C Run-time Functions Can Use Far Pointers in Medium Model

ID: Q66462

The information in this article applies to:

Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1

SUMMARY

Far pointers to data can be used in calls to the C Run-time library routines written using the medium model. This is often necessary because many Windows-based applications are written using the medium model, but must pass far data pointers to the medium model C run-time library routines. Unless precautions are taken, passing far pointers to medium model C Run-time routines will fail.

To use far pointers in calls to medium model C Run-time routines, the model-independent version of the C Run-time functions must be explicitly specified. A model-independent version of a C Run-time function requires specification of the size of the data pointers required (NEAR or FAR) for both function parameters and return values.

Not all C Run-time routines have model-independent versions. To determine if a routine has a model-independent version, please consult the header file associated with the routine or the C Run-time source manual.

MORE INFORMATION

When an application is compiled using the medium memory model, the C compiler assumes that the application will have one data segment and multiple code segments. Because the application has only one data segment, all pointers to data are assumed to be near pointers. Therefore, when the compiler encounters a C Run-time function in the source code, it automatically assumes that any pointer parameters contain near pointers. The compiler uses the appropriate medium model declaration for the run-time functions. This is appropriate for NEAR data items, but many Windows API functions require or return FAR pointers, such as GlobalLock().

To override the compiler's assumptions specify the model-independent version of the desired routine in the application source code. The header file associated with the routine or the C Run-time source manual can be used to determine which C Run-time routines have model-independent versions. Typically, the model-independent versions of C Run-time functions are preceded by an "_n" or an "_f". For example, strdup(), a memory model-dependent function, has two model-independent derivatives: _nstrdup() for NEAR pointers and _fstrdup() for FAR pointers.

When using the model-independent versions of the C Run-time routines, the compiler will not assume that the application's data is near. Thus, far pointers can be used in medium model applications where near pointers would normally be used.

Unfortunately, not all C Run-time routines offer this flexibility. If the routine does not have a model-independent version, two options are available:

Write a model-independent routine that offers the same functionality as the C Run-time routine.

Copy the data into the default data segment so that near pointers, and the standard C Run-time routines, can be used.

One C Run-time routine that demonstrates model independence is strncpy(). If the application source code includes a line similar to the following


   Char_ptr = strncpy(String1, Const_String2, Count);

the compiler will use the default declaration for the routine. That declaration is found in the header file and resembles the following:


   char *strncpy(char *string1, const char *string2, size_t);

In this situation, the data must be in the default data segment because the routine will use the DS register when referencing both strings.

In the same medium model applications, if one or both of the strings are in a data segment other than the default data segment, modify the same source line as follows:


   Char_ptr = _fstrncpy(String1, Const_String2, Count);

In this case, the compiler will use the following function declaration


   char _far * _far _fstrncpy(char _far *string1,
                              const char _far*strings,
                              size_t count);

and the application will then properly access the data in the far segments.

NOTE: When using the same C Run-time routine in a large model application, the default function declaration will be the model-independent version. Thus, in the example above, the compiler will replace the call to strncpy() with an appropriate call to _fstrncpy().

Additional query words: 3.00 3.10

Keywords : kb16bitonly
Version : WINDOWS:3.0,3.1
Platform : WINDOWS
Issue type :