--- lib_eio_posix/eio_posix_stubs.c.orig
+++ lib_eio_posix/eio_posix_stubs.c
@@ -38,6 +38,39 @@
 #include "fork_action.h"
 #include "eio_unix_stubs.h"
 
+#ifdef __APPLE__
+#include <unistd.h>
+#include <AvailabilityMacros.h>
+#if MAC_OS_X_VERSION_MAX_ALLOWED < 110000
+/* preadv/pwritev are only available on macOS 11.0+.
+   Emulate them with pread/pwrite loops; the file offset is not modified. */
+static ssize_t preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset) {
+  ssize_t total = 0;
+  int i;
+  for (i = 0; i < iovcnt; i++) {
+    ssize_t r = pread(fd, iov[i].iov_base, iov[i].iov_len, offset);
+    if (r < 0) return total > 0 ? total : -1;
+    total += r;
+    offset += r;
+    if ((size_t)r < iov[i].iov_len) break;
+  }
+  return total;
+}
+static ssize_t pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset) {
+  ssize_t total = 0;
+  int i;
+  for (i = 0; i < iovcnt; i++) {
+    ssize_t r = pwrite(fd, iov[i].iov_base, iov[i].iov_len, offset);
+    if (r < 0) return total > 0 ? total : -1;
+    total += r;
+    offset += r;
+    if ((size_t)r < iov[i].iov_len) break;
+  }
+  return total;
+}
+#endif
+#endif /* __APPLE__ */
+
 #ifdef ARCH_SIXTYFOUR
 #define Int63_val(v) Long_val(v)
 #define caml_copy_int63(v) Val_long(v)
--- lib_eio/unix/pty.c.orig
+++ lib_eio/unix/pty.c
@@ -13,6 +13,10 @@
 #include <termios.h>
 #endif
 
+#ifdef __APPLE__
+#include <AvailabilityMacros.h>
+#endif
+
 #include <caml/mlvalues.h>
 #include <caml/alloc.h>
 #include <caml/memory.h>
@@ -31,10 +35,11 @@
 #else
   CAMLparam1(v_unit);
 
-#if defined(__OpenBSD__)
+#if defined(__OpenBSD__) || (defined(__APPLE__) && MAC_OS_X_VERSION_MAX_ALLOWED < 1070)
   /* OpenBSD posix_openpt accepts only O_RDWR | O_NOCTTY,
      so request close-on-exec separately via fcntl.
-     https://man.openbsd.org/posix_openpt.3 */
+     https://man.openbsd.org/posix_openpt.3
+     Same on macOS < 10.7, which predates O_CLOEXEC. */
   int pty_fd = posix_openpt(O_RDWR | O_NOCTTY);
   if (pty_fd < 0)
     caml_uerror("posix_openpt", Nothing);
@@ -70,7 +75,7 @@
     caml_uerror("unlockpt", Nothing);
 
   {
-#if defined(__OpenBSD__)
+#if defined(__OpenBSD__) || (defined(__APPLE__) && MAC_OS_X_VERSION_MAX_ALLOWED < 101304)
     char *name = ptsname(pty_fd);
     if (name == NULL)
       caml_uerror("ptsname", Nothing);
@@ -92,10 +97,22 @@
   if (tty_fd < 0)
     caml_uerror("ioctl(TIOCGPTPEER)", Nothing);
 #else
+#if defined(__APPLE__) && MAC_OS_X_VERSION_MAX_ALLOWED < 1070
+  tty_fd = open(String_val(v_name), O_RDWR | O_NOCTTY);
+  if (tty_fd < 0)
+    caml_uerror("open", v_name);
+  if (fcntl(tty_fd, F_SETFD, FD_CLOEXEC) < 0) {
+    int e = errno;
+    close(tty_fd);
+    errno = e;
+    caml_uerror("open.fcntl", v_name);
+  }
+#else
   tty_fd = open(String_val(v_name), O_RDWR | O_NOCTTY | O_CLOEXEC);
   if (tty_fd < 0)
     caml_uerror("open", v_name);
 #endif
+#endif
 
   v_ret = caml_alloc_tuple(2);
   Store_field(v_ret, 0, Val_int(tty_fd));