Use libcurl to show the current speed while transferring a file.

CurrentSpeed.c

/* Use libcurl to show the current speed while transferring a file.

Usage: CurrentSpeed

This example downloads a 200MB file from a speedtest server and shows the
current speed during the download. The current speed is updated approximately
once every second based on the last 5 seconds. The file is not saved to disk.

The bulk of the code in this example came from libcurl's lib/progress.c.
https://github.com/curl/curl/blob/curl-7_68_0/lib/progress.c

curl-library mailing list thread:
'use libcurl to obtain the real-time speed of git clone.'
https://curl.haxx.se/mail/lib-2020-02/0041.html

Copyright (C) 2020 Jay Satiro <raysatiro@yahoo.com>
http://curl.haxx.se/docs/copyright.html

https://gist.github.com/jay/3db44871ccbe7cc089a25381135e122f
*/

#ifdef _WIN32
#define _CRT_NONSTDC_NO_DEPRECATE
#define _CRT_SECURE_NO_WARNINGS
#include <windows.h>
#endif

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

#include <curl/curl.h>
#include <curl/mprintf.h>

/* Use curl's msnprintf since Windows (and other OSes) may not have snprintf.
   Windows has _snprintf but it behaves differently. */
#undef msnprintf
#define msnprintf curl_msnprintf

/* This is a best guess to determine if curl_off_t is larger than 32-bits.
   CURL_SIZEOF_CURL_OFF_T is the most accurate but the symbol was deprecated.
   Compilers may do preprocessing math with the largest integer type, however
   curl_off_t *should* be larger than 32-bits when such a type is available. */
#if (defined(CURL_SIZEOF_CURL_OFF_T) && (CURL_SIZEOF_CURL_OFF_T > 4)) || \
    (!defined(CURL_SIZEOF_CURL_OFF_T) && \
     (CURL_OFF_TU_C(0xffffffff) < \
      CURL_OFF_TU_C(0xffffffff) + CURL_OFF_TU_C(1)))
#define LARGE_CURL_OFF_T
#endif

/* Rate unit selector.
 *
 * The point of this function would be to return a string of the input data,
 * but never longer than 5 columns (+ one zero byte).
 * Add suffix k, M, G when suitable...
 *
 * This function was copied from libcurl's lib/progress.c (7.68.0).
 */
static char *max5data(curl_off_t bytes, char *max5)
{
#define ONE_KILOBYTE  CURL_OFF_T_C(1024)
#define ONE_MEGABYTE (CURL_OFF_T_C(1024) * ONE_KILOBYTE)
#define ONE_GIGABYTE (CURL_OFF_T_C(1024) * ONE_MEGABYTE)
#define ONE_TERABYTE (CURL_OFF_T_C(1024) * ONE_GIGABYTE)
#define ONE_PETABYTE (CURL_OFF_T_C(1024) * ONE_TERABYTE)

  memset(max5, 0, 6);

  if(bytes < CURL_OFF_T_C(100000))
    msnprintf(max5, 6, "%5" CURL_FORMAT_CURL_OFF_T, bytes);

  else if(bytes < CURL_OFF_T_C(10000) * ONE_KILOBYTE)
    msnprintf(max5, 6, "%4" CURL_FORMAT_CURL_OFF_T "k", bytes/ONE_KILOBYTE);

  else if(bytes < CURL_OFF_T_C(100) * ONE_MEGABYTE)
    /* 'XX.XM' is good as long as we're less than 100 megs */
    msnprintf(max5, 6, "%2" CURL_FORMAT_CURL_OFF_T ".%0"
              CURL_FORMAT_CURL_OFF_T "M", bytes/ONE_MEGABYTE,
              (bytes%ONE_MEGABYTE) / (ONE_MEGABYTE/CURL_OFF_T_C(10)) );

#ifdef LARGE_CURL_OFF_T

  else if(bytes < CURL_OFF_T_C(10000) * ONE_MEGABYTE)
    /* 'XXXXM' is good until we're at 10000MB or above */
    msnprintf(max5, 6, "%4" CURL_FORMAT_CURL_OFF_T "M", bytes/ONE_MEGABYTE);

  else if(bytes < CURL_OFF_T_C(100) * ONE_GIGABYTE)
    /* 10000 MB - 100 GB, we show it as XX.XG */
    msnprintf(max5, 6, "%2" CURL_FORMAT_CURL_OFF_T ".%0"
              CURL_FORMAT_CURL_OFF_T "G", bytes/ONE_GIGABYTE,
              (bytes%ONE_GIGABYTE) / (ONE_GIGABYTE/CURL_OFF_T_C(10)) );

  else if(bytes < CURL_OFF_T_C(10000) * ONE_GIGABYTE)
    /* up to 10000GB, display without decimal: XXXXG */
    msnprintf(max5, 6, "%4" CURL_FORMAT_CURL_OFF_T "G", bytes/ONE_GIGABYTE);

  else if(bytes < CURL_OFF_T_C(10000) * ONE_TERABYTE)
    /* up to 10000TB, display without decimal: XXXXT */
    msnprintf(max5, 6, "%4" CURL_FORMAT_CURL_OFF_T "T", bytes/ONE_TERABYTE);

  else
    /* up to 10000PB, display without decimal: XXXXP */
    msnprintf(max5, 6, "%4" CURL_FORMAT_CURL_OFF_T "P", bytes/ONE_PETABYTE);

    /* 16384 petabytes (16 exabytes) is the maximum a 64 bit unsigned number
       can hold, but our data type is signed so 8192PB will be the maximum. */

#else

  else
    msnprintf(max5, 6, "%4" CURL_FORMAT_CURL_OFF_T "M", bytes/ONE_MEGABYTE);

#endif

  max5[5] = 0;
  return max5;
}

struct progress {
  CURL *curl;
  curl_off_t lastshow;
  curl_off_t current_speed;
#define CURR_TIME (5 + 1) /* 6 entries for 5 seconds */
  curl_off_t speeder[CURR_TIME];
  curl_off_t speeder_time[CURR_TIME];
  int speeder_c;
};

int progress_callback(void *clientp, curl_off_t dltotal, curl_off_t dlnow,
                      curl_off_t ultotal, curl_off_t ulnow)
{
  struct progress *progress = (struct progress *)clientp;
  double timespent_dbl;     /* total time in seconds and fraction */
  curl_off_t timespent;     /* total time in seconds */
  curl_off_t timespent_ms;  /* total time in milliseconds */

  (void)dltotal;
  (void)ultotal;

  /* Get the total time in seconds, since the start of the transfer.
     CURLINFO_TOTAL_TIME returns a double that is seconds and fraction.
     CURLINFO_TOTAL_TIME_T returns an integer type that is microseconds.
     The latter isn't in old versions of curl so it's not used here. */
  if(curl_easy_getinfo(progress->curl, CURLINFO_TOTAL_TIME, &timespent_dbl))
    return 1;
  timespent = (curl_off_t)timespent_dbl;
  timespent_ms = (curl_off_t)(timespent_dbl * 1000);

  /* Calculate the current transfer speed at most once a second, and show it.
   *
   * This was copied from libcurl's progress_calc() in lib/progress.c (7.68.0).
   * Some minor modifications were made for type, timestamp and rate output.
   */
  if(progress->lastshow != timespent) {
    int countindex; /* amount of seconds stored in the speeder array */
    int nowindex = progress->speeder_c % CURR_TIME;
    char max5[6];

    progress->lastshow = timespent;

    /* Let's do the "current speed" thing, with the dl + ul speeds
       combined. Store the speed at entry 'nowindex'. */
    progress->speeder[nowindex] = dlnow + ulnow;

    /* remember the exact time for this moment */
    progress->speeder_time[nowindex] = timespent_ms;

    /* advance our speeder_c counter, which is increased every time we get
       here and we expect it to never wrap as 2^32 is a lot of seconds! */
    progress->speeder_c++;

    /* figure out how many index entries of data we have stored in our speeder
       array. With N_ENTRIES filled in, we have about N_ENTRIES-1 seconds of
       transfer. Imagine, after one second we have filled in two entries,
       after two seconds we've filled in three entries etc. */
    countindex = ((progress->speeder_c >= CURR_TIME) ?
                  CURR_TIME : progress->speeder_c) - 1;

    /* first of all, we don't do this if there's no counted seconds yet */
    if(countindex) {
      int checkindex;
      curl_off_t span_ms;

      /* Get the index position to compare with the 'nowindex' position.
         Get the oldest entry possible. While we have less than CURR_TIME
         entries, the first entry will remain the oldest. */
      checkindex = ((progress->speeder_c >= CURR_TIME) ?
                    progress->speeder_c % CURR_TIME : 0);

      /* Figure out the exact time for the time span */
      span_ms = timespent_ms - progress->speeder_time[checkindex];
      if(0 == span_ms)
        span_ms = 1; /* at least one millisecond MUST have passed */

      /* Calculate the average speed the last 'span_ms' milliseconds */
      {
        curl_off_t amount = progress->speeder[nowindex] -
                            progress->speeder[checkindex];

        if(amount > CURL_OFF_T_C(4294967) /* 0xffffffff/1000 */)
          /* the 'amount' value is bigger than would fit in 32 bits if
             multiplied with 1000, so we use the double math for this */
          progress->current_speed = (curl_off_t)
            ((double)amount/((double)span_ms/1000.0));
        else
          /* the 'amount' value is small enough to fit within 32 bits even
             when multiplied with 1000 */
          progress->current_speed = amount*CURL_OFF_T_C(1000)/span_ms;
      }
    }
    else {
      /* the first second we use the average */
      double ulspeed, dlspeed;

      if(curl_easy_getinfo(progress->curl, CURLINFO_SPEED_UPLOAD, &ulspeed) ||
         curl_easy_getinfo(progress->curl, CURLINFO_SPEED_DOWNLOAD, &dlspeed))
        return 1;

      progress->current_speed = (curl_off_t)(ulspeed + dlspeed);
    }

    fprintf(stderr, "\rCurrent speed: %s",
            max5data(progress->current_speed, max5));
  }

  return 0;
}

size_t write_callback(char *ptr, size_t size, size_t nmemb, void *userdata)
{
  (void)ptr;
  (void)userdata;
  return (size * nmemb);
}

int main(int argc, char *argv[])
{
  CURLcode result = CURLE_OK;
  struct progress progress;
  CURL *curl;

  (void)argv;
  (void)argc;

  result = curl_global_init(CURL_GLOBAL_ALL);
  if(result) {
    fprintf(stderr, "Error: libcurl: (%d) %s\n", result,
            curl_easy_strerror(result));
    return EXIT_FAILURE;
  }

  if(atexit(curl_global_cleanup)) {
    fprintf(stderr, "Error: atexit failed to register curl_global_cleanup.\n");
    return EXIT_FAILURE;
  }

  curl = curl_easy_init();
  if(!curl) {
    fprintf(stderr, "Error: libcurl: curl_easy_init failed.\n");
    return EXIT_FAILURE;
  }

  curl_easy_setopt(curl, CURLOPT_CAINFO, "curl-ca-bundle.crt");
  curl_easy_setopt(curl, CURLOPT_URL,
    "https://ny2.testmy.net/b/download?special=1&tt=1&st=st&nfw=1&s=200MB");
  curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 0L);
  curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_callback);
  curl_easy_setopt(curl, CURLOPT_XFERINFOFUNCTION, progress_callback);
  curl_easy_setopt(curl, CURLOPT_XFERINFODATA, &progress);

  memset(&progress, 0, sizeof(progress));
  progress.curl = curl;
  result = curl_easy_perform(curl);
  if(progress.lastshow)
    printf("\n");

  if(result) {
    fprintf(stderr, "Error: libcurl: (%d) %s\n", result,
            curl_easy_strerror(result));
    return EXIT_FAILURE;
  }

  return EXIT_SUCCESS;
}