Echelon Neuron C - Manual (Page 247)

Neuron C Programmer’s Guide 235
As for the I/O operations on the display, assume that the display has a Neurowire
device interface. The custom system image could be accompanied with an include
file for the benefit of whatever Neuron C application program uses it. The
include file can contain the Neurowire I/O declarations and the function
definitions necessary to support the display’s I/O. It can even contain a routine
to be called from the application's when(reset) task that initializes the
appropriate function pointers and memory buffer pointers. Then, the custom
system image can access the functions and memory buffers as necessary without
further intervention from the application program, and without the application
program’s being responsible for passing some special parameters each time it
wants to interact with the custom system image display management software.
This indirection through function pointers is necessary for a custom system
image, because a custom system image cannot contain any unresolved external
symbols when the linker creates it. However, a library of display management
software could just access such I/O functions as discussed above directly by name,
without having to resort to function pointers.