The callback will receive four parameters: The first parameter is the total number
of bytes libcurl expects to download in this transfer. The second parameter is the number of
bytes downloaded so far. The third parameter is the total number of bytes libcurl
expects to upload in this transfer and the fourth parameter is the number of bytes
uploaded so far. If you pass the optional
userdata argument, the value you pass in
userdata will be passed to your callback function as the fifth parameter. The
parameter can be of any type.
Unknown/unused argument values passed to the callback will be set to zero (like if you only download data, the upload size will remain 0). Many times the callback will be called one or more times first, before it knows the data sizes so a program must be made to handle that.
Returning a non-zero value from this callback will cause libcurl to abort the
transfer and return
If you transfer data with the multi interface, this function will not be called during periods of idleness unless you call the appropriate libcurl function that performs transfers.
#CURLOPT_NOPROGRESS must be set to 0 to make this function actually