CURLOPT_ALTSVC(3) Library Functions Manual CURLOPT_ALTSVC(3)

CURLOPT_ALTSVC - alt-svc cache filename

#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_ALTSVC, char *filename);

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.

NULL. The alt-svc cache is not read nor written to file.


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");

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 8443 h3 443 "20190808 06:18:37" 1 0

The fields of that line are:

ALPN id for the source origin
Hostname for the source origin
Port number for the source origin
ALPN id for the destination host
Hostname for the destination host
Port number for the destination host
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)

Added in 7.64.1

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


2024-05-22 libcurl