Re: Minor doc updates.

[Zack Weinberg <zack at codesourcery dot com>]

On Thu, Aug 01, 2002 at 01:28:55PM -0400, Phil Edwards wrote:
> On Tue, Jul 30, 2002 at 08:50:56AM -0700, Zack Weinberg wrote:
> > We ought to be checking the return values here.  It is vanishingly
> > unlikely that time() will fail, but localtime() might not be able to
> > find its timezone database, and then we'll try to dereference a null
> > pointer.
> > 
> > "??:??:??" and "??? ?? ????" seem like reasonable failure-mode strings
> > to return.  Anyone have a better idea?
> 
> Those seem quite reasonable, as long as they're put into a FAQ or a manual.
> (Something that can be found in an online search, at least.)

What do you think of this patch?  I documented the strings, and I have
GCC issue a warning when time/localtime fail.  I've done cursory
testing by LD_PRELOAD-ing a fake time() that always fails:

$ cat test.c
__DATE__
__TIME__
__DATE__
__TIME__

$ LD_PRELOAD=./time-fails.so  ./cc1 -quiet -E -P test.c
test.c:1:1: warning: could not determine date and time: Bad address
"??? ?? ????"
"??:??:??"
"??? ?? ????"
"??:??:??"

	* cppmacro.c (_cpp_builtin_macro_text) [BT_TIME, BT_DATE]:
	Check for failing time()/localtime(), issue a warning, and
	make __TIME__ and __DATE__ expand to fallback strings.

	* doc/cpp.texi, doc/extend.texi: Document behavior of __DATE__
	and __TIME__ when the date and time cannot be determined.

===================================================================
Index: cppmacro.c
--- cppmacro.c	26 Jul 2002 16:29:31 -0000	1.119
+++ cppmacro.c	1 Aug 2002 19:12:29 -0000
@@ -207,17 +207,37 @@ _cpp_builtin_macro_text (pfile, node)
 	     storage.  We only do this once, and don't generate them
 	     at init time, because time() and localtime() are very
 	     slow on some systems.  */
-	  time_t tt = time (NULL);
-	  struct tm *tb = localtime (&tt);
+	  time_t tt;
+	  struct tm *tb = NULL;
 
-	  pfile->date = _cpp_unaligned_alloc (pfile,
-					      sizeof ("\"Oct 11 1347\""));
-	  sprintf ((char *) pfile->date, "\"%s %2d %4d\"",
-		   monthnames[tb->tm_mon], tb->tm_mday, tb->tm_year + 1900);
+	  /* (time_t) -1 is a legitimate value for "number of seconds
+	     since the Epoch", so we have to do a little dance to
+	     distinguish that from a genuine error.  */
+	  errno = 0;
+	  tt = time(NULL);
+	  if (tt != (time_t)-1 || errno == 0)
+	    tb = localtime (&tt);
 
-	  pfile->time = _cpp_unaligned_alloc (pfile, sizeof ("\"12:34:56\""));
-	  sprintf ((char *) pfile->time, "\"%02d:%02d:%02d\"",
-		   tb->tm_hour, tb->tm_min, tb->tm_sec);
+	  if (tb)
+	    {
+	      pfile->date = _cpp_unaligned_alloc (pfile,
+						  sizeof ("\"Oct 11 1347\""));
+	      sprintf ((char *) pfile->date, "\"%s %2d %4d\"",
+		       monthnames[tb->tm_mon], tb->tm_mday, tb->tm_year + 1900);
+
+	      pfile->time = _cpp_unaligned_alloc (pfile,
+						  sizeof ("\"12:34:56\""));
+	      sprintf ((char *) pfile->time, "\"%02d:%02d:%02d\"",
+		       tb->tm_hour, tb->tm_min, tb->tm_sec);
+	    }
+	  else
+	    {
+	      cpp_errno (pfile, DL_WARNING,
+			 "could not determine date and time");
+		
+	      pfile->date = U"\"??? ?? ????\"";
+	      pfile->time = U"\"??:??:??\"";
+	    }
 	}
 
       if (node->value.builtin == BT_DATE)
===================================================================
Index: doc/cpp.texi
--- doc/cpp.texi	17 Jul 2002 21:31:42 -0000	1.38
+++ doc/cpp.texi	1 Aug 2002 19:12:30 -0000
@@ -1780,10 +1780,16 @@ the preprocessor is being run.  The stri
 characters and looks like @code{@w{"Feb 12 1996"}}.  If the day of the
 month is less than 10, it is padded with a space on the left.
 
+If GCC cannot determine the current date, it will emit a warning message
+(once per compilation) and __DATE__ will expand to @code{@w{"??? ?? ????"}}.
+
 @item __TIME__
 This macro expands to a string constant that describes the time at
 which the preprocessor is being run.  The string constant contains
 eight characters and looks like @code{"23:59:01"}.
+
+If GCC cannot determine the current time, it will emit a warning message
+(once per compilation) and __TIME__ will expand to @code{@w{"??:??:??"}}.
 
 @item __STDC__
 In normal operation, this macro expands to the constant 1, to signify
===================================================================
Index: doc/extend.texi
--- doc/extend.texi	31 Jul 2002 00:11:31 -0000	1.92
+++ doc/extend.texi	1 Aug 2002 19:12:32 -0000
@@ -366,7 +366,9 @@ directive (6.10.6).}
 @cite{The definitions for @code{__DATE__} and @code{__TIME__} when
 respectively, the date and time of translation are not available (6.10.8).}
 
-GCC assumes that the date and time is always available.
+If the date and time are not available, @code{__DATE__} expands to
+@code{@w{"??? ?? ????"}} and @code{__TIME__} expands to
+@code{@w{"??:??:??"}}.
 
 @end itemize