From:	George Foot <mert0407 AT sable DOT ox DOT ac DOT uk>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: CWSDPMI: My application crash on "Win95 Command Prompt Only"
Date:	15 Jun 1998 15:02:47 GMT
Organization:	Oxford University, England
Lines:	45
Message-ID:	<6m3d2n$qoq$3@news.ox.ac.uk>
References:	<01bd9474$0e94ec80$92c809c0 AT chessa>
NNTP-Posting-Host:	sable.ox.ac.uk
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

On 10 Jun 1998 13:31:03 GMT in comp.os.msdos.djgpp Alberto Chessa
<achessa AT fiar DOT finmeccanica DOT it> wrote:

: Hi,
: I'm in troubble with an little Allegro based program. It run under Win95
: DOS Box but It won't run under "Win95 Command Prompt Only" mode with
: CWSDPMI DOS Extender (I need to test the pure performace of my program with
: no intrusion at all).

: It reports a "Page fault" error.

You are probably trying to dereference a NULL pointer -- for example,
one that is uninitialised, or had a value assigned from a `malloc'
that failed.  By default, djgpp's startup code arranges for this to
cause the error you see; Windows 95's DPMI server does not support
this, so the error does not show up.  You can turn off this feature
(see `crt0.h' in the `include' directory), but I don't recommend this
-- the crash is due to a bug in your program, and you should fix it.

If you run `symify' while the call frame traceback is still on the
screen, it will tell you what functions were being executed when the
problem occured.  If you compile with extra debugging information (`-g') then
it will also tell you what filename and line number was reached in
each function in the list.  The first entry in the list is the actual
location where the problem occured; entries further down the list are
the locations of calls to the function immediately above them in the
list.

If you are so inclined, you could learn to use a debugger; then you
can examine the values of variables in your program when the crash
occurs, which may help you to find the NULL pointer.  Some types of
programs don't behave well when run from debuggers, though, so other
problems may arise if you try this.

References to djgpp FAQ:
    Null pointer dereference crashes v2.0 programs: Section 9.1
    SYMIFY, a program to read crash traceback: Section 9.2
    Debugging: Chapter 12

Also see `readme.1st' for brief information on debugging.

-- 
george DOT foot AT merton DOT oxford DOT ac DOT uk

xu do tavla fo la lojban  --  http://xiron.pc.helsinki.fi/lojban/lojban.html

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