Many libcurl functions return a CURLcode. That is a special libcurl typedefed
variable for error codes. It returns CURLE_OK (which has the value zero) if
everything is fine and dandy and it returns a non-zero number if a problem was
detected. There are almost one hundred CURLcode errors in use, and you can
find them all in the curl/curl.h header file and documented in the
libcurl-errors man page.
You can convert a CURLcode into a human readable string with the
curl_easy_strerror() function—but be aware that these errors are rarely
phrased in a way that is suitable for anyone to expose in a UI or to an end
user:
const char *str = curl_easy_strerror( error );
printf("libcurl said %s\n", str);
Another way to get a slightly better error text in case of errors is to set
the CURLOPT_ERRORBUFFER option to point out a buffer in your program and
then libcurl stores a related error message there before it returns an error:
char error[CURL_ERROR_SIZE]; /* needs to be at least this big */
CURLcode ret = curl_easy_setopt(handle, CURLOPT_ERRORBUFFER, error);