PRB: Apparent _memval() Failure Caused by printf() Allocation

MS-DOS                 | OS/2       | WINDOWS

The information in this article applies to:

• The C Run-time (CRT), included with:

      - Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax
      - Microsoft C for OS/2, versions 6.0, and 6.0a
      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft C/C++ for OS/2, version 7.0
      - Microsoft Visual C++ for Windows, versions 1.0 and 1.5
  

SYMPTOMS

In Microsoft C, the printf() function makes a call to malloc() the first time it is called within a program. Because of this allocation, _memavl() may seem to return an incorrect value in some cases.

If a call to _memavl() is made within or before the first printf() statement in a program, subsequent malloc() calls cannot allocate as much memory as _memavl() suggests.

RESOLUTION

The workaround is to make the call to printf() before your call to _memavl(), or make another call to _memavl() after printf() to determine your true maximum allocation.

MORE INFORMATION

The following code demonstrates the problem:

/* Compile options needed: none
*/

#include<malloc.h>
#include<stdio.h>

char *ptr;

void main(void)

// printf("If included, this line solves the problem.\n");

   printf("memory available=%u \n",mavl=_memavl());

   if ((ptr=(char *)malloc(mavl))==NULL)
     printf("Not as much memory available as we thought!");