---
name: "CURLOPT_TCP_FASTOPEN"
section: "3"
platform: "linux"
package: "curl"
version: "8.23.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "macos"]
url: "https://new.carta.tech/CURLOPT_TCP_FASTOPEN"
---

# CURLOPT\_TCP\_FASTOPEN(3)

TCP Fast Open

## 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 enabled. TCP Fast Open is also known to be problematic on or across certain networks.

## DEFAULT

0

## PROTOCOLS

This functionality affects tcp only

## 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

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)](/curl_easy_setopt/3.md)* returns a CURLcode indicating success or error.

CURLE\_OK (0) means everything was OK, non-zero means an error occurred, see *[libcurl-errors(3)](/libcurl-errors/3.md)*.

## SEE ALSO

[**CURLOPT\_SSL\_FALSESTART**(3)](/CURLOPT_SSL_FALSESTART/3.md)