Date: Wed, 22 May 1996 08:42:29 +0200 (IST)
From: Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
To: Juanjo Erauskin <juanjo AT jet DOT es>
Cc: djgpp AT delorie DOT com
Subject: Re: maximum open files for program
In-Reply-To: <31A0845F.92C@jet.es>
Message-Id: <Pine.SUN.3.91.960522080824.1173E-100000@is>
Mime-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII


On Mon, 20 May 1996, Juanjo Erauskin wrote:

> 	When the program call system() function, return this error: 	
> foo.exe: cannot open

This is a misfeature of MS-DOS that has (almost) nothing to do with DJGPP. 
The error message is printed by the stub of the child program when it
tries to open `foo.exe' in order to read the COFF info (which it needs to
know how the program should be set up in memory).  If the parent program
has used up more than 20 file handles, that DOS Open call will fail.  The
reason is that the child inherits the open files of its parent, so it too
has 20 used handles.  While DJGPP allows you to open much more than 20
files, it does so only when you call one of the high-level library
functions (like fopen and open); the stub cannot call these because it
reads the .exe file before it even switches into protected mode.  And
while the parent might have increased its maximum file handle count beyond
20, the DOS Exec call used to spawn the child arranges it to have only the
default 20 handles (so only they are really inherited).  The bottom line
is that when the stub calls Int 21h/AX=3D00h, DOS finds that all the 20
file handles are in use and returns with an error. 

Your test program doesn't tell enough to know what kind of application 
needs to keep a large number of files open when it calls `system'.  If 
that is a real need, I can think of a few work-arounds:

	1) If you can get away with less than 15 open files, you should 
have no problems.

	2) Close one of the unused standard handles (3 and 4, connected 
by default to AUX and PRN, come to mind) *before* you begin opening the 
real files you need.  This will work if you only need 16 or 17 files open 
when you call `system'.

	3) Open a few dummy files *before* you open the real files.  Close
those dummy files immediately before you call `system'.  This will cause
some of the handles in the first 20 slots to be free, but push some of the
real files into the portion of the handle table that isn't inherited by
the child (a rare program needs to inherit any handle but the first 3
standard ones, though). 

Note that I didn't have time to test the above work-arounds, so I cannot 
promise that they really work (I might have missed something).