Re: [patch] limit and document unnamed fields.

[DJ Delorie <dj at redhat dot com>]

> I shouldn't have thought it would be more than a few lines of code (a
> couple of recursive functions) to make these recursive checks.

I agree, but that's still more difficult than just documenting that
it's undefined ;-) I figure it would involve one recursive function
and some sort of hash to keep track of what field names we've seen in
the current "scope".

> OK, send the revised patch that keeps the test in grokfield but makes the
> documentation say the case of duplicate names is undefined and I'll review
> it.

2001-10-03  DJ Delorie  <dj@redhat.com>

	* c-decl.c (grokfield): Make sure the only unnamed fields
	we're allowing are either structs or unions.
	* doc/extend.texi: Add documentation for the unnamed field
	extension.

	* gcc.dg/20011008-1.c: New.
	* gcc.dg/20011008-2.c: New.

Index: c-decl.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/c-decl.c,v
retrieving revision 1.250
diff -p -3 -r1.250 c-decl.c
*** c-decl.c	2001/10/07 18:02:43	1.250
--- c-decl.c	2001/10/08 18:39:55
*************** grokfield (filename, line, declarator, d
*** 5366,5371 ****
--- 5366,5383 ----
  {
    tree value;
  
+   if (declarator == NULL_TREE && width == NULL_TREE)
+     {
+       /* This is an unnamed decl.  We only support unnamed
+ 	 structs/unions, so check for other things and refuse them.  */
+       if (TREE_CODE (TREE_VALUE (declspecs)) != RECORD_TYPE
+ 	  && TREE_CODE (TREE_VALUE (declspecs)) != UNION_TYPE)
+ 	{
+ 	  error ("unnamed fields of type other than struct or union are not allowed");
+ 	  return NULL_TREE;
+ 	}
+     }
+ 
    value = grokdeclarator (declarator, declspecs, width ? BITFIELD : FIELD, 0);
  
    finish_decl (value, NULL_TREE, NULL_TREE);
Index: doc/extend.texi
===================================================================
RCS file: /cvs/gcc/gcc/gcc/doc/extend.texi,v
retrieving revision 1.27
diff -p -3 -r1.27 extend.texi
*** extend.texi	2001/10/02 23:15:55	1.27
--- extend.texi	2001/10/08 18:40:40
*************** extensions, accepted by GCC in C89 mode 
*** 433,438 ****
--- 433,439 ----
  * Vector Extensions::   Using vector instructions through built-in functions.
  * Other Builtins::      Other built-in functions.
  * Pragmas::             Pragmas accepted by GCC.
+ * Unnamed Fields::      Unnamed struct/union fields within structs/unions.
  @end menu
  @end ifset
  @ifclear INTERNALS
*************** produce warnings for the listed variable
*** 4502,4507 ****
--- 4503,4549 ----
  that of the @code{unused} attribute, except that this pragma may appear
  anywhere within the variables' scopes.
  @end table
+ 
+ @node Unnamed Fields
+ @section Unnamed struct/union fields within structs/unions.
+ @cindex struct
+ @cindex union
+ 
+ For compatibility with other compilers, GCC allows you to define
+ a structure or union that contains, as fields, structures and unions
+ without names.  For example:
+ 
+ @example
+ struct @{
+   int a;
+   union @{
+     int b;
+     float c;
+   @};
+   int d;
+ @} foo;
+ @end example
+ 
+ In this example, the user would be able to access members of the unnamed
+ union with code like @samp{foo.b}.  Note that only unnamed structs and
+ unions are allowed, you may not have, for example, an unnamed
+ @code{int}.
+ 
+ You must never create such structures that cause ambiguous field definitions.
+ For example, this structure:
+ 
+ @example
+ struct @{
+   int a;
+   struct @{
+     int a;
+   @};
+ @} foo;
+ @end example
+ 
+ It is ambiguous which @code{a} is being referred to with @samp{foo.a}.
+ Such constructs are not supported and must be avoided.  In the future,
+ such constructs may be detected and treated as compilation errors.
  
  @node C++ Extensions
  @chapter Extensions to the C++ Language

----------------------------------------
/* { dg-do compile } */
/* { dg-options "-O0" } */

struct { int; int q; } a; /* { dg-error "Unnamed" } */
struct { union {int x;}; int q; } b;
struct { struct {int x;}; int q; } c;
union { union {int x;}; int q; } d;
union { struct {int x;}; int q; } e;

----------------------------------------
/* { dg-do run } */
/* { dg-options "-O0" } */

struct { union {int x; int y;}; int q; } b;
union { struct {int x;}; int q; } e;

main()
{
  b.y = 10;
  b.x = 15;
  if (b.y != 15)
    abort();

  e.x = 10;
  e.q = 15;
  if (e.x != 15)
    abort();

  exit(0);
}