curl_easy_recv(3)

💅 curl_easy_recv — bestie's guide to raw socket reads

okay so this is from the Library Functions Manual (yeah, the libcurl one 🫶) and it's literally the page for curl_easy_recv(3). Sit down, grab a strawberry drink 🍓, we're learning to read raw bytes off an "easy" connection.

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✨ NAME ✨

curl_easy_recv — receives raw data on an "easy" connection 💌

✨ SYNOPSIS ✨

#include <curl/curl.h>

CURLcode curl_easy_recv(CURL *curl, void *buffer, size_t buflen, size_t *n);

🌸 DESCRIPTION 🌸

So this function receives raw data from the established connection. That's it, that's the vibe. You can pair it with curl_easy_send(3) to implement custom protocols using libcurl — like, your own protocol, your own rules, main character energy 🔥.

This is particularly useful if you use proxies and/or SSL encryption, because libcurl takes care of proxy negotiation and connection setup for you. You don't have to do the boring parts. Iconic. 👑

• buffer is a pointer to your buffer memory that gets populated by the received data.
• buflen is the maximum amount of data you can get in that buffer.
• The variable n points to receives the number of received bytes.

To establish the connection, set CURLOPT_CONNECT_ONLY(3) option before calling curl_easy_perform(3) or curl_multi_perform(3).

⚠️ Note that curl_easy_recv(3) does not work on connections that were created without this option. No option, no bytes. Periodt.

The call returns CURLE_AGAIN if there is no data to read — the socket is used in non-blocking mode internally. When CURLE_AGAIN is returned, use your operating system facilities like select(2) to wait for data. The socket may be obtained using curl_easy_getinfo(3) with CURLINFO_ACTIVESOCKET(3).

💀 Wait on the socket only if curl_easy_recv(3) returns CURLE_AGAIN. The reason for this is libcurl or the SSL library may internally cache some data, therefore you should call curl_easy_recv(3) until all data is read which would include any cached data.

Furthermore if you wait on the socket and it tells you there is data to read, curl_easy_recv(3) may return CURLE_AGAIN if the only data that was read was for internal SSL processing, and no other data is available. Yeah. It's giving "read the room" 😭 the library is doing secret SSL stuff behind your back.

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🌈 PROTOCOLS 🌈

This functionality affects all supported protocols ✨

🎀 EXAMPLE 🎀

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    CURLcode result;
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
    /* Do not do the transfer - only connect to host */
    curl_easy_setopt(curl, CURLOPT_CONNECT_ONLY, 1L);
    result = curl_easy_perform(curl);

    if(result == CURLE_OK) {
      char buf[256];
      size_t nread;
      curl_socket_t sockfd;

      /* Extract the socket from the curl handle - we need it for waiting. */
      result = curl_easy_getinfo(curl, CURLINFO_ACTIVESOCKET, &sockfd);

      /* read data */
      result = curl_easy_recv(curl, buf, sizeof(buf), &nread);
    }
  }
}

Look at that little guy go 🦋 connect only, grab the socket, read the data.

⭐ AVAILABILITY ⭐

Added in curl 7.18.2 — vintage but still slaying.

💖 RETURN VALUE 💖

• On success, returns CURLE_OK, stores the received data into buffer, and the number of bytes it actually read into *n. 🎉
• On failure, returns the appropriate error code. 🙈
• The function may return CURLE_AGAIN. In this case, use your operating system facilities to wait until data can be read, and retry. 🔁
• Reading exactly 0 bytes indicates a closed connection. Bye bestie 👋
• If there is no socket available to use from the previous transfer, this function returns CURLE_UNSUPPORTED_PROTOCOL. Not me showing up with no socket 😭

🫶 SEE ALSO 🫶

• curl_easy_getinfo(3)
• curl_easy_perform(3)
• curl_easy_send(3)
• curl_easy_setopt(3)

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okay that's the whole page, go read some raw bytes, you're gonna ate 💅✨

libcurl · 2026-10-12 · curl_easy_recv(3) — mwah, sending you off with a kiss from the libcurl manual 🎀🦋💜