Basic's SETMEM Doesn't Free C Far Allocation; _fmalloc,

SUMMARY

When reducing far-heap size with the SETMEM statement in Basic and then calling a Microsoft C or QuickC function that does a far allocation, the reduced far-heap memory is not recoverable with SETMEM after returning to the QuickBasic program. C routines do far-memory allocation with "_fmalloc" and "_ffree" functions. This is expected behavior for the following reasons:

The C _fmalloc obtains memory from MS-DOS. Once far memory is obtained by a C program, it becomes part of the C far heap.
The _ffree does not release the memory back to MS-DOS, but rather stores it in the C far heap along with a linked-list data structure that C uses to manage its heap space.

Thus, SETMEM cannot recover this memory.

This information applies to Microsoft QuickBasic versions 4.00, 4.00b, and 4.50 for MS-DOS; to Microsoft Basic Compiler versions 6.00 and 6.00b for MS-DOS and MS OS/2; and to Microsoft Basic PDS version 7.00 for MS-DOS and MS OS/2.

MORE INFORMATION

To recover the memory in the Basic program, the C routine should do a huge allocation with the "halloc" and "hfree" functions. The C huge allocation routines differ from the far allocation routines. The halloc function always requests the memory directly from MS-DOS. The hfree function returns the memory directly back to MS-DOS. Once a C routine has executed an hfree, this memory can be recovered from a Basic program with SETMEM.

There is one disadvantage to using huge allocation. With huge allocation, there is no memory management involved on the part of the C routines. This can lead to memory fragmentation if the C routines do repeated allocations and frees.

The C routine should be carefully constructed to avoid this fragmentation. For more information on C memory management, please see page 114, "Memory Management: A Two-Step Process," "The Waite Group's Microsoft C Bible."

Code Example

/***********************************************************/ 
/* The following C function HugeMemTest can be called from */ 
/* Basic to demonstrate that huge allocation returns       */ 
/* memory to the Basic program.
/* Compile this routine with the medium memory model.      */ 
/* Do not compile with the huge memory model, since this   */ 
/* causes the code not to work and the memory won't be     */ 
/* freed.                                                  */ 
/***********************************************************/ 

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

void
HugeMemTest(void)
{
  long huge *lalloc;
  printf("Inside of C routine\n");

  lalloc = (long huge *)halloc(10000L,sizeof(long));
  if (lalloc == NULL)
     printf("\nInsufficient memory available\n");
  else
     printf("Memory successfully allocated\n");
  hfree(lalloc);

  printf("Leaving C routine\n");
}

'***********************************************************
'* This Basic routine releases memory to DOS using         *
'* SETMEM. It then calls a C routine that does huge        *
'* allocation and a huge free. SETMEM is used to recover   *
'* the memory and success or failure is reported.          *
'***********************************************************
DECLARE SUB HugeMemTest CDECL ()

' Report far heap size before the C call.
CLS
BeforeCall = FRE(-1)
PRINT "AVAILABLE MEMORY ON THE FAR HEAP:     ", BeforeCall
Storage = SETMEM(-50000)
PRINT "SIZE OF FAR HEAP AFTER SETMEM:        ", FRE(-1)

' Call to the C routine.
PRINT
CALL HugeMemTest
PRINT : PRINT : PRINT : PRINT

' Report far heap size after call to C.
PRINT "SIZE OF FAR HEAP AFTER CALL TO C:     ", FRE(-1)
Storage = SETMEM(50000)
AfterCall = FRE(-1)
PRINT "AVAILABLE MEMORY ON THE FAR HEAP : ", AfterCall

' Report success or failure.
IF AfterCall <= BeforeCall THEN
   PRINT "SETMEM FUNCTIONED PROPERLY"
ELSE
   PRINT "SETMEM DID NOT FUNCTION PROPERLY"
END IF
END

Basic's SETMEM Doesn't Free C Far Allocation; _fmalloc, _ffree (42596)

SUMMARY

MORE INFORMATION

Code Example