curl/docs/libcurl/opts/CURLOPT_TCP_FASTOPEN.md
Daniel Stenberg 54ded66618
docs/libcurl: improve easy setopt examples
- always assign the curl_easy_perform() return code (and declare
  the necessary 'result' variable for it)

- always call curl_easy_cleanup() on the created easy handles

Closes #21364
2026-04-18 23:34:39 +02:00

70 lines
1.4 KiB
Markdown

---
c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
SPDX-License-Identifier: curl
Title: CURLOPT_TCP_FASTOPEN
Section: 3
Source: libcurl
See-also:
- CURLOPT_SSL_FALSESTART (3)
Protocol:
- TCP
Added-in: 7.49.0
---
# NAME
CURLOPT_TCP_FASTOPEN - TCP Fast Open
# SYNOPSIS
~~~c
#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 enabled. TCP
Fast Open is also known to be problematic on or across certain networks.
# DEFAULT
0
# %PROTOCOLS%
# EXAMPLE
~~~c
int main(void)
{
CURL *curl = curl_easy_init();
if(curl) {
CURLcode result;
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
curl_easy_setopt(curl, CURLOPT_TCP_FASTOPEN, 1L);
result = curl_easy_perform(curl);
curl_easy_cleanup(curl);
}
}
~~~
# NOTES
This option is only supported on Linux and macOS 10.11 or later.
# %AVAILABILITY%
# 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).