CURLMOPT_MAX_HOST_CONNECTIONS man page
CURLMOPT_MAX_HOST_CONNECTIONS — set max number of connections to a single host
CURLMcode curl_multi_setopt(CURLM *handle, CURLMOPT_MAX_HOST_CONNECTIONS, long max);
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(3). When the limit is reached, the sessions will be pending until a connection becomes available. If CURLMOPT_PIPELINING(3) 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(3) 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.
Added in 7.30.0
Returns CURLM_OK if the option is supported, and CURLM_UNKNOWN_OPTION if not.
CURLMOPT_MAXCONNECTS(3), CURLMOPT_MAX_PIPELINE_LENGTH(3), CURLMOPT_MAX_TOTAL_CONNECTIONS(3), CURLMOPT_PIPELINING(3), curl_multi_setopt(3), CURLOPT_PIPEWAIT(3).