Sender: rich AT phekda DOT freeserve DOT co DOT uk
Message-ID: <3BE26372.CFFD55CA@phekda.freeserve.co.uk>
Date: Fri, 02 Nov 2001 09:12:18 +0000
From: Richard Dawe <rich AT phekda DOT freeserve DOT co DOT uk>
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To: DJGPP workers <djgpp-workers AT delorie DOT com>
Subject: Patch for info on assertions, using __bss_count
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Reply-To: djgpp-workers AT delorie DOT com

Hello.

Below is rev 2 of the patch on __bss_count. I also added some info on
assertions. Thanks to DJ, Eli for your help.

Bye, Rich =]

-- 
Richard Dawe
http://www.phekda.freeserve.co.uk/richdawe/

Index: src/docs/kb/develop.txi
===================================================================
RCS file: /cvs/djgpp/djgpp/src/docs/kb/develop.txi,v
retrieving revision 1.5
diff -c -3 -r1.5 develop.txi
*** src/docs/kb/develop.txi     2001/03/31 13:44:50     1.5
--- src/docs/kb/develop.txi     2001/11/02 09:10:24
***************
*** 77,82 ****
--- 77,152 ----
  @sc{gs} register instead of @sc{fs}, to avoid clobbering @sc{fs} which
  might be used by the application.
  
+ @subsection Assertions
+ @cindex Assertions
+ 
+ Assertions should not generally be used in library code.  To catch an
error,
+ the code should check for the error and return an error code.  Using
+ @code{assert} (@pxref{assert, , assert, libc}) causes the debug
+ and final release (@samp{NDEBUG} defined) versions of the code to
differ,
+ so that the final release is not as well tested.
+ 
+ @subsection Coping with restarted programs
+ @cindex coping with restarted programs
+ @cindex @code{__bss_count} and restarted programs
+ 
+ Uninitialised static and global data is placed in the @code{.bss}
section.
+ @dfn{BSS} stands for Block Started by Symbol.  The @code{.bss} section
+ is zeroed by the start-up code.
+ 
+ Initialised static and global data is placed in the @code{.data}
section.
+ 
+ So it would seem that one can rely on static and global variables being
+ initialised to zero or some specified value.  Unfortunately this may not
+ be true, where programs are unexecuted and then restarted ---
+ Emacs is the primary example.
+ 
+ Part of Emacs's build procedure is that Emacs unexecutes (dumps) itself
to
+ a file.  The dumping process records the value of each variable at
+ the time of the dump to a file.  The symbols in the @code{.bss} section
+ are moved to the @code{.data} section.  The values at the time of the
dump
+ are recorded, not the original values.  So if the library code is
+ relying on the value of a static or global variable, e.g. to see whether
+ it needs to initialise some device or allocate memory, then it may break
+ when the program restarts.
+ 
+ Fortunately there is a way that library code can detect a restart.
+ The variable @code{__bss_count} contains a counter of the number of
times
+ that the program has been started --- it is incremented by the start-up
+ code.  A routine can store @code{__bss_count} and then check whether it
needs
+ to initialise its data by comparing @code{__bss_count} with its stored
value.
+ Here is an example:
+ 
+ @example
+ #include <string.h>
+ #include <libc/bss.h>
+ 
+ extern int do_something_interesting (const int n);
+ 
+ static int my_bss_count = -1;
+ static int my_array[20];
+ 
+ int
+ myfunc (const int n)
+ @{
+   if (my_bss_count != __bss_count)
+     @{
+       my_bss_count = __bss_count;
+       memset(my_array, 0, sizeof(my_array));
+     @}
+ 
+   if (n >= 20)
+     return 0;
+ 
+   if (!my_array[n])
+     my_array[n] = do_something_interesting(n);
+ 
+   return(my_array[n]);
+ @}
+ @end example
+ 
+ For more details see @file{src/libc/crt0/crt1.c} in the DJGPP libc
sources.
+ 
  @section Texinfo documentation
  
  @subsection Formatting