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CURLOPT_XFERINFOFUNCTION(3) curl_easy_setopt options
CURLOPT_XFERINFOFUNCTION - callback to progress meter function
int progress_callback(void *clientp,
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_XFERINFOFUNCTION,
Pass a pointer to your callback function, which should match the
prototype shown above.
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
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
By default, libcurl has an internal progress meter. That's rarely
wanted by users.
Added in 7.32.0. This callback replaces CURLOPT_PROGRESSFUNCTION(3)
libcurl 7.76.0 November 4, 2020 CURLOPT_XFERINFOFUNCTION(3)