curl / libcurl / API / Examples / post-callback.c


 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 * Copyright (C) Daniel Stenberg, <>, et al.
 * This software is licensed as described in the file COPYING, which
 * you should have received as part of this distribution. The terms
 * are also available at
 * You may opt to use, copy, modify, merge, publish, distribute and/or sell
 * copies of the Software, and permit persons to whom the Software is
 * furnished to do so, under the terms of the COPYING file.
 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 * KIND, either express or implied.
 * SPDX-License-Identifier: curl
/* <DESC>
 * Issue an HTTP POST and provide the data through the read callback.
 * </DESC>
#include <stdio.h>
#include <string.h>
#include <curl/curl.h>
/* silly test data to POST */
static const char data[]="Lorem ipsum dolor sit amet, consectetur adipiscing "
  "elit. Sed vel urna neque. Ut quis leo metus. Quisque eleifend, ex at "
  "laoreet rhoncus, odio ipsum semper metus, at tempus ante urna in mauris. "
  "Suspendisse ornare tempor venenatis. Ut dui neque, pellentesque a varius "
  "eget, mattis vitae ligula. Fusce ut pharetra est. Ut ullamcorper mi ac "
  "sollicitudin semper. Praesent sit amet tellus varius, posuere nulla non, "
  "rhoncus ipsum.";
struct WriteThis {
  const char *readptr;
  size_t sizeleft;
static size_t read_callback(char *dest, size_t size, size_t nmemb, void *userp)
  struct WriteThis *wt = (struct WriteThis *)userp;
  size_t buffer_size = size*nmemb;
  if(wt->sizeleft) {
    /* copy as much as possible from the source to the destination */
    size_t copy_this_much = wt->sizeleft;
    if(copy_this_much > buffer_size)
      copy_this_much = buffer_size;
    memcpy(dest, wt->readptr, copy_this_much);
    wt->readptr += copy_this_much;
    wt->sizeleft -= copy_this_much;
    return copy_this_much; /* we copied this many bytes */
  return 0; /* no more data left to deliver */
int main(void)
  CURL *curl;
  CURLcode res;
  struct WriteThis wt;
  wt.readptr = data;
  wt.sizeleft = strlen(data);
  /* In windows, this will init the winsock stuff */
  res = curl_global_init(CURL_GLOBAL_DEFAULT);
  /* Check for errors */
  if(res != CURLE_OK) {
    fprintf(stderr, "curl_global_init() failed: %s\n",
    return 1;
  /* get a curl handle */
  curl = curl_easy_init();
  if(curl) {
    /* First set the URL that is about to receive our POST. */
    curl_easy_setopt(curl, CURLOPT_URL, "");
    /* Now specify we want to POST data */
    curl_easy_setopt(curl, CURLOPT_POST, 1L);
    /* we want to use our own read function */
    curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_callback);
    /* pointer to pass to our read function */
    curl_easy_setopt(curl, CURLOPT_READDATA, &wt);
    /* get verbose debug output please */
    curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
      If you use POST to an HTTP 1.1 server, you can send data without knowing
      the size before starting the POST if you use chunked encoding. You
      enable this by adding a header like "Transfer-Encoding: chunked" with
      CURLOPT_HTTPHEADER. With HTTP 1.0 or without chunked transfer, you must
      specify the size in the request.
      struct curl_slist *chunk = NULL;
      chunk = curl_slist_append(chunk, "Transfer-Encoding: chunked");
      res = curl_easy_setopt(curl, CURLOPT_HTTPHEADER, chunk);
      /* use curl_slist_free_all() after the *perform() call to free this
         list again */
    /* Set the expected POST size. If you want to POST large amounts of data,
    curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, (long)wt.sizeleft);
      Using POST with HTTP 1.1 implies the use of a "Expect: 100-continue"
      header.  You can disable this header with CURLOPT_HTTPHEADER as usual.
      NOTE: if you want chunked transfer too, you need to combine these two
      since you can only set one list of headers with CURLOPT_HTTPHEADER. */
    /* A less good option would be to enforce HTTP 1.0, but that might also
       have other implications. */
      struct curl_slist *chunk = NULL;
      chunk = curl_slist_append(chunk, "Expect:");
      res = curl_easy_setopt(curl, CURLOPT_HTTPHEADER, chunk);
      /* use curl_slist_free_all() after the *perform() call to free this
         list again */
    /* Perform the request, res will get the return code */
    res = curl_easy_perform(curl);
    /* Check for errors */
    if(res != CURLE_OK)
      fprintf(stderr, "curl_easy_perform() failed: %s\n",
    /* always cleanup */
  return 0;


This source code example is simplified and ignores return codes and error checks to a large extent. We do this to highlight the libcurl function calls and related options and reduce unrelated code.

A real-world application will of course properly check every return value and exit correctly at the first serious error.