curl_easy_setopt.3: expand the PROGRESSFUNCTION section

function(void *clientp, double dltotal, double dlnow, double ultotal, double

ulnow); \fP. This function gets called by libcurl instead of its internal

equivalent with a frequent interval during operation (roughly once per second

or sooner) no matter if data is being transferred or not.  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). Returning a non-zero value from

this callback will cause libcurl to abort the transfer and return

\fICURLE_ABORTED_BY_CALLBACK\fP.

or sooner) no matter if data is being transferred or not.

\fIclientp\fP is the pointer set with \fICURLOPT_PROGRESSDATA\fP, it is not

actually 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. \fIdltotal\fP is the total number of bytes

libcurl expects to download in this transfer. \fIdlnow\fP is the number of

bytes downloaded so far. \fIultotal\fP is the total number of bytes libcurl

expects to upload in this transfer. \fIulnow\fP 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).

Returning a non-zero value from this callback will cause libcurl to abort the

transfer and return \fICURLE_ABORTED_BY_CALLBACK\fP.

If you transfer data with the multi interface, this function will not be

called during periods of idleness unless you call the appropriate libcurl