1--- 2c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al. 3SPDX-License-Identifier: curl 4Title: CURLOPT_TRAILERFUNCTION 5Section: 3 6Source: libcurl 7See-also: 8 - CURLOPT_TRAILERDATA (3) 9 - CURLOPT_WRITEFUNCTION (3) 10Protocol: 11 - HTTP 12--- 13 14# NAME 15 16CURLOPT_TRAILERFUNCTION - callback for sending trailing headers 17 18# SYNOPSIS 19 20~~~c 21#include <curl.h> 22 23int curl_trailer_callback(struct curl_slist ** list, void *userdata); 24 25CURLcode curl_easy_setopt(CURL *handle, CURLOPT_TRAILERFUNCTION, 26 curl_trailer_callback *func); 27~~~ 28 29# DESCRIPTION 30 31Pass a pointer to a callback function. 32 33This callback function is called once right before sending the final CR LF in 34an HTTP chunked transfer to fill a list of trailing headers to be sent before 35finishing the HTTP transfer. 36 37You can set the userdata argument with the CURLOPT_TRAILERDATA(3) 38option. 39 40The trailing headers included in the linked list must not be CRLF-terminated, 41because libcurl adds the appropriate line termination characters after each 42header item. 43 44If you use curl_slist_append(3) to add trailing headers to the *curl_slist* 45then libcurl duplicates the strings, and frees the *curl_slist* once the 46trailers have been sent. 47 48If one of the trailing header fields is not formatted correctly it is ignored 49and an info message is emitted. 50 51The return value can either be **CURL_TRAILERFUNC_OK** or 52**CURL_TRAILERFUNC_ABORT** which would respectively instruct libcurl to 53either continue with sending the trailers or to abort the request. 54 55If you set this option to NULL, then the transfer proceeds as usual 56without any interruptions. 57 58# DEFAULT 59 60NULL 61 62# EXAMPLE 63~~~c 64static int trailer_cb(struct curl_slist **tr, void *data) 65{ 66 /* libcurl frees the list */ 67 *tr = curl_slist_append(*tr, "My-super-awesome-trailer: trailer-stuff"); 68 return CURL_TRAILERFUNC_OK; 69} 70 71int main(void) 72{ 73 CURL *curl = curl_easy_init(); 74 if(curl) { 75 CURLcode res; 76 77 /* Set the URL of the request */ 78 curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/"); 79 /* Now set it as a put */ 80 curl_easy_setopt(curl, CURLOPT_PUT, 1L); 81 82 /* Assuming we have a function that returns the data to be pushed 83 Let that function be read_cb */ 84 curl_easy_setopt(curl, CURLOPT_READFUNCTION, trailer_cb); 85 86 struct curl_slist *headers = NULL; 87 headers = curl_slist_append(headers, "Trailer: My-super-awesome-trailer"); 88 res = curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers); 89 90 /* Set the trailers filling callback */ 91 curl_easy_setopt(curl, CURLOPT_TRAILERFUNCTION, trailer_cb); 92 93 /* Perform the transfer */ 94 res = curl_easy_perform(curl); 95 96 curl_easy_cleanup(curl); 97 98 curl_slist_free_all(headers); 99 } 100} 101~~~ 102# AVAILABILITY 103 104This option was added in curl 7.64.0 and is present if HTTP support is enabled. 105 106# RETURN VALUE 107 108Returns CURLE_OK. 109