comma operator

[John Plocher]

In <918 at helios.toronto.edu> dooley at helios.physics.utoronto.ca (Kevin Dooley) writes:
>Can anybody tell me if there is a usage where the comma is 'better'.

The only place I ever had a real use for the comma operator was when working with
a 3rd party C-ISAM library which had many functions and a large set of error returns.

Call the lib functions copen(), cclose(), cinsert(), cfind()...
These routines would return a value X, where on success X > 0 and was then
used as a magic cookie.  If X < 0 then there were 15 or so error conditions
that could have happened - all but 4 or 5 because of stupid programmer errors
(The routines used structures that HAD to be initialized *just so* &$&%#$%# ).

Output of all this is like:

Trace: copen(structure, FLAG|FLAG, "Filename", blah) => DB_NOTFOUND (File not found)
Trace: ccreate(structure,...) => DB_OK

I actually used __LINE__ and __FILE__ in the ERROR() macro so the output was
more useful still.

    -John Plocher

------------------------------------------------------------------------------------

#ifdef DEBUG

int ERROR_CC;

log(funct, string)
char *funct, *string;
{
	fprintf(stderr,"Trace: %s => %s\n", funct,string);
}

char *errorname( error_cc )
int error_cc;
{
	if (error_cc > 0) return itoa(error_cc);
	else if (error_cc == DB_OK)       return "DB_OK";
	else if (error_cc == DB_NOTFOUND) return "DB_NOTFOUND (File not found)";
	else if (error_cc == DB_BADSTRUCT)return "DB_BADSTRUCT (Invalid DB Struct)";
	else if ...
}

/* Relies on NON-ANSI C Preprocessor behavior ! */

#define ERROR(funct)	(log("funct", errorname(ERROR_CC=funct)), ERROR_CC)
/*
** Call funct() and assign the ret val to ERROR_CC
** Send this value to errorname() so we get an english description of the error
** Send the (string representation) of the function and this english to log()
** Finally, give the poor user the ret value we saved in ERROR_CC
*/

#else	/* not DEBUG */

#define ERROR(funct)	funct

#endif	/* if DEBUG else endif */

main() {
int dbfd;

	init( &structure );
	dbfd = ERROR(copen(structure, FLAG|FLAG, "Filename", blah));
	if (dbfd == DB_NOTFOUND)
	    dbfd = ERROR(ccreate(structure,...));
	if (dbfd < 0)
	    bomb();
}