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From: bloomqui@osiris.cso.uiuc.edu (Kim Bloomquist)
Newsgroups: comp.lang.c
Subject: Turboc 2.0 malloc/free/coreleft
Message-ID: <1991Jan30.172158.2769@ux1.cso.uiuc.edu>
Date: 30 Jan 91 17:21:58 GMT
Sender: news@ux1.cso.uiuc.edu (News)
Organization: University of Illinois at Urbana
Lines: 56

I'm trying to write a Turbo C 2.0 program on a Zenith 386 that
reports the amount of memory (RAM) available after performing
certain operations.  The following program produces the output 
shown below:

---------------------------- tst.c --------------------------
#include <stdio.h>
#include <stdlib.h>
#include <alloc.h>
main()
{
	unsigned long n;
	char *s;

	n = coreleft();
	printf("ram left = %lu bytes free\n", n);
	if ((s = (char *) malloc(80)) == NULL)
	{
		printf("allocation failed\n");
		exit(0);
	}
	printf("buffer allocated\n");
	n = coreleft();
	printf("ram left = %lu bytes free\n", n);
	/* printf("string: ");
	gets(s); */
	free((void *) s);
	printf("buffer deallocated\n");
	n = coreleft();
	printf("ram left = %lu bytes free\n", n);
}

output from run 1: /* commented version */

ram left = 63704 bytes free
buffer allocated
ram left = 63616 bytes free
buffer deallocated
ram left = 63704 bytes free

The program performs as expected.  Memory is allocated to hold s then
free() deallocates s making available the same number of bytes (63,704)
we started with.  However, when the comments are removed from around the
printf/gets statements the following output is produced.  The last call
to coreleft shows only 63,082 bytes free and not the original 63,690 bytes.
Question 1:  What is being allocated to the missing 608 bytes?
Question 2:  Can this memory (the 608 bytes) be deallocated?

output from run2: /* uncommented version */

ram left = 63690 bytes free
buffer allocated
ram left = 63602 bytes free
string: 
buffer deallocated
ram left = 63082 bytes free