diff --git a/ikcp.c b/ikcp.c
index cb03857..4ec38b0 100644
--- a/ikcp.c
+++ b/ikcp.c
@@ -865,7 +865,7 @@ void ikcp_flush(ikcpcb *kcp)
 	count = kcp->ackcount;
 	for (i = 0; i < count; i++) {
 		size = (int)(ptr - buffer);
-		if (size > (int)kcp->mtu) {
+		if (size + IKCP_OVERHEAD > (int)kcp->mtu) {
 			ikcp_output(kcp, buffer, size);
 			ptr = buffer;
 		}
@@ -901,7 +901,7 @@ void ikcp_flush(ikcpcb *kcp)
 	if (kcp->probe & IKCP_ASK_SEND) {
 		seg.cmd = IKCP_CMD_WASK;
 		size = (int)(ptr - buffer);
-		if (size > (int)kcp->mtu) {
+		if (size + IKCP_OVERHEAD > (int)kcp->mtu) {
 			ikcp_output(kcp, buffer, size);
 			ptr = buffer;
 		}
@@ -912,7 +912,7 @@ void ikcp_flush(ikcpcb *kcp)
 	if (kcp->probe & IKCP_ASK_TELL) {
 		seg.cmd = IKCP_CMD_WINS;
 		size = (int)(ptr - buffer);
-		if (size > (int)kcp->mtu) {
+		if (size + IKCP_OVERHEAD > (int)kcp->mtu) {
 			ikcp_output(kcp, buffer, size);
 			ptr = buffer;
 		}
@@ -992,7 +992,7 @@ void ikcp_flush(ikcpcb *kcp)
 			size = (int)(ptr - buffer);
 			need = IKCP_OVERHEAD + segment->len;
 
-			if (size + need >= (int)kcp->mtu) {
+			if (size + need > (int)kcp->mtu) {
 				ikcp_output(kcp, buffer, size);
 				ptr = buffer;
 			}
diff --git a/ikcp.h b/ikcp.h
index 280603a..5626693 100644
--- a/ikcp.h
+++ b/ikcp.h
@@ -12,7 +12,7 @@
 #ifndef __IKCP_H__
 #define __IKCP_H__
 
-#include <stdio.h>
+#include <stddef.h>
 #include <stdlib.h>
 #include <assert.h>
 
@@ -358,8 +358,11 @@ IUINT32 ikcp_check(const ikcpcb *kcp, IUINT32 current);
 
 // when you received a low level packet (eg. UDP packet), call it
 int ikcp_input(ikcpcb *kcp, const char *data, long size);
+
+// flush pending data
 void ikcp_flush(ikcpcb *kcp);
 
+// check the size of next message in the recv queue
 int ikcp_peeksize(const ikcpcb *kcp);
 
 // change MTU size, default is 1400
@@ -383,6 +386,10 @@ int ikcp_sndbuf_count(const ikcpcb *kcp);
 
 void ikcp_log(ikcpcb *kcp, int mask, const char *fmt, ...);
 
+// setup allocator
+void ikcp_allocator(void* (*new_malloc)(size_t), void (*new_free)(void*));
+
+
 #ifdef __cplusplus
 }
 #endif