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        !            23: .TH CURLOPT_UPKEEP_INTERVAL_MS 3 "31 Oct 2018" "libcurl 7.62.0" "curl_easy_setopt options"
        !            24: .SH NAME
        !            25: CURLOPT_UPKEEP_INTERVAL_MS \- connection upkeep interval
        !            26: .SH SYNOPSIS
        !            27: #include <curl/curl.h>
        !            28: 
        !            29: CURLcode curl_easy_setopt(CURL *handle, CURLOPT_UPKEEP_INTERVAL_MS, long upkeep_interval_ms);
        !            30: .SH DESCRIPTION
        !            31: Some protocols have "connection upkeep" mechanisms. These mechanisms usually
        !            32: send some traffic on existing connections in order to keep them alive; this
        !            33: can prevent connections from being closed due to overzealous firewalls, for
        !            34: example.
        !            35: 
        !            36: The user needs to explicitly call \fIcurl_easy_upkeep(3)\fP in order to
        !            37: perform the upkeep work.
        !            38: 
        !            39: Currently the only protocol with a connection upkeep mechanism is HTTP/2: when
        !            40: the connection upkeep interval is exceeded and \fIcurl_easy_upkeep(3)\fP
        !            41: is called, an HTTP/2 PING frame is sent on the connection.
        !            42: 
        !            43: .SH DEFAULT
        !            44: CURL_UPKEEP_INTERVAL_DEFAULT (currently defined as 60000L, which is 60 seconds)
        !            45: .SH EXAMPLE
        !            46: .nf
        !            47: CURL *curl = curl_easy_init();
        !            48: if(curl) {
        !            49:   /* Make a connection to an HTTP/2 server. */
        !            50:   curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
        !            51: 
        !            52:   /* Set the interval to 30000ms / 30s */
        !            53:   curl_easy_setopt(curl, CURLOPT_UPKEEP_INTERVAL_MS, 30000L);
        !            54: 
        !            55:   curl_easy_perform(curl);
        !            56: 
        !            57:   /* Perform more work here. */
        !            58: 
        !            59:   /* While the connection is being held open, curl_easy_upkeep() can be
        !            60:      called. If curl_easy_upkeep() is called and the time since the last
        !            61:      upkeep exceeds the interval, then an HTTP/2 PING is sent. */
        !            62:   curl_easy_upkeep(curl);
        !            63: 
        !            64:   /* Perform more work here. */
        !            65: 
        !            66:   /* always cleanup */
        !            67:   curl_easy_cleanup(curl);
        !            68: }
        !            69: .fi
        !            70: .SH AVAILABILITY
        !            71: Added in 7.62.0
        !            72: .SH RETURN VALUE
        !            73: Returns CURLE_OK