[Sdcc-user] libraries and global variables

[Lawrence J. <law...@jt...>]

This is possibly more of a programming question than an SDCC question, 
but I'm not sure, so I'll start here:

With sdcc created libraries that use global variables, such as circular 
buffers for serial ports, presumably the variables must be declared 
global in the library source, or the source of one of the library 
functions.  These same variables must be declared as extern ... in every 
other place they're used, such as an application that may need access.

I mentioned serial ports above because that's what I'm currently fiddling 
around with on an AT89C2051.  The serial port functions are based on 
those of Wolfgang Esslinger, as mentioned in the sdcc manual.  My first 
question, is that in looking into the contents of _ser.c, there is a bit 
variable:

static bit ser_txBusy;

This is declared outside of any function, hence it is global, I think.  
If this is the case, my question is: Why?

I looked through some old programming documentation on storage classes 
and it isn't ever stated that you can't do this, but this may be because 
nobody would ever want to, unless there's a reason I don't see.

When I was trying to link in functions from a library based on _ser.c, I 
kept getting this stubborn warning about ser_txBusy being declared as 
global and not being defined anywhere, even if I put the declaration as a 
global in my application (meaning right above main).  After a while, I 
started to question why this needs to be static, so I changed the 
declaration to simply read:

bit ser_txBusy;

...and it now compiles, well sort of.  The next problem that cropped 
might be related to sdcc.

The block of variables is as follows:

unsigned char ser_txIndexIn;
unsigned char ser_txIndexOut;
unsigned char ser_rxIndexIn;
unsigned char ser_rxIndexOut;

unsigned char ser_txBuffer[TXBUF];
unsigned char ser_rxBuffer[RXBUF];

static bit ser_txBusy;

What I did, initially, was create discrete functions one per file, with 
the file names identical to the functions.

(John McClintock:  I was already well into this before you responded to 
my post of yesterday, so it was too late to turn back.)

In each file, I declared the above block as: 

extern unsigned char ser_txIndexIn;
extern unsigned char ser_txIndexOut;
extern unsigned char ser_rxIndexIn;
extern unsigned char ser_rxIndexOut;

extern unsigned char ser_txBuffer[TXBUF];
extern unsigned char ser_rxBuffer[RXBUF];

extern static bit ser_txBusy;

...in all files except for ser_init.c, which obviously is the 
initialisation routine.  In that file, I declared without extern.  When I 
tried to then compile a program with only the initialisation routine and 
interrupt handler, I would get a "Multiple definition of _ser_rxBuffer" 
followed by a crash.  It strikes me as a little strange that it wouldn't 
complain about any of the other variables.  I checked and rechecked this 
because I couldn't see where the error was coming from.  Eventually, I 
changed the above block of variables to all be extern in all functions 
and placed a global declaration in the header file for the would-be 
library.  The global declaration was wrapped within:

#ifndef _SERLIB
#define _SERLIB
<global variables go here>
#endif

and the compile step had a -D_SERLIB on it for building the library.  I, 
of course, don't put the -D_SERLIB on the command-line for building the 
application, so the block gets declared there.

Why didn't the global declaration in ser_init.c work?  Should it have 
worked at all?  This would have been much simpler in my books, and since 
you can't do anything without the initialisation step, you must end up 
declaring the global variables there, but not so.



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