CURLOPT_TRAILERFUNCTION(3) Library Functions Manual CURLOPT_TRAILERFUNCTION(3)
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.
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.
You can set the userdata argument with the CURLOPT_TRAILERDATA(3) op-
tion.
The trailing headers included in the linked list must not be CRLF-termi-
nated, because libcurl adds the appropriate line termination characters
after each header item.
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.
If one of the trailing header fields is not formatted correctly it is
ignored and an info message is emitted.
The return value can either be CURL_TRAILERFUNC_OK or CURL_TRAILER-
FUNC_ABORT which would respectively instruct libcurl to either continue
with sending the trailers or to abort the request.
If you set this option to NULL, then the transfer proceeds as usual
without any interruptions.
DEFAULT
NULL
PROTOCOLS
This functionality affects http only
EXAMPLE
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 res;
/* 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");
res = curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
/* Set the trailers filling callback */
curl_easy_setopt(curl, CURLOPT_TRAILERFUNCTION, trailer_cb);
/* Perform the transfer */
res = 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)
libcurl 2026-07-12 CURLOPT_TRAILERFUNCTION(3)
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