Application Owns Memory Block After free() Call

MS-DOS                      | WINDOWS

The information in this article applies to:

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

      - Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, and 6.0ax
      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, and
        1.52
  

SUMMARY

When an application uses the malloc() function to request more memory than is currently available in the heap, the malloc() function requests more memory using an MS-DOS interrupt to provide a larger amount of heap space. If malloc() is able to obtain additional memory, it performs the allocation and returns a pointer to the memory.

When the application no longer needs a block of allocated memory, it calls the free() function which marks the block as available in the heap. Subsequent calls to malloc() can use this area of memory. However, according to MS-DOS, the memory block is still in use because it is a part of the application's heap. If the application calls the system() function to execute a command, the memory is not available to MS-DOS.

Microsoft C version 6.0 introduced the _heapmin() function which returns to the operating system unused space in the heap.

The other alternative is to modify the source code to use the halloc() and hfree() functions which allocate memory directly from the operating system and not through the application's heap.