From: s335194 AT student DOT uq DOT edu DOT au (David Wilson)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: problem with malloc in djgpp?
Date: 12 Mar 1997 22:34:40 GMT
Organization: University of Queensland
Lines: 46
Message-ID: <5g7b20$uc$1@nargun.cc.uq.edu.au>
References: <5g491r$mbg AT lyra DOT csx DOT cam DOT ac DOT uk>
NNTP-Posting-Host: student.uq.edu.au
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

>this is the code i use : (long lines got wrapped with ...'s)

>it reports the memory as described above then says malloc failed. despite telling
>me it had at least 14meg and me only malloc'ing 6 meg of it!

Is the program failing in needmem() or in main(), after the call to
needmem()?  

With DPMI you need to realize that the values returned from
__dpmi_get_free_memory_information (yes it is a long name), and the
corresponding _go32_dpmi_remaining_*() functions, are subject to change at
the whims of Windows/CWSDPMI/... 

What MIGHT be happening is that the DPMI client reports only a small
amount of free memory, but if you malloc() a huge chunk it will get more
memory from the system for you, using virtual memory if appropriate.  So
even if your program only got told it had 4 MB free you still might be
able to allocate a 64MB buffer.

When I first got DJGPP I wrote a program that allocated a 50 meg buffer
and filled it with zeros.  It worked fine (on a 16 MB machine), but I
didn't have any checks for free memory first.  Try this program and see if
it works:

--------[]--------

#include <stdio.h>
#include <string.h>

#define SIZE 50*1024*1024
main()
{
  char* p = (char*) malloc(SIZE);
  p ? memset(p, 0, SIZE), printf("worked\n") : printf("failed\n");
  return 0;
}

--------[]--------

(apologies for obfuscation!)

Possible solution to problem:  try the 14-meg malloc first, and if it
fails, then report free memory to user and abort.

--Dave