CURLOPT_TRAILERFUNCTION(3)

Okay so this is CURLOPT_TRAILERFUNCTION(3) from the Library Functions Manual, and it’s giving “callback for sending trailing headers” energy. If you’re here, you’re probably doing HTTP chunked transfer stuff and want to know how to add extra headers at the end. Let’s go, bestie. 💖

🌸 NAME 🌸

CURLOPT_TRAILERFUNCTION — callback for sending trailing headers ✨

🎀 SYNOPSIS 🎀

#include <curl.h>

int curl_trailer_callback(struct curl_slist ** list, void *userdata);

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_TRAILERFUNCTION,
                          curl_trailer_callback *func);

💅 DESCRIPTION 💅

Pass a pointer to a callback function, okay? That’s the whole vibe.

This callback function is called once right before sending the final CR LF in an HTTP chunked transfer to fill a list of trailing headers to be sent before finishing the HTTP transfer. Iconic.

You can set the userdata argument with the CURLOPT_TRAILERDATA(3) option. 🫶

🚨 Important caveat, bestie: the trailing headers included in the linked list must not be CRLF-terminated, because libcurl adds the appropriate line termination characters after each header item. Do not do it yourself.

If you use curl_slist_append(3) to add trailing headers to the curl_slist, then libcurl duplicates the strings, and frees the curl_slist once the trailers have been sent. So it handles the cleanup, no cap.

If one of the trailing header fields is not formatted correctly, it is ignored and an info message is emitted. Noted. 📝

The return value can either be CURL_TRAILERFUNC_OK or CURL_TRAILERFUNC_ABORT, which would respectively instruct libcurl to either continue with sending the trailers or to abort the request. So return CURL_TRAILERFUNC_OK to keep it moving, or CURL_TRAILERFUNC_ABORT to yeet the request.

If you set this option to NULL, then the transfer proceeds as usual without any interruptions. Chill. 🧊

🍓 DEFAULT 🍓

NULL — that’s the default, bestie.

🌈 PROTOCOLS 🌈

This functionality affects http only. Simple.

🦋 EXAMPLE 🦋

Here’s the code, bestie — code is sacred:

extern size_t read_cb(char *ptr, size_t size,
                      size_t nmemb, void *userdata);

static int trailer_cb(struct curl_slist **tr, void *data)
{
  /* libcurl frees the list */
  *tr = curl_slist_append(*tr, "My-super-awesome-trailer: trailer-stuff");
  return CURL_TRAILERFUNC_OK;
}

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    CURLcode result;

    /* Set the URL of the request */
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/");
    /* Now set it as a put */
    curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);

    /* Assuming we have a function that returns the data to be pushed
       Let that function be read_cb */
    curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_cb);

    struct curl_slist *headers = NULL;
    headers = curl_slist_append(headers, "Trailer: My-super-awesome-trailer");
    result = curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);

    /* Set the trailers filling callback */
    curl_easy_setopt(curl, CURLOPT_TRAILERFUNCTION, trailer_cb);

    /* Perform the transfer */
    result = curl_easy_perform(curl);

    curl_easy_cleanup(curl);

    curl_slist_free_all(headers);
  }
}

⭐ AVAILABILITY ⭐

Added in curl 7.64.0.

👑 RETURN VALUE 👑

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).

💜 SEE ALSO 💜

• CURLOPT_TRAILERDATA(3)
• CURLOPT_WRITEFUNCTION(3)

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From libcurl, dated 2026-10-12 — mwah, slay, bye 💅✨