API-libcurl cannot use CURLOPT_RANGE or any other functions to resume to download files

HI lazyCoder,

I have to read more carefully too... or not. 🤔 As seems on libcurl, headers formats are swapped by mistake. Request range there is formatted rather as for response. I have no idea why, just checked; now as seems documentation is incomplete or... there is some bug if we assume documentation is correct.

Anyway, my test setup follows (if can be useful):

/*****************************************************************************
 *                        Test curl partial download
 *                        ==========================
 *
 * Simulated breakage of the connection and next continuation from the place
 * where broke.
 *
 * range_test.c
 * Author: Здравко
 * https://www.dropboxforum.com/t5/user/viewprofilepage/user-id/422790
 *
 *****************************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <curl/curl.h>

const char *access_token = ""; // Put here valid token for the test!

// const char *download_point = "http://127.0.0.1:8080/"; // netcat -l -p 8080
const char *download_point = "https://content.dropboxapi.com/2/files/download";
const char *param_template = "Dropbox-API-Arg: {\"path\":\"%s\"}";

int main(int argc, char **argv) {
  const char *dropboxPath;
  const char *localPath;
  char *param;
  CURL *curl;
  CURLcode res;
  struct curl_slist *param_list = NULL;
  FILE *output;
  long current_pos;
  char rangeBuf[48];

  if (argc != 3) {
    fputs("2 command arguments needed - dropbox file path (or file id) and "
          "local file path!", stderr);
    return EXIT_FAILURE;
  }
  dropboxPath = argv[1];
  localPath = argv[2];
  param = (char*)malloc(strlen(param_template) + strlen(dropboxPath) + 1);
  sprintf(param, param_template, dropboxPath);
  printf("Will try read from '%s'\nand write to '%s'\n\n",
         dropboxPath, localPath);

  curl_global_init(CURL_GLOBAL_ALL);

  puts("Starting break simultaion...");
  output = fopen(localPath, "wb");
  if (output == NULL) {
    free(param);
    curl_global_cleanup();
    fputs("Cannot open target file for write - first step!", stderr);
    return EXIT_FAILURE;
  }
  curl = curl_easy_init();
  if (curl == NULL) {
    fclose(output);
    free(param);
    curl_global_cleanup();
    fputs("Cannot open cURL handle - first step!", stderr);
    return EXIT_FAILURE;
  }
  param_list = curl_slist_append(NULL, param);
  param_list = curl_slist_append(param_list, "Range: bytes=0-199");
  curl_easy_setopt(curl, CURLOPT_URL, download_point);
  curl_easy_setopt(curl, CURLOPT_POST, 1L);
  curl_easy_setopt(curl, CURLOPT_HTTPAUTH, CURLAUTH_BEARER);
  curl_easy_setopt(curl, CURLOPT_XOAUTH2_BEARER, access_token);
  curl_easy_setopt(curl, CURLOPT_HTTPHEADER, param_list);
  curl_easy_setopt(curl, CURLOPT_MIMEPOST, NULL); // Set empty post
  curl_easy_setopt(curl, CURLOPT_WRITEDATA, output);
  res = curl_easy_perform(curl);
  curl_slist_free_all(param_list);
  param_list = NULL;
  curl_easy_cleanup(curl);
  if (res != CURLE_OK) {
    fclose(output);
    free(param);
    curl_global_cleanup();
    fprintf(stderr, "Error while download - first step: %s!\n",
            curl_easy_strerror(res));
    return EXIT_FAILURE;
  }
  printf("Current file size: %ld\n", ftell(output));
  fclose(output);
  puts("Simultaion completed.\n");

  puts("Starting resume download...");
  output = fopen(localPath, "ab");
  if (output == NULL) {
    free(param);
    curl_global_cleanup();
    fputs("Cannot open target file for write - second step!", stderr);
    return EXIT_FAILURE;
  }
  current_pos = ftell(output);
  printf("Resume position is %ld\n", current_pos);
  curl = curl_easy_init();
  if (curl == NULL) {
    fclose(output);
    free(param);
    curl_global_cleanup();
    fputs("Cannot open cURL handle - second step!", stderr);
    return EXIT_FAILURE;
  }
  param_list = curl_slist_append(NULL, param);
  sprintf(rangeBuf, "Range: bytes=%ld-", current_pos);
  param_list = curl_slist_append(param_list, rangeBuf);
  curl_easy_setopt(curl, CURLOPT_URL, download_point);
  curl_easy_setopt(curl, CURLOPT_POST, 1L);
  curl_easy_setopt(curl, CURLOPT_HTTPAUTH, CURLAUTH_BEARER);
  curl_easy_setopt(curl, CURLOPT_XOAUTH2_BEARER, access_token);
  curl_easy_setopt(curl, CURLOPT_HTTPHEADER, param_list);
  curl_easy_setopt(curl, CURLOPT_MIMEPOST, NULL); // Set empty post
  curl_easy_setopt(curl, CURLOPT_WRITEDATA, output);
  res = curl_easy_perform(curl);
  curl_slist_free_all(param_list);
  param_list = NULL;
  curl_easy_cleanup(curl);
  if (res != CURLE_OK) {
    fclose(output);
    free(param);
    curl_global_cleanup();
    fprintf(stderr, "Error while download - second step: %s!\n",
            curl_easy_strerror(res));
    return EXIT_FAILURE;
  }
  printf("Current file size: %ld\n", ftell(output));
  fclose(output);
  puts("Download resume completed.\n");

  curl_global_cleanup();
  free(param);
  return EXIT_SUCCESS;
}

Good luck.