diff --git a/src/include/lwip/altcp.h b/src/include/lwip/altcp.h
index b63f1530..11acaff3 100644
--- a/src/include/lwip/altcp.h
+++ b/src/include/lwip/altcp.h
@@ -80,7 +80,8 @@ struct altcp_pcb {
   u8_t pollinterval;
 };
 
-/** @ingroup altcp */
+/** @ingroup altcp
+ *  Struct containing an allocator and its state. */
 typedef struct altcp_allocator_s {
   /** Allocator function */
   altcp_new_fn  alloc;