CURLcode curl_easy_setopt(CURL *handle, CURLOPT_PRE_PROXY, char *preproxy);
To specify port number in this string, append :[port] to the end of the host name. The proxy's port number may optionally be specified with the separate option CURLOPT_PROXYPORT(3). If not specified, libcurl will default to using port 1080 for proxies.
A pre proxy is a SOCKS proxy that curl connects to before it connects to the HTTP(S) proxy specified in the CURLOPT_PROXY option. The pre proxy can only be a SOCKS proxy.
The pre proxy string should be prefixed with [scheme]:// to specify which kind of socks is used. Use socks4://, socks4a://, socks5:// or socks5h:// (the last one to enable socks5 and asking the proxy to do the resolving, also known as CURLPROXY_SOCKS5_HOSTNAME type) to request the specific SOCKS version to be used. Otherwise SOCKS4 is used as default.
Setting the pre proxy string to "" (an empty string) will explicitly disable the use of a pre proxy.
The application does not have to keep the string around after setting this option.
When you set a host name to use, do not assume that there's any particular single port number used widely for proxies. Specify it!
CURL *curl = curl_easy_init();
if(curl) {
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/file.txt");
curl_easy_setopt(curl, CURLOPT_PREPROXY, "socks4://socks-proxy:1080");
curl_easy_setopt(curl, CURLOPT_PROXY, "http://proxy:80");
curl_easy_perform(curl);
}