Subroutine layout in C

[Eric S. Raymond]

In article <2800002 at uxg.cso.uiuc.edu>, phil at uxg.cso.uiuc.edu writes:
> I want to write a subroutine in C, called S.  I want S to be known outside.
> I also want to have two subroutines X and Y to be known ONLY to S (not known
> outside of S).  Either can be called by S, and each calls the other in a
> recursive way.  I also need to share several variables entirely within
> this context (shared between S, X, Y).  They can be static.  There will
> only be 1 instance of S (and therefore also of X and Y, but that should
> be hidden).  Main program M should be able to call S, but any references
> to X and Y will not be resolved by the module S.
> 
> How do I lay out the arrangement of source for S?  An example would be
> appreciated.  Thanks.

Let xreturn, yreturn and sreturn be the return types of X, Y and S. Then the
simplest way to accomplish this is:

static int sharevar1;
static char *sharevar2;

static xreturn X()
{
     yreturn Y();	/* this is the trick; forward-declare Y() */

    /* text of X */
}

static yreturn Y()
{
    /* text of Y */
}

sreturn S()
{
    /* text of S */
}

You might want to put S() up front and forward-declare X() and Y(). That would
look like:

sreturn S()
{
     xreturn X();
     yreturn Y();

    /* text of S */
}

I use a #define forward /*empty*/ so I can write:

sreturn S()
{
     forward xreturn X();
     forward yreturn Y();

    /* text of S */
}

Be aware that there is some theological variation among C compilers on whether
forwards in functions remain in effect for only function scope until the end of
the module file. ANSI C, I believe, assumes the former, and writing as if the
former were true does no harm on compilers that assume the latter.

I gather you're a novice. Congratulations on having learned concern for scope
and visibility issues early. Too many people never break the bad habit C
encourages of leaving their functions and data promiscuously exposed.
-- 
      Eric S. Raymond                     (the mad mastermind of TMN-Netnews)
      Email: eric at snark.uu.net                       CompuServe: [72037,2306]
      Post: 22 S. Warren Avenue, Malvern, PA 19355      Phone: (215)-296-5718