> Please can someone faimilar with C kindly help clear up my confusion
here?

Well I don't really claim familiarity with C, but have done a little bit
with it.

> I'm declaring a couple of SPI function prototypes, see below. Not
> wishing to waste too much time testing awkward things, shouldn't the
> common variables (sync_mode, bus_mode etc.) be specified more uniquely
> for each SPI channel? Will the following, although it compiles ok,
cause
> ambiguity when it comes to execution? I can't tell whether prototyping
a
> function is the same as declaring a variable by name, nor how it's
> allocating the names following "unsigned char".
> 
> Many thanks,
> Matt
> 
> 
> 
> void OpenSPI1
> (
> unsigned char sync_mode,
> unsigned char bus_mode,
> unsigned char smp_phase
> );
> unsigned char WriteSPI1(unsigned char data_out );
> 
> void OpenSPI2
> (
> unsigned char sync_mode,
> unsigned char bus_mode,
> unsigned char smp_phase
> );
> unsigned char WriteSPI2(unsigned char data_out );
> 
> void Open1USART
> (
> unsigned char config,
> unsigned int spbrg
> );

Note that the variables declared inside each function are not declared
'static' so each time the function gets called they are recreated on the
stack, and are unique to that function. So the variables inside OpenSPI1
are different variables to OpenSPI2, even though they have the same
name. The scope of each name is limited to the code inside that
function. The variables are cleared to all zeros on creation on the
stack if I understand the C standard correctly, but may need correction
here. If you attempt to look at the values of these variables with a
debugger such as MPLAB/ICD combination, you get strange results unless
you are single stepping through the code inside that function.

When the function terminates by executing a 'return' the variables are
removed from the stack, and no longer exist. At this stage your debugger
will again go strange with the values of these variables (IIRC it gives
an error message that says they are 'Out of Scope'). If the other
routine is now called the variables for that function are automatically
created on the stack, and are unique to that function, even if they
happened to reside at the same position in the stack.

If the variables are declared 'static' in addition to the other
attributes they have, then they are created in the normal RAM area, and
are persistent between calls to the function, i.e. they keep their last
value from the last time the function was called, or are uninitialized
before the first call of the function unless initialisation values are
declared. But they are still separate variables even though they have
the same name. I don't know how the debugger identifies which one you
wish to watch, I haven't had occasion to go down this road at this
stage.


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