Descriptor is the abstraction of the UNIX file descriptor.
public int descriptor;
void close;
void dealloc;
descriptor if it is not -1.
id init;
[self init -1] to avoid ever closing file descriptor 0 by
accident.
protected id init int fd;
fd as the descriptor.
int type-of-file;
FILE_TYPE_* constants for the receiving File.
Signal a file_error and return FILE_TYPE_NONEXISTENT if not open.