CURLOPT_SSL_ENABLE_ALPN(3) Introduction to Library Functions

NAME

CURLOPT_SSL_ENABLE_ALPN - Application Layer Protocol Negotiation

SYNOPSIS

#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSL_ENABLE_ALPN, long npn);

DESCRIPTION

Pass a long as parameter, 0 or 1 where 1 is for enable and 0 for
disable. This option enables/disables ALPN in the SSL handshake (if
the SSL backend libcurl is built to use supports it), which can be
used to negotiate http2.

DEFAULT

1, enabled

PROTOCOLS

This functionality affects all TLS based protocols: HTTPS, FTPS,
IMAPS, POP3S, SMTPS etc.

All TLS backends support this option.

EXAMPLE

int main(void)
{
CURL *curl = curl_easy_init();
if(curl) {
CURLcode result;
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/");
curl_easy_setopt(curl, CURLOPT_SSL_ENABLE_ALPN, 0L);
result = curl_easy_perform(curl);
curl_easy_cleanup(curl);
}
}

AVAILABILITY

Added in curl 7.36.0

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_SSL_ENABLE_NPN(3), CURLOPT_SSL_OPTIONS(3)

libcurl 2026-01-08 CURLOPT_SSL_ENABLE_ALPN(3)