CURLcode curl_easy_setopt(CURL *handle, CURLOPT_STREAM_WEIGHT, long weight);
When using HTTP/2, this option sets the individual weight for this particular stream used by the easy handle. Setting and using weights only makes sense and is only usable when doing multiple streams over the same connections, which thus implies that you use CURLMOPT_PIPELINING(3).
This option can be set during transfer and will then cause the updated weight info get sent to the server the next time an HTTP/2 frame is sent to the server.
See section 5.3 of RFC 7540 for protocol details: https://httpwg.github.io/specs/rfc7540.html#StreamPriority
Streams with the same parent should be allocated resources proportionally based on their weight. So if you have two streams going, stream A with weight 16 and stream B with weight 32, stream B will get two thirds (32/48) of the available bandwidth (assuming the server can send off the data equally for both streams).
CURL *curl = curl_easy_init();
CURL *curl2 = curl_easy_init(); /* a second handle */
if(curl) {
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/one");
curl_easy_setopt(curl, CURLOPT_STREAM_WEIGHT, 10L);
/* the second has twice the weight */
curl_easy_setopt(curl2, CURLOPT_URL, "https://example.com/two");
curl_easy_setopt(curl2, CURLOPT_STREAM_WEIGHT, 20L);
/* then add both to a multi handle and transfer them! */
}