GNURLOPT_TRAILERFUNCTION(3)

GNURLOPT_TRAILERFUNCTION(3) 🎀 — from the curl_easy_setopt options manual, bestie ✨

okay so this whole page is about the trailer callback, and it’s lowkey iconic. no cap.

🌸 NAME 🌸

• CURLOPT_TRAILERFUNCTION — set callback for sending trailing headers, periodt 💅

✨ 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 💖

So you pass a pointer to a callback function. That callback gets called once right before sending the final CR LF in an HTTP chunked transfer, and its whole job is to fill a list of trailing headers to be sent before finishing the HTTP transfer. slay.

You can set the userdata argument with the CURLOPT_TRAILERDATA option, iykyk.

The trailing headers included in the linked list must not be CRLF-terminated, because libcurl will add the appropriate line termination characters after each header item. it’s giving “let libcurl handle the line endings.”

If you use curl_slist_append to add trailing headers to the curl_slist then libcurl will duplicate the strings, and will free the curl_slist and the duplicates once the trailers have been sent. 🫶

If one of the trailing headers is not formatted correctly (i.e. HeaderName: headerdata) it will be ignored and an info message will be emitted. not me getting ignored for bad formatting 😭

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. choose your fighter.

If you set this option to NULL, then the transfer proceeds as usual without any interruptions. chill vibes only.

🍓 DEFAULT 🍓

NULL — literally the default, bestie.

🌈 PROTOCOLS 🌈

HTTP — that’s the one listed here, periodt.

🦋 EXAMPLE 🦋

here’s the example, verbatim and all:

       #include <gnurl/curl.h>

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

       CURL *curl = curl_easy_init(); if(curl) {
         /* Set the URL of the request */
         curl_easy_setopt(curl, CURLOPT_URL, "http://example.com/");
         /* Now set it as a put */
         curl_easy_setopt(curl, CURLOPT_PUT, 1L);

         /* Assuming we have a function that will return 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-awsome-
       trailer");
         res = curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);

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

         /* Perform the request, res will get the return code */
         res = curl_easy_perform(curl);

         curl_easy_cleanup(curl);

         curl_slist_free_all(headers); }

👑 AVAILABILITY 👑

This option was added in curl 7.64.0 and is present if HTTP support is enabled. iconic behavior.

💜 SEE ALSO 💜

• CURLOPT_TRAILERDATA(3), — go read that page next, bestie 🫶

🎀 that’s the whole page! source/version: libcurl 7.64.0 · date: 14 Aug 2018 · manual: GNURLOPT_TRAILERFUNCTION(3) — mwah, bestie 💋✨