CURLOPT_ALTSVC - alt-svc cache filename

Added in curl 7.64.1

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

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.

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); } }

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)

CURLOPT_ALTSVC - alt-svc cache filename

This functionality affects http only

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).

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.

CURLOPT_ALTSVC_CTRL(3), CURLOPT_CONNECT_TO(3), CURLOPT_COOKIEFILE(3), CURLOPT_RESOLVE(3) libcurl 2025-06-16 CURLOPT_ALTSVC(3)

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