---
name: "CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS"
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_HAPPY_EYEBALLS_TIMEOUT_MS"
---

# CURLOPT\_HAPPY\_EYEBALLS\_TIMEOUT\_MS(3)

head start for IPv6 for happy eyeballs

## NAME

CURLOPT\_HAPPY\_EYEBALLS\_TIMEOUT\_MS - head start for IPv6 for happy eyeballs

## SYNOPSIS

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

## DESCRIPTION

Happy eyeballs is an algorithm that attempts to connect to both IPv4 and IPv6 addresses for dual-stack hosts, preferring IPv6 first for *timeout* milliseconds. If the IPv6 address cannot be connected to within that time then a connection attempt is made to the IPv4 address in parallel. The first connection to be established is the one that is used.

The range of suggested useful values for *timeout* is limited. Happy Eyeballs RFC 6555 says "It is RECOMMENDED that connection attempts be paced 150-250 ms apart to balance human factors against network load." libcurl currently defaults to 200 ms. Firefox and Chrome currently default to 300 ms.

## DEFAULT

CURL\_HET\_DEFAULT (currently defined as 200L)

## PROTOCOLS

All except FILE

## 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_HAPPY_EYEBALLS_TIMEOUT_MS, 300L);
    curl_easy_perform(curl);
    /* always cleanup */
    curl_easy_cleanup(curl);
  }
}
```

## AVAILABILITY

Added in 7.59.0

## RETURN VALUE

Returns CURLE\_OK

## SEE ALSO

[**CURLOPT\_CONNECTTIMEOUT\_MS**(3)](/CURLOPT_CONNECTTIMEOUT_MS/3.md), [**CURLOPT\_LOW\_SPEED\_LIMIT**(3)](/CURLOPT_LOW_SPEED_LIMIT/3.md), [**CURLOPT\_TIMEOUT**(3)](/CURLOPT_TIMEOUT/3.md)