NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/std/c/178

Text File | 1996-08-06 | 3.0 KB | 80 lines

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From: [e-mail] (Tanmoy Bhattacharya)
Newsgroups: comp.lang.c.moderated,comp.std.c
Subject: Re: Function returning structure
Date: 24 Jan 1996 11:07:12 -0600
Organization: Los Alamos National Laboratory
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References: <4e2ki8$[e-mail]>
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--text follows this line--

Note cross-post to comp.std.c

In article <4e2ki8$[e-mail]> [e-mail] (Bob
Nelson) writes:
<snip>
   /*
   Consider the following code -- in which a structure member is of type
   array of char and an instance of that structure is returned from a
   a function.

   The Standard [6.3.2.3] says that the dot operator designates a member
   of a structure. (It goes on to add -- in its example text -- that f().x
   is a valid postfix expression though not an lvalue, presuming that
   f is a function returning a structure and that x is member).

   With that in mind -- does the standard have anything that might
   disqualify member selection (in a non-assignment context) if that
   member is of type array and that structure is returned by a function?

There is nothing wrong selecting that element. However, this element
does not decay to a pointer to its first element. As a result, very
few things can be done to it, except casting it to type void. In
particular, one cannot index it with the [], assign it to a pointer
variable, or pass it to printf or scanf.

   One current, popular compiler fails to compile this code -- diagnosing
   an "invalid use of non-lvalue array".
   */
<snip>
       printf("%s\n", func().c);

begin comp.std.c question {
  Is the following strictly conformant?

  #include <stdarg.h>
  #include <stdio.h>
  typedef struct a {char b[5];} a;
  typedef char c[5];
  int f(char *fmt, ...) { char *s;
                          va_list ap; va_start(ap,x);
                          s = va_arg(ap, c); va_end(ap);
                          return printf(fmt,s); }
  int F(char *fmt, ...) { return 0; }
  a g(void) { static a y; return y; }
  int main(void) { f ( "%s", g().b ); return 0; }

  i.e. can non-lvalue arrays be passed to a function and accessed by
  the stdarg macros there? I do not think so, because the `return value'
  of va_arg is not an lvalue, and hence does not decay to a pointer if
  it has an array type.

  But what if one replaces f by F in main? i.e. can a non-lvalue array
  be passed to a vararg function at all? There seems to be no
  prohibition against that.
}

Cheers
Tanmoy
--
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