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From: I1090801%DBSTU1.BITNET@CUNYVM.CUNY.EDU
Newsgroups: comp.lang.c
Subject: Check for function results
Message-ID: <10530@brl-adm.ARPA>
Date: Tue, 24-Nov-87 23:49:14 EST
Article-I.D.: brl-adm.10530
Posted: Tue Nov 24 23:49:14 1987
Date-Received: Sat, 28-Nov-87 07:49:20 EST
Sender: news@brl-adm.ARPA
Lines: 35

I am a student of computer science and I have to write a C-compiler.
The problem I came across is: does a C-compiler have to generate code
to check if a function really delivers a result in all cases?
Example: In the following (nonsense) function
   int f()
   {
        int a;
        a = 1;              /* only to set a to a non-garbage value */
        if (...)
             return a;
        else
             ...            /* statements without any return */
   }
a value is delivered in the then part but in the else part nothing
happens and the program just runs out of the end of the function.
So, in this case, some garbage is delivered. In this example you can
easily see this, but things get worse if the function is more
complicated. I think it isn't easy for a compiler to see all this
at compile-time, so things should be checked during runtime of a
program. My idea is to generate code that sets an internal flag
to FALSE at the start of the function. Each return(expression) should
set this flag to TRUE and then jump to the end of the function where
the return is handled. Here the flag should be checked and an error-
message be generated if it is still FALSE. This way it can be deteced
if the way through the function passed a return(expression).
I wonder if all this is really necessary, but I can't find anything
about this problem in K&R. Do other compilers check this? I think,
some strange errors can be detected by this method.

Maybe someone on the net can help me.

                                      Ulf Gruene
                                I1090801@DBSTU1.BITNET
                         Technische Universitaet Braunschweig
                                     West Germany