curl / libcurl / API / curl_easy_setopt / CURLOPT_ALTSVC

CURLOPT_ALTSVC explained

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.

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.

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);
  }
}

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:

ALPN id for the source origin

www.example.comp

Hostname for the source origin

8443

Port number for the source origin

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.

Boolean (1 or 0) if "persist" was set for this entry

Integer priority value (not currently used)

Availability

Added in curl 7.64.1

Return value

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