Mail Archives: djgpp/1996/09/16/11:45:10
At 08:30 16/09/96 -0400, kagel AT quasar DOT bloomberg DOT com wrote:
> From: Thomas Nilsson <thoni AT softlab DOT se>
> Date: Fri, 13 Sep 1996 19:23:43 +0200
>
> I have an application which I mainly develop on Unix, however thanks to
> the wonderful DJGPP it's very easy to port to PC's, thanks all who have
> contributed in the DJGPP project!!
>[SNIP]
> The structure is declared static at the top level of the module and in
> the beginning of the file, but still gdb says 'No symbol xxxx in current
> context'. The structure *should* not be used before that point in the
> running of the program, but it is not zero as I would have expected.
>
>Static and global variables are not initialized or zero'd unless explicitly
>initialized in the code. You have just been lucky because UNIX zeros the
Excerpt from K&R Book 2nd ed. pg 40:
"...External and static variables are initialized to zero by default."
I have no ANSI/ISO standard handy by now, but it is on the standard as well
(I think on section four).
>memory pages it assigns to a process so the garbage you have been getting just
>happened to be the zeros you wanted. DOS does not zero memory nor does the
>DPMI spec require this (IMHO) so DJGPP does not zero uninitialized memory.
>
> This being strange, but still I wanted to investigate this structure, so
> I removed the static declaration to make it more visible, and thought
> that I might get at it already at the start of the program. And of
> course I could, *but* now it is zeroised correctly and not destroyed so
> the program works perfectly...
>
>Just lucky that globals are probably allocated before statics so now your
>structure is on a different memory page with different garbage.
>
>You really should convert the definition of the structure to an initialized
>definition. Never rely on the value of a variable you have not explicitly
>set.
>
> Is this a bug, or if not, how do I find out when the structure is
> trashed. Note however that Purify (a lovely memory access analyser for
> many plattforms) does not indicate any problems on Unix (so at least
> it's not a general bug, but then which ones are...)
>
>Purify reports no problem because none exists you are just suffering from GIGO.
>
> ------------------------------------------------------------------------
>
>--
>Art S. Kagel, kagel AT quasar DOT bloomberg DOT com
>
>A proverb is no proverb to you 'till life has illustrated it. -- John Keats
>
>
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