+/**
+ * Rewrite the tail of an existing string, starting at a given index.
+ *
+ * Overwrites the contents of *str starting at \p start with newly formatted
+ * text, including a new null-terminator. Allocates more memory as necessary.
+ *
+ * This can be used to append formatted text when the length of the existing
+ * string is already known, saving a strlen() call.
+ *
+ * \sa ralloc_asprintf_append
+ *
+ * \param str The string to be updated.
+ * \param start The index to start appending new data at.
+ * \param fmt A printf-style formatting string
+ *
+ * \p str will be updated to the new pointer unless allocation fails.
+ * \p start will be increased by the length of the newly formatted text.
+ *
+ * \return True unless allocation failed.
+ */
+bool ralloc_asprintf_rewrite_tail(char **str, size_t *start,
+ const char *fmt, ...)
+ PRINTFLIKE(3, 4);
+
+/**
+ * Rewrite the tail of an existing string, starting at a given index.
+ *
+ * Overwrites the contents of *str starting at \p start with newly formatted
+ * text, including a new null-terminator. Allocates more memory as necessary.
+ *
+ * This can be used to append formatted text when the length of the existing
+ * string is already known, saving a strlen() call.
+ *
+ * \sa ralloc_vasprintf_append
+ *
+ * \param str The string to be updated.
+ * \param start The index to start appending new data at.
+ * \param fmt A printf-style formatting string
+ * \param args A va_list containing the data to be formatted
+ *
+ * \p str will be updated to the new pointer unless allocation fails.
+ * \p start will be increased by the length of the newly formatted text.
+ *
+ * \return True unless allocation failed.
+ */
+bool ralloc_vasprintf_rewrite_tail(char **str, size_t *start, const char *fmt,
+ va_list args);
+