diff --git a/src/util/io.c b/src/util/io.c
index c6446826..50c0fd6c 100644
--- a/src/util/io.c
+++ b/src/util/io.c
@@ -81,6 +81,7 @@ read_all (
     return recvd_total;
 }
 
+__attribute__ ((visibility("hidden")))
 ssize_t
 write_all (
     SOCKET fd,
diff --git a/src/util/io.h b/src/util/io.h
index 25dd5c45..fec391d8 100644
--- a/src/util/io.h
+++ b/src/util/io.h
@@ -70,6 +70,7 @@ read_all (
  * are detected. This is currently limited to interrupted system calls and
  * short writes.
  */
+__attribute__ ((visibility("hidden")))
 ssize_t
 write_all (
     SOCKET fd,