CURLOPT_ALTSVC(3) Library Functions Manual CURLOPT_ALTSVC(3)
NAME
CURLOPT_ALTSVC - alt-svc cache filename
SYNOPSIS
#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_ALTSVC, char *filename);
DESCRIPTION
Pass in a pointer to a filename to instruct libcurl to use that file as
the Alt-Svc cache to read existing cache contents from and possibly also
write it back to after a transfer, unless CURLALTSVC_READONLYFILE is set
in CURLOPT_ALTSVC_CTRL(3).
Specify a blank filename ("") to make libcurl not load from a file at
all.
The application does not have to keep the string around after setting
this option.
Using this option multiple times makes the last set string override the
previous ones. Set it to NULL to disable its use again.
SECURITY CONCERNS
libcurl cannot fully protect against attacks where an attacker has write
access to the same directory where it is directed to save files. This is
particularly sensitive if you save files using elevated privileges.
DEFAULT
NULL. The alt-svc cache is not read nor written to file.
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);
}
}
FILE FORMAT
A text based file with one line per alt-svc entry and each line consists
of nine space-separated fields.
An example line could look like
h2 www.example.com 8443 h3 second.example.com 443 "20190808 06:18:37" 1 0
The fields of that line are:
h2 ALPN id for the source origin
www.example.comp
Hostname for the source origin
8443 Port number for the source origin
h3 ALPN id for the destination host
second.example.com
Hostname for the destination host
443 Port number for the destination host
2019* Expiration date and time of this entry within double quotes. The
date format is "YYYYMMDD HH:MM:SS" and the time zone is GMT.
1 Boolean (1 or 0) if "persist" was set for this entry
0 Integer priority value (not currently used)
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_CTRL(3), CURLOPT_CONNECT_TO(3), CURLOPT_COOKIEFILE(3),
CURLOPT_RESOLVE(3)
libcurl 2026-07-12 CURLOPT_ALTSVC(3)
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