CURLMOPT_MAX_HOST_CONNECTIONS(3) Library Functions Manual CURLMOPT_MAX_HOST_CONNECTIONS(3)

CURLMOPT_MAX_HOST_CONNECTIONS - max number of connections to a single host

#include <curl/curl.h>
CURLMcode curl_multi_setopt(CURLM *handle, CURLMOPT_MAX_HOST_CONNECTIONS,
                            long max);

Pass a long to indicate max. The set number is used as the maximum amount of simultaneously open connections to a single host (a host being the same as a hostname + port number pair). For each new session to a host, libcurl might open a new connection up to the limit set by CURLMOPT_MAX_HOST_CONNECTIONS(3). When the limit is reached, new sessions are kept pending until a connection becomes available.

The default max value is 0, unlimited. 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 are queued up waiting for their chance. When that happens, the CURLOPT_TIMEOUT_MS(3) timeout is 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(3) timeout is however treated as a per-connect timeout.

Changing this value when there are transfers in progress is possible, and the new value is then used the next time checks are performed. Lowering the value does however not close down any active transfers, it simply does not allow new ones to get made.

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int main(void)
{
  CURLM *m = curl_multi_init();
  /* do no more than 2 connections per host */
  curl_multi_setopt(m, CURLMOPT_MAX_HOST_CONNECTIONS, 2L);
}

Added in 7.30.0

Returns CURLM_OK if the option is supported, and CURLM_UNKNOWN_OPTION if not.

CURLMOPT_MAXCONNECTS(3), CURLMOPT_MAX_TOTAL_CONNECTIONS(3)

2024-05-22 libcurl