CURLOPT_SSL_ENABLE_ALPN(3) Introduction to Library Functions CURLOPT_SSL_ENABLE_ALPN(3)

CURLOPT_SSL_ENABLE_ALPN - Application Layer Protocol Negotiation

#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSL_ENABLE_ALPN, long npn);

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.

1, enabled

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

All TLS backends support this option.

int main(void)
{


  CURL *curl = curl_easy_init();


  if(curl) {


    CURLcode res;


    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/");


    curl_easy_setopt(curl, CURLOPT_SSL_ENABLE_ALPN, 0L);


    res = curl_easy_perform(curl);


    curl_easy_cleanup(curl);


  }
}

Added in curl 7.36.0

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_SSL_ENABLE_NPN(3), CURLOPT_SSL_OPTIONS(3)

2025-05-02 libcurl