret_null(&d, "example");
    else
    {
        /* This is unexpectedly working. */
    }

The example above would expand to:

    if (dynstr_append(__VA_ARGS__) != DYNSTR_OK) return false;
    else
    {
        /* This is unexpectedly working. */
    }

Which is valid C yet allows possibly unexpected behaviour. The solution
is then to enclose these conditions into a `do while (0)` statement.

dynstr.c

@@ -141,6 +141,6 @@ void dynstr_free(struct dynstr *const d)
     if (d->str)
     {
         free(d->str);
-        memset(d, 0, sizeof *d);
+        dynstr_init(d);
     }
 }

include/dynstr.h

@@ -30,27 +30,35 @@
 /**
  * Convenience macro that calls dynstr_append and returns NULL if failed.
  */
-#define dynstr_append_or_ret_null(...) if (dynstr_append(__VA_ARGS__) != DYNSTR_OK) return NULL
+#define dynstr_append_or_ret_null(...) \
+   do {if (dynstr_append(__VA_ARGS__) != DYNSTR_OK) return NULL;} while (0)
 
 /**
  * Convenience macro that calls dynstr_append and returns false if failed.
  */
-#define dynstr_append_or_ret_false(...) if (dynstr_append(__VA_ARGS__) != DYNSTR_OK) return false
+#define dynstr_append_or_ret_false(...) \
+   do {if (dynstr_append(__VA_ARGS__) != DYNSTR_OK) return false;} while (0)
 
 /**
  * Convenience macro that calls dynstr_append and returns its error code if failed.
  */
-#define dynstr_append_or_ret(...) {const enum dynstr_err err = dynstr_append(__VA_ARGS__); if (err != DYNSTR_OK) return err;}
+#define dynstr_append_or_ret(...) \
+   { \
+      const enum dynstr_err err = dynstr_append(__VA_ARGS__);  \
+      if (err != DYNSTR_OK) return err; \
+   }
 
 /**
  * Convenience macro that calls dynstr_append and returns zero if failed.
  */
-#define dynstr_append_or_ret_zero(...) if (dynstr_append(__VA_ARGS__) != DYNSTR_OK) return 0
+#define dynstr_append_or_ret_zero(...) \
+   do {if (dynstr_append(__VA_ARGS__) != DYNSTR_OK) return 0;} while (0)
 
 /**
  * Convenience macro that calls dynstr_append and returns one if failed.
  */
-#define dynstr_append_or_ret_nonzero(...) if (dynstr_append(__VA_ARGS__) != DYNSTR_OK) return 1
+#define dynstr_append_or_ret_nonzero(...) \
+   do {if (dynstr_append(__VA_ARGS__) != DYNSTR_OK) return 1;} while (0)
 
 /**
  * Dynamic string type used for this library.
@@ -65,8 +73,11 @@ struct dynstr
 
 /**
  * List of errors that this library might return.
- * @note If using GNU C extensions, this errors can be translated into other
- * types by using the convenience macros above.
+ * @note Following the tradition, success code is guaranteed to always be
+ * zero.
+ * @attention Error codes are guaranteed to be non-zero, but their ordering
+ * might change, so please use descriptive error names instead of their integer
+ * equivalents.
  */
 enum dynstr_err
 {
@@ -77,8 +88,10 @@ enum dynstr_err
 };
 
 /**
- * Reportedly, it initializes an empty string with zero length.
- * @attention Always call this function when creating a dynstr instance.
+ * This function initializes string pointer to NULL and sets zero length.
+ * @attention Always call this function before using an instance.
+ * For future compatibility, please avoid initializing instances by calling
+ * @c memset or similar and use this function instead.
  * @param d Dynamic string to be initialized.
  */
 void dynstr_init(struct dynstr *d);
@@ -116,7 +129,8 @@ enum dynstr_err dynstr_prepend(struct dynstr *d, const char *format, ...);
  * @return Returns one of the following error codes:
  *    # DYNSTR_OK if successful.
  *    # DYNSTR_ERR_ALLOC if no more memory is available.
- *    # DYNSTR_ERR_INIT if destination dynamic string was not initialized.
+ *    # DYNSTR_ERR_INIT if destination dynamic string was either not
+ *      initialized or already contains data.
  *    # DYNSTR_ERR_SRC if source dynamic string has no length or data.
  * @note This function has the same effect as calling:
  * @code
@@ -131,9 +145,11 @@ enum dynstr_err dynstr_dup(struct dynstr *dst, const struct dynstr *src);
 /**
  * This function frees memory used by the dynamic string.
  * @param d Dynamic string to be freed.
- * @attention Attempting to call this function on an uninitialized or empty
- * instance is undefined behaviour.
- * @attention Parameters inside the struct are reset once memory is freed.
+ * @note This function does nothing on empty, initialized instances.
+ * @attention Calling this function on an uninitialized instance leads to
+ * undefined behaviour.
+ * @attention Once memory is freed, @ref dynstr_init is called so it can be
+ * used again.
  */
 void dynstr_free(struct dynstr *d);