From: "John M. Aldrich" <fighteer AT cs DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: failed malloc/callocs :(
Date: Sun, 04 May 1997 20:57:09 +0000
Organization: Two pounds of chaos and a pinch of salt
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Message-ID: <336CF825.5F47@cs.com>
References: <5ki242$qe8 AT bore DOT pipex DOT net>
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To: djgpp AT delorie DOT com
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nikki wrote:
> 
> this works fine on a 32 meg machine under win95 or dos (with cwsdpmi)
> on a 16 meg machine it gives the insufficient memory error, despite having
> plenty of memory ( 120 meg virtual, 14 meg real).
> 
> there are no other (large) memory allocations in the program, this is the only
> one. so why does it fail? and how come needmem() said it would be fine?
> 
> does anyone have any suggestions as to how to improve needmem() so it always
> works, and also how to make this run on a 16meg machine?

The answer is simple - don't use _go32_dpmi_remaining_*_memory().  They
aren't reliable enough to use for such a task, because they only reflect
the state of the operating system and DPMI host, not the internal state
of the program itself.  The DJGPP memory allocation routines recycle
allocated memory in order to save time.  This means that once your
program grabs a chunk of memory, it's allocated forever as far as the
DPMI host is concerned.  If you free() it, it just goes back onto the
list of free blocks in malloc()'s internal structures, and is not
actually freed to the system.

But for the task you are attempting, you shouldn't need to use such a
clumsy workaround.  malloc() and calloc() both return NULL pointers if
they fail to allocate the desired amount of memory.  A simple test will
suffice:

if ( ( ptr = malloc( 1024 * 1024 ) ) == NULL )
{
    fprintf( stderr, "malloc failed\n" );
    exit( 1 );
}

All the problems of handling the internal structures, grabbing memory
from the DPMI host, adjusting the free memory values, are all done for
you.  There's no need to reinvent the wheel.

-- 
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| John M. Aldrich, aka Fighteer I  |     mailto:fighteer AT cs DOT com     |
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