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.TH CURLOPT_TCP_FASTOPEN 3 "16 Feb 2016" libcurl libcurl
.SH NAME
CURLOPT_TCP_FASTOPEN \- TCP Fast Open
.SH SYNOPSIS
.nf
#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_TCP_FASTOPEN, long enable);
.fi
.SH DESCRIPTION
Pass a long as parameter set to 1L to enable or 0 to disable.

TCP Fast Open (RFC 7413) is a mechanism that allows data to be carried in the
SYN and SYN-ACK packets and consumed by the receiving end during the initial
connection handshake, saving up to one full round-trip time (RTT).

Beware: the TLS session cache does not work when TCP Fast Open is enabled. TCP
Fast Open is also known to be problematic on or across certain networks.
.SH DEFAULT
0
.SH PROTOCOLS
All
.SH EXAMPLE
.nf
int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
    curl_easy_setopt(curl, CURLOPT_TCP_FASTOPEN, 1L);
    curl_easy_perform(curl);
  }
}
.fi
.SH AVAILABILITY
Added in 7.49.0. This option is currently only supported on Linux and macOS
10.11 or later.
.SH RETURN VALUE
Returns CURLE_OK if fast open is supported by the operating system, otherwise
returns CURLE_NOT_BUILT_IN.
.SH SEE ALSO
.BR CURLOPT_SSL_FALSESTART "(3), "