CURLOPT\_SSL\_ENABLE\_ALPN(3) Library Functions Manual CURLOPT\_SSL\_ENABLE\_ALPN(3)

# [NAME](#NAME)

CURLOPT\_SSL\_ENABLE\_ALPN - Application Layer Protocol Negotiation

# [SYNOPSIS](#SYNOPSIS)

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

# [DESCRIPTION](#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](#DEFAULT)

1, enabled

# [PROTOCOLS](#PROTOCOLS)

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

All TLS backends support this option.

# [EXAMPLE](#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](#AVAILABILITY)

Added in curl 7.36.0

# [RETURN VALUE](#RETURN_VALUE)

[*curl\_easy\_setopt(3)*](/man/os/curl/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)*](/man/os/curl/libcurl-errors.3).

# [SEE ALSO](#SEE_ALSO)

[**CURLOPT\_SSL\_ENABLE\_NPN**(3)](/man/os/curl/CURLOPT_SSL_ENABLE_NPN.3), [**CURLOPT\_SSL\_OPTIONS**(3)](/man/os/curl/CURLOPT_SSL_OPTIONS.3)

2026-10-19 libcurl

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Document: CURLOPT_SSL_ENABLE_ALPN(3) from curl 8.23.0-r1 (Chainguard OS)
Source: https://github.com/curl/curl.git