Cannot Allocate More Than 64K with Calloc()

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 5.1, 6.0, 6.0a, and 6.0ax
      - Microsoft C for OS/2, versions 5.1, 6.0, and 6.0a
      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft Visual C++ for Windows, versions 1.0 and 1.5
  

In Microsoft C, although calloc() takes two unsigned integers as parameters, it does not allocate more than 64K. Calloc() determines the size you are attempting to allocate by multiplying the two arguments. If the size is greater than 64K, it will return NULL.

The confusion typically arises because, with malloc(), it is impossible to ask for more than 64K because of the size of the argument malloc() takes. Malloc() takes an unsigned int as an argument; therefore, the largest number you can pass it cannot be greater than 64K. This is not the case with calloc(), and the assumption could be made that you CAN allocate more than 64K with calloc(). This is not the case.