diff --git a/plugin/protocol/http2/src/http2_common.cpp b/plugin/protocol/http2/src/http2_common.cpp
index b8b2a2c..0eb2521 100644
--- a/plugin/protocol/http2/src/http2_common.cpp
+++ b/plugin/protocol/http2/src/http2_common.cpp
@@ -151,7 +151,8 @@ int inflate_init(struct z_stream_st **strm, int gzip)
 	(*strm)->zst.avail_in = 0;
 	(*strm)->zst.next_in = Z_NULL;
 
-	int ret = 0;
+	// Z_OK stand for 0; Z_ERRNO stand for -1.
+	int ret = Z_ERRNO;
 	if (gzip == HTTP2_CONTENT_ENCODING_GZIP)
 		ret = inflateInit2(&((*strm)->zst), MAX_WBITS + 16);
 	else if (gzip == HTTP2_CONTENT_ENCODING_DEFLATE)
@@ -168,7 +169,7 @@ int inflate_init(struct z_stream_st **strm, int gzip)
 	return ret;
 }
 
-int inflate_br_read(struct z_stream_st **strm, const uint8_t *source, int len,
+static int inflate_br_read(struct z_stream_st **strm, const uint8_t *source, int len,
 				    char **dest, int *outlen)
 {
 #define CHUNK	 (1024 * 1024 * 4)
@@ -210,7 +211,7 @@ finish:
 	return ret;
 }
 
-int inflate_gzip_read(struct z_stream_st **strm, char **dest, int *outlen)
+static int inflate_gzip_read(struct z_stream_st **strm, char **dest, int *outlen)
 {
 	#define CHUNK    (1024 * 1024 * 4)
     int  ret = -1;
@@ -269,7 +270,7 @@ int deflate_init(struct z_stream_st **strm, int gzip)
 	if (*strm != NULL)
 		return Z_OK;
 
-	int ret = 0;
+	int ret = 0; // 0 stand for Z_OK
 
 	*strm = ALLOC(struct z_stream_st, 1);
 	assert(*strm);
@@ -304,7 +305,7 @@ int deflate_init(struct z_stream_st **strm, int gzip)
 	return ret;
 }
 
-int deflate_br_write(struct z_stream_st **strm,
+static int deflate_br_write(struct z_stream_st **strm,
     				 const unsigned char * source, size_t slen,
     				 struct evbuffer * evbuf, int end)
 {
@@ -353,7 +354,7 @@ int deflate_br_write(struct z_stream_st **strm,
 
 }
 
-int defalta_gzip_write(struct z_stream_st **strm, const uint8_t *source, int slen,
+static int defalta_gzip_write(struct z_stream_st **strm, const uint8_t *source, int slen,
 					   struct evbuffer * evbuf, int end)
 {
 #define SZ_IOVEC    2
@@ -440,7 +441,7 @@ void deflate_finished(struct z_stream_st **strm)
 			BrotliEncoderDestroyInstance((*strm)->brenc_state);
 			(*strm)->brenc_state = NULL;
 			goto finish;
-			
+
 		}
 		(void) deflateEnd(&((*strm)->zst));
 finish: