diff --git a/Changes b/Changes
index e673d5e71..771120afb 100644
--- a/Changes
+++ b/Changes
@@ -66,6 +66,10 @@ Standard library:
 - Add optimized composition operators |> and @@ in Pervasives
 
 
+Other libraries:
+- PR#5568: add O_CLOEXEC flag to Unix.openfile, so that the returned
+  file descriptor is created in close-on-exec mode
+
 Runtime system:
 * PR#6019: more efficient implementation of caml_modify() and caml_initialize().
   The new implementations are less lenient than the old ones: now,
diff --git a/otherlibs/unix/open.c b/otherlibs/unix/open.c
index 097a0455b..ecee01389 100644
--- a/otherlibs/unix/open.c
+++ b/otherlibs/unix/open.c
@@ -17,6 +17,9 @@
 #include <signals.h>
 #include "unixsupport.h"
 #include <string.h>
+#ifdef HAS_UNISTD
+#include <unistd.h>
+#endif
 #include <fcntl.h>
 
 #ifndef O_NONBLOCK
@@ -31,16 +34,31 @@
 #ifndef O_RSYNC
 #define O_RSYNC 0
 #endif
+#ifndef O_CLOEXEC
+#define NEED_CLOEXEC_EMULATION
+#define O_CLOEXEC 0
+#endif
 
-static int open_flag_table[] = {
+static int open_flag_table[14] = {
   O_RDONLY, O_WRONLY, O_RDWR, O_NONBLOCK, O_APPEND, O_CREAT, O_TRUNC, O_EXCL,
-  O_NOCTTY, O_DSYNC, O_SYNC, O_RSYNC, 0
+  O_NOCTTY, O_DSYNC, O_SYNC, O_RSYNC,
+  0, /* O_SHARE_DELETE, Windows-only */
+  O_CLOEXEC
 };
 
+#ifdef NEED_CLOEXEC_EMULATION
+static int open_cloexec_table[14] = {
+  0, 0, 0, 0, 0, 0, 0, 0,
+  0, 0, 0, 0,
+  0,
+  1
+};
+#endif
+
 CAMLprim value unix_open(value path, value flags, value perm)
 {
   CAMLparam3(path, flags, perm);
-  int ret, cv_flags;
+  int fd, cv_flags;
   char * p;
 
   cv_flags = convert_flag_list(flags, open_flag_table);
@@ -48,9 +66,17 @@ CAMLprim value unix_open(value path, value flags, value perm)
   strcpy(p, String_val(path));
   /* open on a named FIFO can block (PR#1533) */
   enter_blocking_section();
-  ret = open(p, cv_flags, Int_val(perm));
+  fd = open(p, cv_flags, Int_val(perm));
   leave_blocking_section();
   stat_free(p);
-  if (ret == -1) uerror("open", path);
-  CAMLreturn (Val_int(ret));
+  if (fd == -1) uerror("open", path);
+#if defined(NEED_CLOEXEC_EMULATION) && defined(FD_CLOEXEC)
+  if (convert_flag_list(flags, open_cloexec_table) != 0) {
+    int flags = fcntl(fd, F_GETFD, 0);
+    if (flags == -1 ||
+        fcntl(fd, F_SETFD, flags | FD_CLOEXEC) == -1)
+      uerror("open", path);
+  }
+#endif
+  CAMLreturn (Val_int(fd));
 }
diff --git a/otherlibs/unix/unix.ml b/otherlibs/unix/unix.ml
index 8a69ca7b4..a89783146 100644
--- a/otherlibs/unix/unix.ml
+++ b/otherlibs/unix/unix.ml
@@ -227,6 +227,7 @@ type open_flag =
   | O_SYNC
   | O_RSYNC
   | O_SHARE_DELETE
+  | O_CLOEXEC
 
 type file_perm = int
 
diff --git a/otherlibs/unix/unix.mli b/otherlibs/unix/unix.mli
index a9b551041..6c4202246 100644
--- a/otherlibs/unix/unix.mli
+++ b/otherlibs/unix/unix.mli
@@ -242,6 +242,9 @@ type open_flag =
                                    O_SYNC/O_DSYNC) *)
   | O_SHARE_DELETE              (** Windows only: allow the file to be deleted
                                    while still open *)
+  | O_CLOEXEC                   (** Set the close-on-exec flag on the
+                                   descriptor returned by {!openfile} *)
+
 (** The flags to {!Unix.openfile}. *)
 
 
diff --git a/otherlibs/unix/unixLabels.mli b/otherlibs/unix/unixLabels.mli
index 91636c6a8..7494d4f7b 100644
--- a/otherlibs/unix/unixLabels.mli
+++ b/otherlibs/unix/unixLabels.mli
@@ -239,6 +239,8 @@ type open_flag = Unix.open_flag =
   | O_SYNC                      (** Writes complete as `Synchronised I/O file integrity completion' *)
   | O_RSYNC                     (** Reads complete as writes (depending on O_SYNC/O_DSYNC) *)
   | O_SHARE_DELETE              (** Windows only: allow the file to be deleted while still open *)
+  | O_CLOEXEC                   (** Set the close-on-exec flag on the
+                                   descriptor returned by {!openfile} *)
 (** The flags to {!UnixLabels.openfile}. *)
 
 
diff --git a/otherlibs/win32unix/open.c b/otherlibs/win32unix/open.c
index 40ea34b63..a2ffa4e1e 100644
--- a/otherlibs/win32unix/open.c
+++ b/otherlibs/win32unix/open.c
@@ -16,22 +16,26 @@
 #include "unixsupport.h"
 #include <fcntl.h>
 
-static int open_access_flags[13] = {
+static int open_access_flags[14] = {
   GENERIC_READ, GENERIC_WRITE, GENERIC_READ|GENERIC_WRITE,
-  0, 0, 0, 0, 0, 0, 0, 0, 0, 0
+  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
 };
 
-static int open_create_flags[13] = {
-  0, 0, 0, 0, 0, O_CREAT, O_TRUNC, O_EXCL, 0, 0, 0, 0, 0
+static int open_create_flags[14] = {
+  0, 0, 0, 0, 0, O_CREAT, O_TRUNC, O_EXCL, 0, 0, 0, 0, 0, 0
 };
 
-static int open_share_flags[13] = {
-  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, FILE_SHARE_DELETE
+static int open_share_flags[14] = {
+  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, FILE_SHARE_DELETE, 0
+};
+
+static int open_cloexec_flags[14] = {
+  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1
 };
 
 CAMLprim value unix_open(value path, value flags, value perm)
 {
-  int fileaccess, createflags, fileattrib, filecreate, sharemode;
+  int fileaccess, createflags, fileattrib, filecreate, sharemode, cloexec;
   SECURITY_ATTRIBUTES attr;
   HANDLE h;
 
@@ -55,9 +59,10 @@ CAMLprim value unix_open(value path, value flags, value perm)
   else
     fileattrib = FILE_ATTRIBUTE_NORMAL;
 
+  cloexec = convert_flag_list(flags, open_cloexec_flags);
   attr.nLength = sizeof(attr);
   attr.lpSecurityDescriptor = NULL;
-  attr.bInheritHandle = TRUE;
+  attr.bInheritHandle = cloexec ? FALSE : TRUE;
 
   h = CreateFile(String_val(path), fileaccess,
                  sharemode, &attr,
diff --git a/otherlibs/win32unix/unix.ml b/otherlibs/win32unix/unix.ml
index a8a3dcc2a..cff003994 100644
--- a/otherlibs/win32unix/unix.ml
+++ b/otherlibs/win32unix/unix.ml
@@ -169,6 +169,7 @@ type open_flag =
   | O_SYNC
   | O_RSYNC
   | O_SHARE_DELETE
+  | O_CLOEXEC
 
 type file_perm = int