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CURLOPT_H...TIMEOUT_MS(3) Library Functions Manual CURLOPT_H...TIMEOUT_MS(3) NAME CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS - head start for IPv6 for happy eye- balls 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 par- allel. The first connection to be established is the one that is used. The range of suggested useful values for timeout is limited. Happy Eye- balls 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 de- fault to 300 ms. DEFAULT CURL_HET_DEFAULT (currently defined as 200L) PROTOCOLS This functionality affects all supported protocols 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 curl 7.59.0 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). SEE ALSO CURLOPT_CONNECTTIMEOUT_MS(3), CURLOPT_LOW_SPEED_LIMIT(3), CURLOPT_TIME- OUT(3) libcurl 2025-06-03 CURLOPT_H...TIMEOUT_MS(3)
NAME | SYNOPSIS | DESCRIPTION | DEFAULT | PROTOCOLS | EXAMPLE | AVAILABILITY | RETURN VALUE | SEE ALSO
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