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Date:   Wed, 15 Sep 2021 10:37:58 -0700
From:   Wei Wang <>
        Alejandro Colomar <>,
        Michael Kerrisk <>
Cc:, Yuchung Cheng <>,
        Eric Dumazet <>
Subject: [patch] send.2: Add MSG_FASTOPEN flag

MSG_FASTOPEN flag is available since Linux 3.7. Add detailed description
in the manpage according to RFC7413.

Signed-off-by: Wei Wang <>
Reviewed-by: Yuchung Cheng <> 
Reviewed-by: Eric Dumazet <>
 man2/send.2 | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/man2/send.2 b/man2/send.2
index fd28fed90..a40ae6214 100644
--- a/man2/send.2
+++ b/man2/send.2
@@ -252,6 +252,33 @@ data on sockets that support this notion (e.g., of type
 the underlying protocol must also support
 .I out-of-band
+.BR MSG_FASTOPEN " (since Linux 3.7)"
+Attempts TCP Fast Open (RFC7413) and sends data in the SYN like a
+combination of
+.BR connect (2)
+.BR write (2)
+, by performing an implicit
+.BR connect (2)
+operation. It blocks until the data is buffered and the handshake
+has completed.
+For a non-blocking socket, it returns the number of bytes buffered
+and sent in the SYN packet. If the cookie is not available locally,
+it returns
+, and sends a SYN with a Fast Open cookie request automatically.
+The caller needs to write the data again when the socket is connected.
+On errors, it returns the same errno as
+.BR connect (2)
+if the handshake fails. This flag requires enabling TCP Fast Open
+client support on sysctl net.ipv4.tcp_fastopen.
+Refer to
+socket option in
+.BR tcp (7)
+for an alternative approach.
 .SS sendmsg()
 The definition of the
 .I msghdr

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