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Subject: RE: finding out how much memory is available with malloc, gcc, cygwin
Date: Tue, 19 Aug 2003 11:26:22 +0530
Message-ID: <DC91351678E31E4FB24B551221F507EC6D6CBC@blr-itp-msg.wipro.com>
From: "Kiran Bacche" <kiran DOT bacche AT wipro DOT com>
To: "Jim Holder" <j_r_holder AT yahoo DOT com>, <cygwin AT cygwin DOT com>
Content-Transfer-Encoding: 8bit


Hi Jim
   Well in almost all current Oses, each application is allocated a
virtual memory of 2^32 Bytes. This block is divided into many segments
like data, text, bss, stack and heap. 
The max. size of heap depends varies from one application to another as
the other segments vary from one application to another. And also it is
OS dependent. Some OS may restrict it's heap from  growing beyond some
MBs, and some may allow applications to readjust the heap size. (thru
calls similar to unix brk() call).

  OK that's some theory. Coming to your applciation, if the intent is to
find out the "physical memory" present, I think some kernel level
programming is required. Please refer "Linux Device Drivers" By Rubini
of the O'Reilly Team.

But if the intent is to really find out how many bytes can be malloced
by an application, I think the answer will be different on different
Oses.

Hope this helps.

Cheers
  Kiran


-----Original Message-----
From: Jim Holder [mailto:j_r_holder AT yahoo DOT com] 
Sent: Tuesday, August 19, 2003 3:25 AM
To: cygwin AT cygwin DOT com
Subject: finding out how much memory is available with malloc, gcc,
cygwin


A program similar to the one listed below works for me on Linux but not
under
Cygwin.  The idea is to find out how memory I can allocate on the
machine.  The
Cygwin version keeps going (past 700MB), reporting that it has allocated
more
memory than the machine has.  What am I doing wrong?  Thanks.

#include <stdlib.h>
#include <stdio.h>
int main(void)
{
    char *p;
    int loop = 1;
    long allocated = 0L;
    const long PIECE = 1000L;
    const long TENMB = 10000000L;
    long nextTenMB = TENMB;

    while(loop)
    {
        p = (char*)malloc(PIECE);
        if(p == NULL)
        {
            printf("p == NULL\n");
            loop = 0;
        }
        if(p == (char*)0)
        {
            printf("p == (char*)0\n");
            loop = 0;
        }
        if(loop == 0)
            break;
        allocated += PIECE;
        if(allocated >= nextTenMB)
        {
            printf("%ld\n", allocated);
            nextTenMB += TENMB;
        }
    }
    printf("final:  %ld\n", allocated);
    return 0;
}

Some environment particulars:

    -  cygwin 1.3.19-1
    -  gcc 3.2-3
    -  windows 2000, version 5.0, service pack 4

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