Question

I have to trasfer files using FTP protocol and libcurl in c. It works fine, but I have some problems.

1) If a transfer is started, but at a certain point I disconnect from the network, my program remains in the libcurl function, the speed goes to 0 and I can’t do anything else. I tried to set timeout (CURLOPT_TIMEOUT), but it’s just a timeout on the transfer time.

2) The second problem I have, which is linked to the first one is, how can I know if the trasfer if successfully completed?

My trasfer code is:

struct FtpFile {

  const char *filename;
  FILE *stream;
};
   long int size;

static size_t my_fwrite(void *buffer, size_t size, size_t nmemb, void *stream)
{
  struct FtpFile *out=(struct FtpFile *)stream;
  if(out && !out->stream) {
    /* open file for writing */
    out->stream=fopen(out->filename, "ab"); 
    if(!out->stream)
      return -1; /* failure, can't open file to write */
  }
  return fwrite(buffer, size, nmemb, out->stream);
}

int sz;

int main(void)
{
  CURL *curl;
  CURLcode res;
  char prova;

   struct stat statbuf;
   FILE *stream;

   /* apro il file per leggere la dimensione*/
   if ((stream = fopen("Pdf.pdf", "rb")) 
       == NULL)
   {
      fprintf(stderr, "Nessun file da aprire, il download partirà da zero.\n");
   }
   else
   {
   /* Ricevo informazioni sul file */
   fstat(fileno(stream), &statbuf);
   fclose(stream);
   size = statbuf.st_size;
   printf("Dimensione del file in byte: %ld\n", size);
   }

  struct FtpFile ftpfile={
    "Pdf.pdf", /* name to store the file as if succesful */
    NULL
  };

  curl_global_init(CURL_GLOBAL_DEFAULT);

  curl = curl_easy_init();
  if(curl) {

    curl_easy_setopt(curl, CURLOPT_URL, "ftp://....");
    /* Define our callback to get called when there's data to be written */
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, my_fwrite);
    /* Set a pointer to our struct to pass to the callback */
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, &ftpfile);

    /* Switch on full protocol/debug output */
    curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L); 

    /*Pass a long as parameter. It contains the offset in number of bytes that you want the transfer to start from.   
    Set this option to 0 to make the transfer start from the beginning (effectively disabling resume). For FTP, set  
    this option to -1 to make the transfer start from the end of the target file (useful to continue an interrupted 
    upload).

    When doing uploads with FTP, the resume position is where in the local/source file libcurl should try to resume 
    the upload from and it will then append the source file to the remote target file. */

    if(stream == NULL)
    {
        curl_easy_setopt(curl, CURLOPT_RESUME_FROM, 0);
    }
    else
    {
        curl_easy_setopt(curl, CURLOPT_RESUME_FROM, size);
    }

    /*Used to show file progress*/
    curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 0);

    res = curl_easy_perform(curl);

    /* always cleanup */
    curl_easy_cleanup(curl);

    if(CURLE_OK != res) {
      /* we failed */
      fprintf(stderr, "curl told us %d\n", res);
    }
  }

  if(ftpfile.stream)
    fclose(ftpfile.stream); /* close the local file */

  curl_global_cleanup();

  return 0;
}

Answer 1

I have recently answered a similar question to this.

1) this is by design. if you want to timeout the connection, try using these instead:

curl_easy_setopt(curl, CURLOPT_LOW_SPEED_LIMIT, dl_lowspeed_bytes);    
curl_easy_setopt(curl, CURLOPT_LOW_SPEED_TIME, dl_lowspeed_time);

If your download rate falls below your desired threshold, you can check the connectivity & take whatever action you see fit.

NB: Added in 7.25.0: CURLOPT_TCP_KEEPALIVE, CURLOPT_TCP_KEEPIDLE

so these might be another suitable alternative for you.

2) like this:

curl_easy_getinfo(curl, CURLINFO_CONTENT_LENGTH_DOWNLOAD, &dl_bytes_remaining);
curl_easy_getinfo(curl, CURLINFO_SIZE_DOWNLOAD, &dl_bytes_received);
if (dl_bytes_remaining == dl_bytes_received)
    printf("our work here is done ;)\n");