Set error buffer for error messages
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_ERRORBUFFER, char *buf);
Pass a char * to a buffer that the libcurl may store human readable error messages in on failures or problems. This may be more helpful than just the return code from curl_easy_perform(3) and related functions. The buffer must be at least CURL_ERROR_SIZE bytes big.
You must keep the associated buffer available until libcurl no longer needs it. Failing to do so will cause very odd behavior or even crashes. libcurl will need it until you call curl_easy_cleanup(3) or you set the same option again to use a different pointer.
If the library does not return an error, the buffer may not have been touched. Do not rely on the contents in those cases.