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.TH CURLOPT_ALTSVC_CTRL 3 "November 04, 2020" "libcurl 7.74.0" "curl_easy_setopt options"

.SH NAME
CURLOPT_ALTSVC_CTRL \- control alt-svc behavior
.SH SYNOPSIS
.nf
#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);
.fi
.SH DESCRIPTION
Populate the long \fIbitmask\fP with the correct set of features to instruct
libcurl how to handle Alt-Svc for the transfers using this handle.

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.

Setting any bit will enable the alt-svc engine.
.IP "CURLALTSVC_READONLYFILE"
Do not write the alt-svc cache back to the file specified with
\fICURLOPT_ALTSVC(3)\fP even if it gets updated. By default a file specified
with that option will be read and written to as deemed necessary.
.IP "CURLALTSVC_H1"
Accept alternative services offered over HTTP/1.1.
.IP "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.
.IP "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.
.SH DEFAULT
Alt-Svc handling is disabled by default. If \fICURLOPT_ALTSVC(3)\fP is set,
\fICURLOPT_ALTSVC_CTRL(3)\fP 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.
.SH PROTOCOLS
HTTPS
.SH EXAMPLE
.nf
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);
}
.fi
.SH AVAILABILITY
Added in 7.64.1
.SH RETURN VALUE
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
.SH "SEE ALSO"
.BR CURLOPT_ALTSVC "(3), " CURLOPT_CONNECT_TO "(3), " CURLOPT_RESOLVE "(3), "