datno - the data file number. In V12 the file number typedef was formally changed from COUNT, a two-byte value to FILNO, a four-byte value. Refer to this link for compatibility details. Four Byte File Numbering
bufptr - pointer to a buffer where to store the definition (after successful return, it can be cast to A_STFIELDS)
bufsiz - size of the bufptr.
Return
The size of bufptr actually used.
uerr_cod needs to be checked if an error condition occurred.
A return value of 0 with uerr_cod 0 indicates that no field has been defined.
uerr_cod set to VBSZ_ERR indicates bufsiz is too small. Return value contains required size.
