api doc: clarify OSMO_NAME_C_IMPL() required FUNC_BUF signature
Change-Id: Ibe722d38d28e5590a35e856dd15c2538e6ee0a3e
diff --git a/include/osmocom/core/utils.h b/include/osmocom/core/utils.h
index 40f4446..e637786 100644
--- a/include/osmocom/core/utils.h
+++ b/include/osmocom/core/utils.h
@@ -297,7 +297,9 @@
* \param[in] INITIAL_BUFSIZE Which size to first talloc from ctx -- a larger size makes a reallocation less likely, a
* smaller size allocates less unused bytes, zero allocates once but still runs the string composition twice.
* \param[in] ON_ERROR String constant to copy on error rc returned by FUNC_BUF, or NULL to return NULL.
- * \param[in] FUNC_BUF Name of a function with signature foo_buf(char *buf, size_t buflen, ...).
+ * \param[in] FUNC_BUF Name of a function with signature int foo_buf(char *buf, size_t buflen, ...).
+ * The function must return the strlen() that it would write to a sufficiently large buffer or
+ * negative on error, like snprintf().
* \param[in] FUNC_BUF_ARGS Additional arguments to pass to FUNC_BUF after the buf and buflen.
*/
#define OSMO_NAME_C_IMPL(CTX, INITIAL_BUFSIZE, ON_ERROR, FUNC_BUF, FUNC_BUF_ARGS...) \