---
name: "CURLOPT_TCP_FASTOPEN"
section: "3"
platform: "macos"
package: "curl"
version: "166.0.0.0.1"
release: "macOS 26.5"
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

All

## 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);
  }
}
```

## AVAILABILITY

Added in 7.49.0. This option is currently only supported on Linux and macOS 10.11 or later.

## RETURN VALUE

Returns CURLE\_OK if fast open is supported by the operating system, otherwise returns CURLE\_NOT\_BUILT\_IN.

## SEE ALSO

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