CURLOPT_TCP_FASTOPEN(3) Library Functions Manual CURLOPT_TCP_FASTOPEN(3)
NAME
CURLOPT_TCP_FASTOPEN - TCP Fast Open
SYNOPSIS
#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_TCP_FASTOPEN, long enable);
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 en-
abled. TCP Fast Open is also known to be problematic on or across cer-
tain networks.
DEFAULT
0
PROTOCOLS
This functionality affects tcp only
EXAMPLE
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);
}
}
NOTES
This option is only supported on Linux and macOS 10.11 or later.
AVAILABILITY
Added in curl 7.49.0
RETURN VALUE
curl_easy_setopt(3) returns a CURLcode indicating success or error.
CURLE_OK (0) means everything was OK, non-zero means an error occurred,
see libcurl-errors(3).
SEE ALSO
CURLOPT_SSL_FALSESTART(3)
libcurl 2026-07-12 CURLOPT_TCP_FASTOPEN(3)
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