diff --git a/include/asterisk/channel.h b/include/asterisk/channel.h
index 87f5efea88..03613aaddd 100644
--- a/include/asterisk/channel.h
+++ b/include/asterisk/channel.h
@@ -310,8 +310,21 @@ struct ast_channel_tech {
 struct ast_channel_spy_list;	/*!< \todo Add explanation here */
 struct ast_channel_whisper_buffer;	/*!< \todo Add explanation here */
 
+/*!
+ * The high bit of the frame count is used as a debug marker, so
+ * increments of the counters must be done with care.
+ * Please use c->fin = FRAMECOUNT_INC(c->fin) and the same for c->fout.
+ */
 #define	DEBUGCHAN_FLAG  0x80000000
-#define	FRAMECOUNT_INC(x)	( ((x) & DEBUGCHAN_FLAG) | ((x++) & ~DEBUGCHAN_FLAG) )
+
+/* XXX not ideal to evaluate x twice... */
+#define	FRAMECOUNT_INC(x)	( ((x) & DEBUGCHAN_FLAG) | (((x)+1) & ~DEBUGCHAN_FLAG) )
+
+/*!
+ * The current value of the debug flags is stored in the two
+ * variables global_fin and global_fout (declared in main/channel.c)
+ */
+extern unsigned long global_fin, global_fout;
 
 enum ast_channel_adsicpe {
 	AST_ADSI_UNKNOWN,