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diff --git a/gl/lib/nonblocking.h b/gl/lib/nonblocking.h
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+/* Non-blocking I/O for pipe or socket descriptors.
+   Copyright (C) 2011-2020 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+#ifndef _NONBLOCKING_H
+#define _NONBLOCKING_H
+
+#include <stdbool.h>
+
+/* Non-blocking I/O is an I/O mode by which read(), write() calls avoid
+   blocking the current thread.  When non-blocking is enabled:
+   - A read() call returns -1 with errno set to EAGAIN when no data or EOF
+     information is immediately available.
+   - A write() call returns -1 with errno set to EAGAIN when it cannot
+     transport the requested amount of data (but at most one pipe buffer)
+     without blocking.
+   Non-blocking I/O is most useful for character devices, pipes, and sockets.
+   Whether it also works on regular files and block devices is platform
+   dependent.
+
+   There are three modern alternatives to non-blocking I/O:
+     - use select() or poll() followed by read() or write() if the descriptor
+       is ready,
+     - call read() or write() in separate threads,
+     - use <aio.h> interfaces.  */
+
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+/* Return 1 if I/O to the descriptor DESC is currently non-blocking, 0
+   it is blocking, or -1 with errno set if fd is invalid or blocking
+   status cannot be determined (such as with sockets on mingw).  */
+extern int get_nonblocking_flag (int desc);
+
+/* Specify the non-blocking flag for the descriptor DESC.
+   Return 0 upon success, or -1 with errno set upon failure.
+   The default depends on the presence of the O_NONBLOCK flag for files
+   or pipes opened with open() or on the presence of the SOCK_NONBLOCK
+   flag for sockets.  */
+extern int set_nonblocking_flag (int desc, bool value);
+
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _NONBLOCKING_H */