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From: vinoski@apollo.HP.COM (Stephen Vinoski)
Newsgroups: comp.sys.apollo
Subject: Re: function prototype mystery?
Message-ID: <5102cefd.20b6d@apollo.HP.COM>
Date: 16 Apr 91 17:28:00 GMT
References: <1991Apr11.164655.17092@milton.u.washington.edu>
Sender: root@apollo.HP.COM
Reply-To: vinoski@apollo.HP.COM (Stephen Vinoski)
Organization: Hewlett-Packard Apollo Division - Chelmsford, MA
Lines: 67

In article <1991Apr11.164655.17092@milton.u.washington.edu> etb@milton.u.washington.edu (Eric Bushnell) writes:
>I'm using the apollo system call proc2_$get_info(),
>which is defined in proc2.h as:
>
>void proc2_$get_info(
>        uid_$t          &p2_uid,
>        proc2_$info_t   *info,
>        pinteger        &info_buf_len,  /* size of info buffer (bytes) */
>        status_$t       *sts
>);
>
>What do the &'s do to the first and third arguments?

That indicates that the arguments are to be passed by reference.  This
is a Domain/C extension borrowed from C++.  Domain/Pascal passes
arguments by reference by default, and this extension allows C and
Pascal to interface very easily, much easier than with the std_$call()
stuff from the pre-sr10 days.

>It looks to me like the function expects addresses, but if
>I pass addresses to it, the compiler (cc 6.7.m, OS10.3) complains
>that the argument type conflicts with the declaration.

You don't need to pass addresses.  The compiler will take care of
everything for you.  For instance, given this function:

  void incr(int &x)
  {
    x++;
  }

This function can be called like so:

  int j = 5;
  incr(j);

The variable j will now contain the value 6.  This can be done with
pointers:

  void incr(int *x)
  {
    (*x)++;
  }

but this version of incr() must be called like this:

  int j = 5;
  incr(&j);

Note, however, that unlike C++, Domain/C will not generate a temporary
in the case where a constant is passed by reference, so the following
will cause an access violation:

  incr(5);

A constant can safely be passed by reference if the function does not
modify its argument.

All of this is covered in section 5.3.2 of the Domain C Language
Reference (002093-A00) (that's where this example came from).


-steve
| Steve Vinoski  (508)256-0176 x5904       | Internet: vinoski@apollo.hp.com  |
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