diff --git a/racket/src/racket/src/port.c b/racket/src/racket/src/port.c
index a2c5dedb41..4229ef703e 100644
--- a/racket/src/racket/src/port.c
+++ b/racket/src/racket/src/port.c
@@ -6554,11 +6554,13 @@ static int try_get_fd_char(int fd, int *ready)
   unsigned char buf[1];
 
   old_flags = fcntl(fd, F_GETFL, 0);
-  fcntl(fd, F_SETFL, old_flags | MZ_NONBLOCKING);
+  if (!(old_flags & MZ_NONBLOCKING))
+    fcntl(fd, F_SETFL, old_flags | MZ_NONBLOCKING);
   do {
     c = read(fd, buf, 1);
   } while ((c == -1) && errno == EINTR);
-  fcntl(fd, F_SETFL, old_flags);
+  if (!(old_flags & MZ_NONBLOCKING))
+    fcntl(fd, F_SETFL, old_flags);
 
   if (c < 0) {
     *ready = 0;
@@ -6857,11 +6859,13 @@ static intptr_t fd_get_string_slow(Scheme_Input_Port *port,
       int old_flags;
 
       old_flags = fcntl(fip->fd, F_GETFL, 0);
-      fcntl(fip->fd, F_SETFL, old_flags | MZ_NONBLOCKING);
+      if (!(old_flags & MZ_NONBLOCKING))
+        fcntl(fip->fd, F_SETFL, old_flags | MZ_NONBLOCKING);
       do {
         bc = read(fip->fd, target + target_offset, target_size);
       } while ((bc == -1) && errno == EINTR);
-      fcntl(fip->fd, F_SETFL, old_flags);
+      if (!(old_flags & MZ_NONBLOCKING))
+        fcntl(fip->fd, F_SETFL, old_flags);
 
       if ((bc == -1) && (errno == EAGAIN)) {
         none_avail = 1;
@@ -8125,7 +8129,8 @@ static intptr_t flush_fd(Scheme_Output_Port *op,
       intptr_t amt;
 
       flags = fcntl(fop->fd, F_GETFL, 0);
-      fcntl(fop->fd, F_SETFL, flags | MZ_NONBLOCKING);
+      if (!(flags & MZ_NONBLOCKING))
+        fcntl(fop->fd, F_SETFL, flags | MZ_NONBLOCKING);
 
       amt = buflen - offset;
 
@@ -8141,7 +8146,8 @@ static intptr_t flush_fd(Scheme_Output_Port *op,
       } while ((len == -1) && (errno == EAGAIN) && (amt > 0));
 
       errsaved = errno;
-      fcntl(fop->fd, F_SETFL, flags);
+      if (!(flags & MZ_NONBLOCKING))
+        fcntl(fop->fd, F_SETFL, flags);
 
       full_write_buffer = (errsaved == EAGAIN);
 #endif