curl / libcurl / API / curl_easy_setopt / CURLOPT_XFERINFOFUNCTION

CURLOPT_XFERINFOFUNCTION explained

Name

CURLOPT_XFERINFOFUNCTION - progress meter callback

Synopsis

#include <curl/curl.h>
 
int progress_callback(void *clientp,
                      curl_off_t dltotal,
                      curl_off_t dlnow,
                      curl_off_t ultotal,
                      curl_off_t ulnow);
 
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_XFERINFOFUNCTION,
                          progress_callback);

Description

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 gets called 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, 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 is about to transfer and has already 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 are set to zero (like if you only download data, the upload size remains 0). Many times the callback is called one or more times first, before it knows the data sizes so a program must be made to handle that.

Return zero from the callback if everything is fine.

Return 1 from this callback to make libcurl abort the transfer and return CURLE_ABORTED_BY_CALLBACK.

If your callback function returns CURL_PROGRESSFUNC_CONTINUE it makes libcurl to continue executing the default progress function.

If you transfer data with the multi interface, this function is not 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 get called.

Default

NULL - use the internal progress meter. That is rarely wanted by users.

Protocols

This functionality affects all supported protocols

Example

struct progress {
  char *private;
  size_t size;
};
 
static size_t progress_callback(void *clientp,
                                curl_off_t dltotal,
                                curl_off_t dlnow,
                                curl_off_t ultotal,
                                curl_off_t ulnow)
{
  struct progress *memory = clientp;
  printf("my ptr: %p\n", memory->private);
 
  /* use the values */
 
  return 0; /* all is good */
}
 
int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    struct progress data;
 
    /* pass struct to callback  */
    curl_easy_setopt(curl, CURLOPT_XFERINFODATA, &data);
 
    /* enable progress callback getting called */
    curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 0L);
 
    curl_easy_setopt(curl, CURLOPT_XFERINFOFUNCTION, progress_callback);
  }
}

Availability

Added in curl 7.32.0

Return value

Returns CURLE_OK.