Switching to Digital Mars C++

What's in This Chapter

• Portable Programming Practices
  • Object compatibility with other compilers.
  • Using third party libraries with Digital Mars C++.
  • Recompiling for Digital Mars C++.
• Converting from Microsoft
• Converting from Borland

• Use ANSI standard library functions wherever possible and avoid compiler-specific extensions.
• Do not rely on the location, alignment, or size of objects in memory. In addition to the inevitable implementation dependencies of memory models, the Digital Mars C++ compiler assigns different widths to some of the standard data types.
• Do not rely on objects of the same size being treated as if they had the same type. Digital Mars C++ enforces stricter type checking than either Microsoft C++ or Borland C++, and will generate warnings or errors in these instances. In addition, loosely typed code will often fail under 32-bit compilation. For example, Digital Mars C++ defines the WORD type as an unsigned short, and the UINT type as an unsigned int. If these types are treated identically in your code, you can use the -Jm compiler option to relax type checking.
• Use manifest constants wherever possible, instead of relying on explicit (and possibly compiler or environment dependent) values.
• Write function prototypes for all your user-defined functions. See Digital Mars C++ Language Implementation for more information.
• Can avoid linking files compiled with different compilers. Before you begin conversion, delete all .obj and .lib files created with your old compiler for which you have the source code. Also, be sure that the INCLUDE and LIB environment variables reference the Digital Mars C++ directories so you do not accidentally compile with one vendor's headers and link with the other's libraries.

Object Level Compatibility

• The Digital Mars C++ compiler's "helper" functions (long multiply, float, and so on) are different than their Borland or Microsoft counterparts. If you do not want to recode calls to these functions, remove the other vendor's helper functions from their libraries and include them in your converted code.
• Digital Mars' floating point libraries are reentrant; our object code is therefore different than Microsoft's or Borland's for functions with C linkage. You can work around this problem by rewriting affected functions to pass a pointer to their return value as a parameter, or by using C++, FORTRAN, or Pascal linkage instead of C linkage.
• The layout of the struct_iob in stdio. h is different for Digital Mars C++. Therefore, you cannot link buffered I/ O functions compiled with Digital Mars' stdio. h with buffered I/ O functions compiled with a different stdio. h. To avoid problems, compile all modules in a program with the same version of stdio. h.
• By default, Digital Mars C++ aligns structure members on 16-bit boundaries in 16-bit compilations; Microsoft and Borland C++ do not. For binary compatibility, compile with the -a option to suppress structure member alignment.

Using Third-Party Libraries

If you have the source to the third-party library, try recompiling it with Digital Mars C++. A better solution is to obtain the Digital Mars version of the library from the vendor. Recompiling for Digital Mars C++ In many cases, converting programs written for another compiler to Digital Mars C++ can be as easy as recompiling them. Here are some steps you can take to ease recompilation:

• Compile without the global optimizer until your program runs without errors. A working program can occasionally fail when optimized. For more information, see Optimizing Code.
• In Digital Mars C++, character data is signed by default. If your code depends on char data being unsigned, compile with either the -J or -Ju options. -J treats chars as unsigned, not sign extended. -Ju treats chars as unsigned, sign extended.
• If your code does not follow strict ANSI type checking rules, use the -Jm option, which relaxes type checking.

Compile time warnings

dmc -w2 -w6 myfile.cpp 

What's in This Chapter

• Keywords
• Predefined macros
• Header files
• Libraries
• MFC
• Memory model support
• Assembly language interface
• Compiler options

Keywords

Digital Mars C++ supports most of Microsoft C's nonstandard keywords. The following table lists Microsoft's keyword extensions and their Digital Mars counterparts:

Predefined Macros

Digital Mars C++ supports most of Microsoft C's nonstandard macros. The following table lists Microsoft's extended macros and their Digital Mars counterparts:

Header Files

Library Functions

Digital Mars C++ supports the great majority of Microsoft Visual C++ Version 1.5 library functions. The table below lists the only Microsoft functions that the Digital Mars C++ run-time library does not support:

Microsoft Foundation Classes

Digital Mars' versions of the Microsoft Foundation Class Library are licenced from Microsoft Corporation.

When compiling code that uses the Microsoft Foundation Classes, use the -gf option (see Compiling Code).

Memory Models

• In the Digital Mars Compact and Large models, unlike their Microsoft counterparts, the stack segment and data segment are not the same. Near pointers in these models are relative to the data segment and cannot be used to access automatic or parameter variables. Check your assembly language source code to ensure that SS is not assumed to be equal to DS.
• Microsoft C only creates a far data segment if the amount of data exceeds 32,767 bytes, or exceeds the value specified via the /Gt option to CL (Digital Mars C++ has a corresponding compiler option, -GT). While data is always assigned to the near data segment by default, functions are near by default in the Compact model and far by default in the Large model in Digital Mars C++.
• Digital Mars' malloc() functions take an unsigned int as an argument for all 16-bit memory models and take an unsigned long for 32-bit models. Therefore, the 16-bit models limit the size of a data object to 64KB blocks. To allocate a block greater than 64KB, call farmalloc, which takes an unsigned long.
• At program startup, a Microsoft C program's heap management run-time allocates _amblksiz bytes of far heap space. Additional heap is allocated in _amblksiz blocks as needed. In contrast, Digital Mars C++ makes a call to DOS for each request for far heap space.
• Unlike Microsoft C, Digital Mars C++ does not allocate heap blocks of zero size if a program requests them. NULL is returned instead.
• Unlike Microsoft C, Digital Mars C++ cleans up free heap space when a program releases a block of memory.
• Digital Mars C++ does not support the Microsoft C _heapadd, _bheapadd, and _nheapadd functions because its heap manager calls DOS each time a program requests far heap space.
• Digital Mars C++ does not support the Microsoft _expand, _bexpand, _fexpand, and _nexpand routines. Use realloc in place of _expand or _nexpand calls (or wherever possible), and farrealloc in place of _bexpand or _fexpand. However, be aware that realloc and farrealloc, unlike _expand, can move the block to a new location to satisfy the reallocation request, in which case they will return the new location of the reallocated block.
• Digital Mars C++ does not support these additional non-standard Microsoft memory management functions: _heapchk, _bheapchk, _fheapchk, _nheapchk, _heapwalk, _bheapwalk, _fheapwalk, _nheapwalk, _heapset, _bheapset, _fheapset, _nheapset, _heapmin, _bheapmin, _fheapmin, _nheapmin.

Huge memory model support

Digital Mars C++ also supports the Microsoft halloc and hfree functions.

Support for based pointers

__based(__segname("_CODE"));
__based(__segname("_DATA")); 
__based(__segname("_STACK")); 

Assembly Language Interface

• DS is not equal to SS for the Compact and Large memory models.
• For C functions (those declared as __cdecl or extern "C"), double and float values are returned in the registers, rather than in a static memory locaton. For C++, FORTRAN, and Pascal functions, double and float values are returned on the stack in a manner compatible with Microsoft C.
• In Digital Mars C++, structs, floats, and doubles are returned by allocating a temporary variable on the stack and passing a hidden pointer to it. The called function copies the return values into this variable and returns a pointer to it.

Compiler Options

To convert your old Microsoft compiler command lines to the most similar Digital Mars command lines possible, use the CL utility. CL automatically converts Microsoft command lines to SC command lines, and then runs SC.

• Keywords
• Predefined macros
• Header files
• Libraries
• Memory model support
• Assembly language interface
• Compiler options

Keywords

Digital Mars C++ supports many of Borland C's nonstandard keywords. The following table lists Borland's extended keywords and their Digital Mars counterparts:

Predefined Macros

Digital Mars C++ supports many of Borland C's nonstandard macros. The following table lists Borland's extended macros and their Digital Mars counterparts:

Header Files

Library Functions

Digital Mars C++ supports most Borland library functions. The table below lists those Borland functions that the Digital Mars C++ run-time library does not support:

Some Digital Mars C++ run-time library functions have different types or numbers of arguments from their Borland counterparts, return different values, or support different defined constants. For information on specific functions see the Runtime Library Reference.

Borland Graphics Interface

Memory Models

• In the Digital Mars Compact and Large models, unlike their Borland counterparts, the stack and data segments are not the same. Near pointers in these models are relative to the data segment and cannot access automatic or parameter variables. Check your assembly language source code to ensure that SS is not assumed to be equal to DS.
• Digital Mars' malloc() functions take an unsigned int as an argument for 16-bit memory models and take an unsigned long for 32-bit models. Therefore, the 16-bit models limit the size of a data object to 64KB blocks. To allocate a block greater than 64KB, call farmalloc, which takes an unsigned long.
• At program startup, Borland's memory-management routine allocates all available DOS memory as one block of far heap space. In contrast, Digital Mars C++ makes a call to DOS for each request for far heap space.
• Digital Mars C++ does not support these additional non-standard Borland memory-management functions: heapcheck, farheapcheck, heapfillfree, farheapfillfree, heapwalk, farheapwalk, coreleft.

Huge memory model support

Digital Mars C++ does not support the Borland halloc and hfree functions. Convert calls to these functions to the corresponding Digital Mars far calls (which have unsigned long argument types and can, therefore, allocate huge blocks) or store the affected data in a different structure (a huge array, for example).

Assembly Language Interface

• DS is not equal to SS for the Compact and Large memory models.
• For C functions (those declared as __cdecl and extern "C"), double and float values are returned in registers, rather than in a static memory location. For C++, FORTRAN, and Pascal functions, double and float values are returned on the stack in a manner compatible with Borland C.
• In Digital Mars C++, structs, floats, and doubles are returned by allocating a temporary variable on the stack and passing a hidden pointer to it. The called function copies the return values into this variable and returns a pointer to it.
• Digital Mars C++ does not support the _FLAGS register variable.
• Digital Mars C++ does not store long double values as 10-byte quantities; they are the same as doubles (8 bytes).

Compiler Options

To convert Borland compiler command lines to the most similar Digital Mars command lines possible, use the BCC utility. BCC automatically converts Borland C++ command lines to SC command lines and then runs SC.