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From: g-rh@cca.CCA.COM (Richard Harter)
Newsgroups: comp.lang.c
Subject: Another portable code question
Message-ID: <16673@cca.CCA.COM>
Date: Sat, 13-Jun-87 04:12:05 EDT
Article-I.D.: cca.16673
Posted: Sat Jun 13 04:12:05 1987
Date-Received: Sat, 20-Jun-87 19:35:58 EDT
Reply-To: g-rh@cca.UUCP (Richard Harter)
Distribution: world
Organization: Computer Corp. of America, Cambridge, MA
Lines: 52


	Here is one that has me puzzled.  We have a C program which
runs on a large variety of UNIX boxes (system V and bsd), VMS, and PRIMOS.
One of the files has code that looks like this:

extern int (*command_table[])();
.....
command_processor() {
....
int (*command)();
....
command = command_table[n];
....
(*command)(args);
....
}

	The declaration for command_table declares it as an array of
pointers to functions returning int.  (Confirmed by cdecl)  The declar-
ation for command declares it as a pointer to a function returning int.
We pick off the appropriate command and execute it.  Now, on a wide
variety of UNIX systems the line "(*command)(args);" may be written
as "(command)(args)".  We have observed:

(a)	On most UNIX machines (VAX BSD and Sun in particular) either
form is acceptable and will compile and execute correctly.  Moreover
lint will accept either form without complaint.  (VMS C also takes
either form without complaint or error.)

(b)	One vendor's compiler (I won't name it because I don't have the
log sheets at hand) gives a compilation warning for (*command)().
(This is a UNIX box.)

(c)	The PRIMOS C compiler rejects (command)(args) and insists
upon (*command)(args)

First Question:  Is the above code correct and legal C, or are we 
overlooking something subtle?

Second Question:  Why do both alternatives pass lint and execute
correctly on most machines?

Third Question:  Are we correct in assuming that the compiler for 
vendor X is broken?

Fourth Question:  We are strongly committed to portable code and would
like the code to be the same, in so far as possible, to be the same for
all operating systems that we deal with.  we have good reasons for using
a jump table.  What is the best way to code this with full portability?
-- 

Richard Harter, SMDS Inc. [Disclaimers not permitted by company policy.]