CURLcode curl_easy_upkeep(CURL *handle);
Some protocols have "connection upkeep" mechanisms. These mechanisms usually send some traffic on existing connections in order to keep them alive; this can prevent connections from being closed due to overzealous firewalls, for example.
Currently the only protocol with a connection upkeep mechanism is HTTP/2: when the connection upkeep interval is exceeded and curl_easy_upkeep(3) is called, an HTTP/2 PING frame is sent on the connection.
This function must be explicitly called in order to perform the upkeep work. The connection upkeep interval is set with CURLOPT_UPKEEP_INTERVAL_MS(3).
On failure, returns the appropriate error code.
CURL *curl = curl_easy_init();
if(curl) {
/* Make a connection to an HTTP/2 server. */
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
/* Set the interval to 30000ms / 30s */
curl_easy_setopt(curl, CURLOPT_UPKEEP_INTERVAL_MS, 30000L);
curl_easy_perform(curl);
/* Perform more work here. */
/* While the connection is being held open, curl_easy_upkeep() can be
called. If curl_easy_upkeep() is called and the time since the last
upkeep exceeds the interval, then an HTTP/2 PING is sent. */
curl_easy_upkeep(curl);
/* Perform more work here. */
/* always cleanup */
curl_easy_cleanup(curl);
}