CURLOPT_ALTSVC(3) Introduction to Library Functions 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.
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.
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.
NULL. The alt-svc cache is not read nor written to file.
This functionality affects http only
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);
}
}
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).
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)
Added in curl 7.64.1
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).
CURLOPT_ALTSVC_CTRL(3), CURLOPT_CONNECT_TO(3), CURLOPT_COOKIEFILE(3),
CURLOPT_RESOLVE(3)
libcurl 2026-01-08 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)
libcurl 2026-01-08 CURLOPT_ALTSVC(3)