help: cygwin on WinNT - allocates 3x memory asked for

[Milos Popovic]

Hi,

I hoped that someone would be willing to help me with resolving a simple 
cygwin-related malloc problem under Win NT 4.0. I have searched the web for 
an answer and haven't seen any related post. I apologize if the answer is 
obvious to posters on this list - I'm not very experienced with cygwin.


PROBLEM: a C program written to allocate x amount of memory actually uses 
about 3 times that much memory, when compiled with gcc (3.2-2) under cygwin 
(1.3.15-2). This does not occur when compiling with a native windows 
compiler (I used lcc) where allocating 50MB increases the system memory 
used by roughly the 50MB.

The program below tries to allocate memory starting with 16MB and doubling 
the allocation until it fails. The getchar statements are there to 
interrupt the program and wait for a keypress so that I can monitor the 
amount of memory used by the system in the Windows NT Task Manager before 
and after each allocation.

The result is that a 512MB allocation actually increases the system memory 
usage by about 1500MB. This ratio of 3 is the same regardless of the size 
of allocation. When compiled with a native compiler (non-cygwin), a 512MB 
allocation increases the used ram by about 512MB. I also tried using 
realloc instead of malloc/free for successive allocations - no difference. 
C++ version with new/delete on g++ - also no difference.  The factor of 3 
is also the same whether you're allocating a vector of chars, doubles, etc.

Any hints? Listing is below. (I'd appreciate it if you'd cc me any reply). 
Thanks a lot!

Milos



/*
    Finds largest power of 2 chunk of memory that can be allocated.
    Milos Popovic, Nov 13, 2000
*/

#define EVER ;;
#define N    1024*1024*16/sizeof(mytype)   // Start with 16MB vector

#include <stdio.h>
#include <stdlib.h>

typedef char mytype;

int main(void)
{
   size_t n = N;
   mytype *x;   // = (char *) malloc(n * sizeof(mytype));

   for(EVER) {
     x = (mytype *) malloc(n * sizeof(mytype));

     if (x == NULL) {
       printf("Can't assign more memory!\n");
       exit(1);
     }

     printf("Allocated %d bytes.\n", n * sizeof(mytype));
     n *= 2;     // Double amount of memory to allocate next time
     printf("Hit enter."); getchar();
     free(x);
               // x = (char *) realloc(x, n * sizeof(mytype));
     printf("Hit enter."); getchar();
   }

   printf("Done.\n");
   return 0;
}


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