Date: Mon, 3 Nov 1997 17:39:10 -0800 (PST)
Message-Id: <199711040139.RAA11038@adit.ap.net>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
To: Peter Palotas <blizzar AT hem1 DOT passagen DOT se>, djgpp AT delorie DOT com
From: Nate Eldredge <eldredge AT ap DOT net>
Subject: Re: Writing to a file from dtor doesn't work!
Precedence: bulk

At 03:53  11/3/1997 -0500, Peter Palotas wrote:
>Take a look at the following tiny bit of code:
>
>#include <stdio.h>
>class A 
>{
>private:
>	FILE *fl;
>public:
>	A(void)
>        {
>        	fl = fopen("test.log", "wt");
>        	fprintf(fl, "In A constructor\n");
>        }
>
>        ~A(void)
>        {
>         	fprintf(fl, "In A destructor\n");
>                fclose(fl);
>        }
>};
>
>A a;
>
>int main(void)
>{
>	return 0;
>}
>
>Okay, now everyone probably sees what this should do. It should write the
>two lines "In A constructor" and "In A destructor" to the file "test.log"
>(provided that it successfully can open the file, but since this is only a
>test program, I don't bother checking that.
>
>Well, the problem is that if you declare an object of 'A' at global scope,
>like I did in this example, nothing gets written in the destructor! i.e.
>the file only contains one line; "In A constructor".
>
>Declaring 'a' (an object of 'A') inside main() however makes everything
>work as it's supposed to.  (Reopening the file in the destructor also works
>fine, so it seems as if the file is closed before we reach the destructor
>or something?)
You got it. See below.
>
>Now is this a bug, or is it normal behaviour? Becuase it doesn't seem
>normal to me, and I haven't found any documentation on it either, so any
>help solving this would be appreciated.

I found the cause, but I still don't know if it's a bug. In
src/libc/ansi/stdlib/exit.c, __stdio_cleanup_hook is called before the
destructors. You guessed it, __stdio_cleanup_hook flushes all streams, and
closes all except stdin, stdout, and stderr (cin, cout, cerr for C++). If
this is not what's Supposed To Happen, it should be easy to change, just
move two lines of code. *Are* files Supposed to work in destructors?

Nate Eldredge
eldredge AT ap DOT net