Q42756: Not Including MALLOC.H in Compact, Large Model Causes Problems

Article: Q42756
Product(s): See article
Version(s): 5.00 5.10 | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | S_QuickC | mspl13_c
Last Modified: 30-MAY-1989

It is vital in the compact- and large-memory models that the malloc()
function be prototyped as returning a four-byte type (preferably a far
pointer, of course). The proper method in Microsoft C is to include
the header file MALLOC.H. This prototypes malloc() as returning a void
pointer.

The default data-pointer size in the compact and large models is 32
bits. The default return type of any function, including malloc, when
it is not prototyped is a 16-bit short integer. Thus, the segment
portion of a far address will be destroyed unless the compiler knows
it is dealing with a 32-bit type.

If you attempt to use the Unix style of including MEMORY.H rather than
MALLOC.H, you will encounter problems at run time. MEMORY.H does not
prototype any of the memory allocation functions; it only prototypes
memory copy and move functions. In a segmented architecture that can
have data and code pointers of different sizes, this may have serious
ramifications. In the case of a large- or compact-model program such
as the following, the call to malloc() will produce the following code
fragment:

    #include <memory.h>
    main()
    {
        char    *addr;
        size_t  nbytes = 100;

        addr = (char *) malloc( nbytes );
    }

The call to malloc() produces the following code fragment:

            push    Word Ptr [nbytes]
            call    _malloc
            add     sp, +02
            cwd
            mov     Word Ptr [addr], AX
            mov     Word Ptr [bp-06], DX

If MALLOC.H had been included, then the CWD (Convert Word to Double)
instruction would not be present. CWD sign extends the AX register
into the DX register in an attempt to convert the 16-bit integer in AX
to a 32-bit data pointer. This trashes the segment returned by
malloc() in DX. The resultant pointer will typically point to segment
0x0000, meaning that in DOS the interrupt table will be trashed and in
OS/2 the program will GP fault when an assignment is made to the
allocated memory.

Note: The compiler will also produce data conversion warnings at
warning level 1 or higher when such a program is compiled. Paying
attention to these warnings generally eliminates problems in
converting C programs from other systems.

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