diff --git a/src/sysdep.c b/src/sysdep.c
index 8b8aa0b8c98..7569c71e860 100644
--- a/src/sysdep.c
+++ b/src/sysdep.c
@@ -3313,7 +3313,7 @@ emacs_read (fildes, buf, nbyte)
 int
 emacs_write (fildes, buf, nbyte)
      int fildes;
-     char *buf;
+     const char *buf;
      unsigned int nbyte;
 {
   register int rtnval, bytes_written;