Q32998: malloc May Truncate Request More Than 64K

Article: Q32998
Product(s): See article
Version(s): 4.00 5.00 5.10 | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 19-JUL-1988

When trying to allocate memory with malloc() and calloc(), the
request for memory will be truncated if it exceeds 64K.
   The maximum number of bytes that can be allocated by malloc is less
than 64K because the allocation routines will consume a certain number
of bytes to track memory allocation within the segment. The maximum
number of bytes you can allocate using malloc or calloc is
approximately 65516.
   Requests for more than 65516 bytes and less than 64K will result in
malloc returning NULL. Requests for more than 64K will potentially
return a pointer but it will be a pointer to a block of a size other than
that requested because the parameter passed to malloc is an unsigned
integer. This integer has a maximum value of 64K; passing a number greater
than this value will result in undefined behavior. If malloc is passed a
parameter greater than 64K, the compiler will issue a data-conversion
warning, which should be heeded.
   If you require more than about 65516 bytes for a single allocation,
you should use halloc().

   The following program demonstrates the behavior described above:

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

int *intarray;

main()
{
    intarray= (int *)malloc(32768*sizeof(int));
    if (intarray == NULL)
        printf("not enough memory, no allocation");
    else
        printf("memory allocated");

    intarray= (int *)malloc(32767*sizeof(int));
    if (intarray == NULL)
        printf("not enough memory, no allocation");
    else
        printf("memory allocated");

}

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