CURLOPT_SSL_ENABLE_ALPN(3)                            Library Functions Manual

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. only

       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                           2026-09-02        CURLOPT_SSL_ENABLE_ALPN(3)