CURLOPT_SSL_ENABLE_ALPN(3)

💅 CURLOPT_SSL_ENABLE_ALPN — it's giving "let's negotiate, bestie" ✨

okay so real quick, this page lives in the Library Functions Manual, and it's all about CURLOPT_SSL_ENABLE_ALPN, which stands for Application Layer Protocol Negotiation (yes the whole thing, no cap 🫶).

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🌸 NAME 🌸

CURLOPT_SSL_ENABLE_ALPN — Application Layer Protocol Negotiation, aka the little handshake conversation that decides what protocol y'all are actually gonna speak 💬

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🎀 SYNOPSIS 🎀

#include <curl/curl.h>

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

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✨ DESCRIPTION ✨

So like, you pass a long as the parameter, and it's either 0 or 1 — 1 means enable, 0 means disable, that's the whole vibe check 🔥

This option enables/disables ALPN in the SSL handshake (but only if the SSL backend libcurl is built to use actually supports it, iykyk — if it doesn't, well, nothing happens 🙈). And what's the point? It can be used to negotiate http2. that's the main character move right there 👑

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💖 DEFAULT 💖

1, enabled — yeah she's on by default, slaying from the start 💅

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🦋 PROTOCOLS 🦋

This functionality affects all TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc. — literally the whole TLS friend group 🌈

All TLS backends support this option. all of them. no exceptions. slay 💜

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🌸 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);
  }
}

(not us turning ALPN off with that 0L — bold choice, iconic honestly 🎀)

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⭐ AVAILABILITY ⭐

Added in curl 7.36.0 — she's been around a minute, respect her legacy 👑

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🧚 RETURN VALUE 🧚

curl_easy_setopt(3) returns a CURLcode indicating success or error, fr fr.

CURLE_OK (0) means everything was OK 💚 — and non-zero means an error occurred 😭, so go see libcurl-errors(3) for the whole breakdown.

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🫶 SEE ALSO 🫶

• CURLOPT_SSL_ENABLE_NPN(3)
• CURLOPT_SSL_OPTIONS(3)

go read those too bestie, they're lowkey related 💖

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mwah 💋 — this page is from libcurl, dated 2026-10-12, and the page itself is CURLOPT_SSL_ENABLE_ALPN(3). now go negotiate some protocols, you got this 🦋✨