Pass a long for the amount. The set number will be used as the maximum number of simultaneously open connections in total using this multi handle. For each new session, libcurl will open a new connection up to the limit set by CURLMOPT_MAX_TOTAL_CONNECTIONS. When the limit is reached, the sessions will be pending until there are available connections. If CURLMOPT_PIPELINING is enabled, libcurl will try to use multiplexing if the host is capable of it.
When more transfers are added to the multi handle than what can be performed due to the set limit, they will be queued up waiting for their chance. When that happens, the CURLOPT_TIMEOUT_MS timeout will be counted inclusive of the waiting time, meaning that if you set a too narrow timeout in such a case the transfer might never even start before it times out.
Even in the queued up situation, the CURLOPT_CONNECTTIMEOUT_MS timeout is however treated as a per-connect timeout.
CURLM *m = curl_multi_init(); /* never do more than 15 connections */ curl_multi_setopt(m, CURLMOPT_MAX_TOTAL_CONNECTIONS, 15L);
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