| isfd | isam file identifier |
| kdsc | pointer to dictinfo or key description structure |
| idx | zero for dictinfo, otherwise index number |
SUCCESS, or a negative value on error, in which case the error code is found in iserrno.
this call serves two somewhat different purposes.
if idx is zero, then it is actually expecting a pointer to a dictinfo struct, which will then be loaded with information about the current file. this is a C-ISAM inherited oddness, please see isisaminfo for a somewhat tidier method.
if idx is non zero then isindexinfo will return the specified key description, where a value of 1 represents the primary, followed by each successive index in the file.