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CURLOPT_ALTSVC_CTRL(3) Library Functions Manual CURLOPT_ALTSVC_CTRL(3) NAME CURLOPT_ALTSVC_CTRL - control alt-svc behavior SYNOPSIS #include <curl/curl.h> #define CURLALTSVC_READONLYFILE (1<<2) #define CURLALTSVC_H1 (1<<3) #define CURLALTSVC_H2 (1<<4) #define CURLALTSVC_H3 (1<<5) CURLcode curl_easy_setopt(CURL *handle, CURLOPT_ALTSVC_CTRL, long bitmask); DESCRIPTION Populate the long bitmask with the correct set of features to instruct libcurl how to handle Alt-Svc for the transfers using this handle. libcurl only accepts Alt-Svc headers over a Secure Transport, meaning HTTPS. It also only completes a request to an alternative origin if that origin is properly hosted over HTTPS. These requirements are there to make sure both the source and the destination are legitimate. Alternative services are only used when setting up new connections. If there exists an existing connection to the host in the connection pool, then that is preferred. If CURLOPT_ALTSVC(3) is set, CURLOPT_ALTSVC_CTRL(3) gets a default value corresponding to CURLALTSVC_H1 | CURLALTSVC_H2 | CURLALTSVC_H3 - the HTTP/2 and HTTP/3 bits are only set if libcurl was built with sup- port for those versions. Setting any bit enables the alt-svc engine. CURLALTSVC_READONLYFILE Do not write the alt-svc cache back to the file specified with CURLOPT_ALTSVC(3) even if it gets updated. By default a file specified with that option is read and written to as deemed nec- essary. CURLALTSVC_H1 Accept alternative services offered over HTTP/1.1. CURLALTSVC_H2 Accept alternative services offered over HTTP/2. This is only used if libcurl was also built to actually support HTTP/2, oth- erwise this bit is ignored. CURLALTSVC_H3 Accept alternative services offered over HTTP/3. This is only used if libcurl was also built to actually support HTTP/3, oth- erwise this bit is ignored. DEFAULT 0 - Alt-Svc handling is disabled PROTOCOLS This functionality affects http only EXAMPLE int main(void) { CURL *curl = curl_easy_init(); if(curl) { curl_easy_setopt(curl, CURLOPT_ALTSVC_CTRL, (long)CURLALTSVC_H1); curl_easy_setopt(curl, CURLOPT_ALTSVC, "altsvc-cache.txt"); curl_easy_perform(curl); } } AVAILABILITY Added in curl 7.64.1 RETURN VALUE curl_easy_setopt(3) returns a CURLcode indicating success or error. CURLE_OK (0) means everything was OK, non-zero means an error occurred, see libcurl-errors(3). SEE ALSO CURLOPT_ALTSVC(3), CURLOPT_CONNECT_TO(3), CURLOPT_RESOLVE(3) libcurl 2025-06-03 CURLOPT_ALTSVC_CTRL(3)
NAME | SYNOPSIS | DESCRIPTION | DEFAULT | PROTOCOLS | EXAMPLE | AVAILABILITY | RETURN VALUE | SEE ALSO
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