CURLOPT\_TCP\_FASTOPEN(3) Library Functions Manual CURLOPT\_TCP\_FASTOPEN(3)

# [NAME](#NAME)

CURLOPT\_TCP\_FASTOPEN - TCP Fast Open

# [SYNOPSIS](#SYNOPSIS)

```
#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_TCP_FASTOPEN, long enable);
```

# [DESCRIPTION](#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](#DEFAULT)

0

# [PROTOCOLS](#PROTOCOLS)

This functionality affects tcp only

# [EXAMPLE](#EXAMPLE)

```
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](#NOTES)

This option is only supported on Linux and macOS 10.11 or later.

# [AVAILABILITY](#AVAILABILITY)

Added in curl 7.49.0

# [RETURN VALUE](#RETURN_VALUE)

[*curl\_easy\_setopt(3)*](/man/os/curl/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)*](/man/os/curl/libcurl-errors.3).

# [SEE ALSO](#SEE_ALSO)

[**CURLOPT\_SSL\_FALSESTART**(3)](/man/os/curl/CURLOPT_SSL_FALSESTART.3)

2026-10-01 libcurl

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Document: CURLOPT_TCP_FASTOPEN(3) from curl 8.22.0-r4 (Chainguard OS)
Source: https://github.com/curl/curl.git