diff --git a/contrib/pgcrypto/pgp-compress.c b/contrib/pgcrypto/pgp-compress.c
index 086bec31ae2..961cf21e748 100644
--- a/contrib/pgcrypto/pgp-compress.c
+++ b/contrib/pgcrypto/pgp-compress.c
@@ -113,7 +113,7 @@ compress_process(PushFilter *next, void *priv, const uint8 *data, int len)
 	/*
 	 * process data
 	 */
-	st->stream.next_in = unconstify(uint8 *, data);
+	st->stream.next_in = data;
 	st->stream.avail_in = len;
 	while (st->stream.avail_in > 0)
 	{
diff --git a/src/bin/pg_basebackup/bbstreamer_gzip.c b/src/bin/pg_basebackup/bbstreamer_gzip.c
index 3bdbfa0bc43..fb25fef1500 100644
--- a/src/bin/pg_basebackup/bbstreamer_gzip.c
+++ b/src/bin/pg_basebackup/bbstreamer_gzip.c
@@ -269,7 +269,7 @@ bbstreamer_gzip_decompressor_content(bbstreamer *streamer,
 	mystreamer = (bbstreamer_gzip_decompressor *) streamer;
 
 	zs = &mystreamer->zstream;
-	zs->next_in = (uint8 *) data;
+	zs->next_in = (const uint8 *) data;
 	zs->avail_in = len;
 
 	/* Process the current chunk */
diff --git a/src/bin/pg_basebackup/walmethods.c b/src/bin/pg_basebackup/walmethods.c
index d780c4055cd..2de11ce9b1c 100644
--- a/src/bin/pg_basebackup/walmethods.c
+++ b/src/bin/pg_basebackup/walmethods.c
@@ -705,7 +705,7 @@ typedef struct TarMethodData
 
 #ifdef HAVE_LIBZ
 static bool
-tar_write_compressed_data(TarMethodData *tar_data, void *buf, size_t count,
+tar_write_compressed_data(TarMethodData *tar_data, const void *buf, size_t count,
 						  bool flush)
 {
 	tar_data->zp->next_in = buf;
@@ -782,8 +782,7 @@ tar_write(Walfile *f, const void *buf, size_t count)
 #ifdef HAVE_LIBZ
 	else if (f->wwmethod->compression_algorithm == PG_COMPRESSION_GZIP)
 	{
-		if (!tar_write_compressed_data(tar_data, unconstify(void *, buf),
-									   count, false))
+		if (!tar_write_compressed_data(tar_data, buf, count, false))
 			return -1;
 		f->currpos += count;
 		return count;
diff --git a/src/include/c.h b/src/include/c.h
index f69d739be57..82f8e9d4c7b 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -75,6 +75,11 @@
 #include <libintl.h>
 #endif
 
+/* Define before including zlib.h to add const decorations to zlib API. */
+#ifdef HAVE_LIBZ
+#define ZLIB_CONST
+#endif
+
 
 /* ----------------------------------------------------------------
  *				Section 1: compiler characteristics