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.

libcurl creates the file to store the alt-svc cache in using default file permissions, meaning that on *nix systems you may need to restrict your umask to prevent other users on the same system to access the file.

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, CURLALTSVC_H1);
    curl_easy_setopt(curl, CURLOPT_ALTSVC, "altsvc-cache.txt");
    curl_easy_perform(curl);
  }
}

HISTORY
CURLALTSVC_* macros became long types in 8.16.0, prior to this version a long cast was necessary when passed to curl_easy_setopt(3).

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)