int progress_callback(void *clientp,
This function gets called by libcurl instead of its internal equivalent with a frequent interval. While data is being transferred it will be called very frequently, and during slow periods like when nothing is being transferred it can slow down to about one call per second.
clientp is the pointer set with CURLOPT_XFERINFODATA(3), it is not used by libcurl but is only passed along from the application to the callback.
The callback gets told how much data libcurl will transfer and has transferred, in number of bytes. dltotal is the total number of bytes libcurl expects to download in this transfer. dlnow is the number of bytes downloaded so far. ultotal is the total number of bytes libcurl expects to upload in this transfer. ulnow is the number of bytes uploaded so far.
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.
If your callback function returns CURL_PROGRESSFUNC_CONTINUE it will cause libcurl to continue executing the default progress function.
Returning any other non-zero value from this callback will cause libcurl to abort the transfer and return CURLE_ABORTED_BY_CALLBACK.
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(3) must be set to 0 to make this function actually get called.