GNURLOPT_ALTSVC_CTRL(3)

💖 NAME 💖

GNURLOPT_ALTSVC_CTRL(3) — control alt-svc behavior. Also hi bestie, this page lives in the curl_easy_setopt options manual, so we are officially in easy-option land ✨

🧚 SYNOPSIS 🧚

#include <gnurl/curl.h>

#define CURLALTSVC_IMMEDIATELY  (1<<0)
#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);

💀 EXPERIMENTAL 💀

Warning: this feature is early code and is marked as experimental. It can only be enabled by explicitly telling configure with --enable-alt-svc. You are advised to not ship this in production before the experimental label is removed.

🌸 DESCRIPTION 🌸

Okay so you populate the long bitmask with the correct set of features to instruct libcurl how to handle Alt-Svc for the transfers using this handle. That is the whole assignment, bestie 🫶

libcurl will only accept Alt-Svc headers over a secure transport, meaning HTTPS. It will also only complete 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. No sketchy little shortcuts, periodt.

Setting any bit will enable the alt-svc engine. Yup, any bit — the engine wakes up and starts paying attention 👑

• CURLALTSVC_IMMEDIATELY — if an Alt-Svc: header is received, this instructs libcurl to switch to one of those alternatives asap rather than to save it and use for the next request. (Not currently supported).
• 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 will be read and written to as deemed necessary.
• CURLALTSVC_H1 — accept alternative services offered over HTTP/1.1.
• CURLALTSVC_H2 — accept alternative services offered over HTTP/2. This will only be used if libcurl was also built to actually support HTTP/2, otherwise this bit will be ignored.
• CURLALTSVC_H3 — accept alternative services offered over HTTP/3. This will only be used if libcurl was also built to actually support HTTP/3, otherwise this bit will be ignored.

🎀 DEFAULT 🎀

Alt-Svc handling is disabled by default. If CURLOPT_ALTSVC(3) is set, CURLOPT_ALTSVC_CTRL(3) has 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 support for those versions.

🦋 PROTOCOLS 🦋

HTTPS

🌈 EXAMPLE 🌈

CURL *curl = curl_easy_init();
if(curl) {
  curl_easy_setopt(curl, CURLOPT_ALTSVC_CTRL, CURLALTSVC_H1);
  curl_easy_setopt(curl, CURLOPT_ALTSVC, "altsvc-cache.txt");
  curl_easy_perform(curl);
}

⭐ AVAILABILITY ⭐

Added in 7.64.1. Iconic vintage, honestly.

💅 RETURN VALUE 💅

Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.

🔥 SEE ALSO 🔥

• CURLOPT_ALTSVC(3), CURLOPT_CONNECT_TO(3), CURLOPT_RESOLVE(3),

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That's the whole page, bestie — source is libcurl 7.64.1, dated 5 Feb 2019, and this experimental little icon is out 💅✨