---
name: "gnutls_transport_set_fastopen"
section: "3"
platform: "linux"
package: "gnutls"
version: "3.8.13-3"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/gnutls_transport_set_fastopen"
---

# gnutls\_transport\_set\_fastopen(3)

API function

## NAME

gnutls\_transport\_set\_fastopen - API function

## SYNOPSIS

**#include \<gnutls/socket.h\>**

**void gnutls\_transport\_set\_fastopen(gnutls\_session\_t** *session***, int** *fd***, struct sockaddr \*** *connect\_addr***, socklen\_t** *connect\_addrlen***, unsigned int** *flags***);**

## ARGUMENTS

- gnutls\_session\_t session

  is a **gnutls\_session\_t** type.

- int fd

  is the session's socket descriptor

- struct sockaddr \* connect\_addr

  is the address we want to connect to

- socklen\_t connect\_addrlen

  is the length of  *connect\_addr*

- unsigned int flags

  must be zero

## DESCRIPTION

Enables TCP Fast Open (TFO) for the specified TLS client session. That means that TCP connection establishment and the transmission of the first TLS client hello packet are combined. The peer's address must be  specified in  *connect\_addr* and  *connect\_addrlen* , and the socket specified by  *fd* should not be connected.

TFO only works for TCP sockets of type AF\_INET and AF\_INET6. If the OS doesn't support TCP fast open this function will result to gnutls using **connect()** transparently during the first write.

## NOTE

This function overrides all the transport callback functions. If this is undesirable, TCP Fast Open must be implemented on the user callback functions without calling this function. When using this function, transport callbacks must not be set, and  **gnutls\_transport\_set\_ptr()** or **gnutls\_transport\_set\_int()** must not be called.

On GNU/Linux TFO has to be enabled at the system layer, that is in /proc/sys/net/ipv4/tcp\_fastopen, bit 0 has to be set.

This function has no effect on server sessions.

## SINCE

3\.5.3

## REPORTING BUGS

Report bugs to \<bugs@gnutls.org\>.\
 Home page: https://www.gnutls.org

## COPYRIGHT

Copyright © 2001-2023 Free Software Foundation, Inc., and others.\
 Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved.

## SEE ALSO

The full documentation for **gnutls** is maintained as a Texinfo manual. If the /usr/share/doc/gnutls/ directory does not contain the HTML form visit

- https://www.gnutls.org/manual/