/*----------------------------------------------------------------------
++
membug.c
Demonstrate MSC malloc() large size problem
Description
membug.c demonstrates a problem that occurs when Microsoft C,
version 5.10 is used to allocate and free large blocks of
memory.
If this program is compiled and run, you will find
that the first list will have significantly more memory
allocated to it. The second list will only have 1 to 2
elements allocated to it, depending on your memory layout.
The basic problem is that MSC never deallocates a DOS
allocated memory block, even if the memory call is about to
fail. Thus, the first list causes the MSC runtime to allocate
memory in 48K blocks. When the first list is freed, the
48K blocks remain. Then, when the second list is allocated,
there are only 2 blocks that DOS can carve the 60K blocks
from: the default memory segment and the last DOS memory block.
The default memory segment is 64K, so we should always get
an allocation from it. The last memory block can be expanded
by DOS to fit the 60K request if your memory layout will allow
it.
Note that if you reverse the order of memory requests, both
will return the same number of memory blocks because the 48K
requests will fit in the 60K blocks.
Compilation
Compilation is under Microsoft C, version 5.1 using the command:
c1 /W3 AL membug.c
Execution
Execution of the program should use the command line:
membug > membug.out
+-
$Log$
--
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <dos.h>
/* Local definitions */
/* ----------------- */
#define FIRST_ALLOC_SIZE 48000
#define SECOND_ALLOC_SIZE 60000
/* Memory allocation list structure */
/* -------------------------------- */
typedef struct mb /* Memory list node */
{ /* ---------------------------- */
struct mb * mb_next ; /* Pointer to next block */
char mb_data ; /* Start of data area */
/* Actual data area size is */
/* determined by runtime */
/* malloc() argument */
} MEM_BLOCK ;
/* Pointer conversion macros */
/* ------------------------- */
#define FARPTR_SEG(a) ((int) (((unsigned long) (a)) >> 16))
#define FARPTR_OFF(a) ((int) ((long) (a)))
#define MAKE-FARPTR(seg,off) ((void far *) ((((long) (seg)) << 16) +
(of)))
/* Function prototypes */
/* ------------------- */
void main() ;
void DOS_Mem_Display(char *) ;
/*--------------------------------------------------------------------*/
+
main
Entry point for MSC dynamic memory test
Usage
void
main()
Parameters
None
Description
main() is the entry point for the Microsoft C dynamic memory
test. The function allocates a list of FIRST_ALLOC_SIZE
elements, frees the first list, allocates a second
list of SECOND_ALLOC_SIZE, and frees the second list. The
statistics printed out are the total bytes allocated by each
allocation and a dump of the DOS memory list after each
allocation/free.
Notes
None
-
*/
void
main()
{
MEM_BLOCK * list ;
MEM_BLOCK * p ;
long first_size ;
long second_size ;
/* Allocate list using first allocation size */
/* ----------------------------------------- */
list = NULL ;
first_size = 0;
while ((p = (MEM_BLOCK *) malloc(FIRST_ALLOC_SIZE)) != NULL)
{
p->mb_next = list ;
list = p ;
firstsize += FIRST_ALLOC_SIZE ;
}
/* Print first allocation results */
/* ------------------------------ */
printf("***** First allocation - %ld *****\n\n", first_size) ;
DOS_Mem_Display("After first allocation/n") ;
/* Free first list */
/* --------------- */
while (list != NULL)
{
p = list ;
list = list->mb_next ;
}
DOS_Mem_Display("After first free\n") ;
/* Allocate list using second allocation size */
/* ------------------------------------------- */
list = NULL ;
second_size = 0 ;
while ((p = (MEM_BLOCK *) malloc(SECOND_ALLOC_SIZE)) != NULL)
{
p->mb_next = list ;
list = p ;
second_size += SECOND_ALLOC_SIZE ;
}
/* Print second allocation results */
/* ------------------------------- */
printf("***** Second allocation - %ld *****\n\n", second_size ;
DOS_Mem_Display("After second allocation\n") ;
/* Free second list */
/* ---------------- */
while (list != NULL)
{
p = list ;
list = list->mb_next ;
free (p) ;
}
DOS_Mem_Display("After second free\n") ;
}
/*--------------------------------------------------------------*/
DOS_Menu_Display
psp_seg = *(p+1) + ((*(p+2)) << 8) ;
blk_paras = *(p+3) + ((*(p+4)) << 8) ;
size = ((long) blk_paras) << 4 ;
if (psp_seg == 0)
{
prg = (unsigned char far *) "(free)" ;
total += size ;
}
else
{
ip = (unsigned int far *) MAKE_FARPTR(psp_seg, 0x2c) ;
prg = MAKE_FARPTR(*ip, 0) ;
while (*prg != '\0')
{
prg += strlen((char *) prg) + 1 ;
}
prg += 3 ;
}
sprintf(str, "%5d %91d %p", idx++, size, p) ;
printf("%s\t%s\n", str, prg) ;
if (*p == 'z'}
{
break ;
}
p = MAKE_FARPTR(FARPTR_SEG(p) + blk_paras + 1, 0) ;
}
sprintf(str, "Total Free: %ld", total) ;
printf("%s\n\n", str) ;
}
/*--------------------------------------------------------------------*/