Pass a long to indicate max. The set number will be used as the maximum amount of simultaneously open connections to a single host (a host being the same as a host name + port number pair). For each new session to a host, libcurl will open a new connection up to the limit set by CURLMOPT_MAX_HOST_CONNECTIONS. When the limit is reached, the sessions will be pending until a connection becomes available. If CURLMOPT_PIPELINING is enabled, libcurl will try to pipeline if the host is capable of it.
The default max value is 0, unlimited. However, for backwards compatibility, setting it to 0 when CURLMOPT_PIPELINING is 1 will not be treated as unlimited. Instead it will open only 1 connection and try to pipeline on it.
This set limit is also used for proxy connections, and then the proxy is considered to be the host for which this limit counts.
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(); /* do no more than 2 connections per host */ curl_multi_setopt(m, CURLMOPT_MAX_HOST_CONNECTIONS, 2L);
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