int im_incheck( in )
int im_outcheck( out )
int im_iocheck( in, out)
IMAGE *in, *out;
IM_PARTIAL: the descriptor is magically turned into an IM_SETBUF descriptor. Memory is allocated and the image generated into that. The old partial callbacks are closed down, and the descriptor reformed as a IM_SETBUF.
IM_OPENOUT: if the descriptor has been written to, it is automatically `rewound,` that is, it is closed and reopened as an IM_MMAPIN descriptor.
IM_SETBUF: just checks that the descriptor has been written to.
See the manual page for im_setupout(3) for a skeleton program.
im_outcheck(3) checks that a descriptor is suitable for WIO output with im_writeline(3). If it sees an IM_PARTIAL image, it turns it magically into an IM_SETBUF image.
im_iocheck(3) simply calls in_incheck(3) for image in and im_outcheck(3) for image out.