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deps/curl/docs/libcurl/opts/CURLOPT_ALTSVC_CTRL.md

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+---
+c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLOPT_ALTSVC_CTRL
+Section: 3
+Source: libcurl
+See-also:
+  - CURLOPT_ALTSVC (3)
+  - CURLOPT_CONNECT_TO (3)
+  - CURLOPT_RESOLVE (3)
+---
+
+# NAME
+
+CURLOPT_ALTSVC_CTRL - control alt-svc behavior
+
+# SYNOPSIS
+
+~~~c
+#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.
+
+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 necessary.
+
+## 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, otherwise 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, otherwise this bit is 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
+
+~~~c
+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 7.64.1
+
+# RETURN VALUE
+
+Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.