dfGetExtraDataSize provides extracting of the size of additional data which was saved in df-file when it was created.
int dfGetExtraDataSize (
Special df-file descriptor, created by dfOpenDfFile function.
Pointer on the variable where the requested value is placed.
Can not be NULL.
If the function succeeds, the return value is zero (EDF_SUCCESS).
If the function fails, the return value is one of the following:
If df-file does not contain additional data then function returns EDF_SUCCESS and *pnExDataSize is "0".