diff --git a/libavcodec/ac3.h b/libavcodec/ac3.h
index 3a14a3b383..bfaf7d0db2 100644
--- a/libavcodec/ac3.h
+++ b/libavcodec/ac3.h
@@ -24,6 +24,9 @@
  * Common code between AC3 encoder and decoder.
  */
 
+#ifndef AC3_H
+#define AC3_H
+
 #include "ac3tab.h"
 
 #define AC3_MAX_CODED_FRAME_SIZE 3840 /* in bytes */
@@ -148,3 +151,5 @@ void ac3_parametric_bit_allocation(AC3BitAllocParameters *s, uint8_t *bap,
                                    int snroffset, int fgain, int is_lfe,
                                    int deltbae,int deltnseg,
                                    uint8_t *deltoffst, uint8_t *deltlen, uint8_t *deltba);
+
+#endif /* AC3_H */