CURLOPT_TCP_FASTOPEN(3)

💅 CURLOPT_TCP_FASTOPEN(3) 💅

okay so we're in the Library Functions Manual, bestie, and today's main character is CURLOPT_TCP_FASTOPEN — the option that literally skips a whole round-trip like it's running late for class. ✨

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🌸 NAME 🌸

CURLOPT_TCP_FASTOPEN — TCP Fast Open, no cap 🚀

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🦋 SYNOPSIS 🦋

#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_TCP_FASTOPEN, long enable);

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💖 DESCRIPTION 💖

So you pass a long as the parameter, and it's so simple: set it to 1L to turn it on, or 0 to turn it off. that's the whole vibe check. 🫶

TCP Fast Open (RFC 7413) is this iconic mechanism where data just... rides along inside the SYN and SYN-ACK packets, and the receiving end consumes it during the initial connection handshake. the result? it saves up to one full round-trip time (RTT). one. whole. RTT. that's giving main character energy honestly. ⭐

⚠️ Beware, bestie: the TLS session cache does not work when TCP Fast Open is enabled. like, at all. 😭 and TCP Fast Open is also known to be problematic on or across certain networks. so like... know before you go. 💀

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🎀 DEFAULT 🎀

0

(off by default — she's lowkey until you tell her otherwise)

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🌈 PROTOCOLS 🌈

This functionality affects tcp only. that's it. that's the whole list. periodt. 💅

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✨ EXAMPLE ✨

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
    curl_easy_setopt(curl, CURLOPT_TCP_FASTOPEN, 1L);
    curl_easy_perform(curl);
  }
}

look at her go 🔥

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🧚 NOTES 🧚

This option is only supported on Linux and macOS 10.11 or later. so if you're on something else… not me being unsupported 🙈

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👑 AVAILABILITY 👑

Added in curl 7.49.0 — a classic, honestly. 🎀

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💜 RETURN VALUE 💜

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

CURLE_OK (0) means everything was OK — slay. non-zero means an error occurred, so go check libcurl-errors(3) and figure out what went wrong, iykyk. 😭

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🌸 SEE ALSO 🌸

• CURLOPT_SSL_FALSESTART(3) — her cousin, also doing too much with the handshake 🦋

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and that's the page, bestie!! 💖 this one came from the libcurl crew, dated 2026-10-12 — mwah, kisses, go forth and fast-open responsibly. 🍓💅🫶