Compiling Code
The Command Line
The compiler comes in two forms, the command line tools and the IDDE. The free downloadable version is run from the command line. The IDDE version comes on the CD. To run the command line tools, open a console window. Under Windows XP, click on [Start] then [Command Prompt].
A Simple Example
To illustrate compiling the classic hello world! program using the console compiler, create a text file hello.c with the contents:
#include <stdio.h> void main() { printf("hello world!\n"); }
Compile and link it with the following command:
dmc hello
which will compile hello.c to create the object file hello.obj which is linked to generate the executable hello.exe. To run the executable type:
hello
which will print:
hello world!
Compilation Process
Digital Mars C++ is an extremely fast one-pass compilation system with highly effective register utilization. An optional global optimization facility provides tight and efficient object code. Run the compiler from within the IDDE or more traditionally using dmc from the console command line.
The compiler executables can compile either C or C++ files. The preprocessor is integrated in; but for specialized applications a separate standalone preprocessor is also included.
As it runs, the compiler reads the source files, executes all preprocessor functions, checks for any syntax errors, optimizes the program, generates machine code, and outputs an object (.obj) file suitable for input to the linker.
Startup code
Programs compiled with Digital Mars C++ call one of several startup functions before passing control to a program's main() routine. (The startup function called depends on what kind of application is being built, as described in __acrtused.) This startup function performs any required initializations such as initializing data and running the static constructors.
There are several files that contain startup functions (all with different names) in several subdirectories of the Digital Mars \dm\src directory.
Compiler Options
Compiler options customizes the way Digital Mars C and C++ compile programs. Specify compiler options via the Build page in the IDDE's Project Settings dialog box or by passing options as command line arguments to the dmc (or sc) control program. All compiler options are optional; the default settings are oriented towards generating a complete compiled program.
Specifying compiler output
By default, Digital Mars C++ compiles and links the source files given on the command line and generates an executable file whose name is the same as the name of the first source file with the extension replaced with .exe. To generate different files or additional files use the dmc options described below or via corresponding options in the Compiler Output subpage on the Build page of the IDDE's Project Settings dialog box.
Support for C language features
Support for C++ language features
Digital Mars C++ supports run time type identification, exception handling. These C++ language features are not enabled by default; to enable them, specify compiler options as described below.
Suppressing warnings and errors
By default, the compiler generates warnings about certain constructs that, while legal, are likely mistakes. Warnings can be disabled individually or as a group. Warnings can also be treated as errors. In the IDDE, these are set via the Compiler Output subpage on the Build page of the IDDE's Project Settings dialog box. See Warnings and associated warning numbers for messages.
Dynamic profiling
See Trace Dynamic Profiling.
Choosing a memory model
When programming for Win32, it is not necessary to choose a memory model. The compiler defaults to compiling for Win32. Compiling 16 bit code or DOS extender code will require choosing a memory model.
Memory model choices trade off between meeting minimum system requirements, maximizing code efficiency, and accessing as much available memory as possible.
For more information, see Choosing a Memory Model. To specify the memory model, use the dmc options described below or use the Memory Models subpage on the Build page of the IDDE's Project Settings dialog box.
For more information on memory allocation, see Using Assembly Language Functions.
Specifies the memory model to compile for. The default for dmc is the Win32 model (-mn).
Note: For Win32, it is imperative that SS==DS==ES at all times. Otherwise the Win32 API functions will crash. (CS need not be equal to SS, FS is reserved for thread-related information, and GS is available for general use.) Thus, SS==DS==ES for the Win32 (-mn) memory model.
Here are some example combinations of memory model specifiers:
The program stack
Win32 programs generally do not need to worry about stack overflows, but other memory model options require some attention to it.
In all memory models, the stack grows downward toward statically allocated data. If the stack grows larger than its allocated size, statically allocated data are destroyed, and the program performs unpredictably. To check if the stack grows past the allocated limit, use the special function called _chkstack.
Call _chkstack from functions that potentially use large amounts of stack space, such as recursive functions. If the stack exceeds the allocated size, _chkstack aborts the program with the message:
Stack overflow
Use the -s compiler option (see below) to generate stack-overflow checking code on entry to every function, although this increases program size and reduces speed. Use stack-overflow checking if you suspect stack-overflow errors (a symptom is a mysterious crash).
Controlling stack size for DOS programsControl a DOS program's stack size in the following ways:
- Use the =nnnn command line option (see below) when running the program.
Include the declaration
unsigned __cdecl _stack = nnnn;
in the program source code, where nnnn is the size in bytes that you want the stack to be. For C++ code, use:
extern "C" unsigned __cdecl _stack = nnnn;
Note that the declaration must appear just as described above to work correctly.
- Modify the c.asm startup module.
For Win16 compilations, use the linker's /STACK switch to set the stack size.
For a DOS program, do not use the linker /STACK parameter to set the stack size; use one of the methods described above. The default stack size is 8192 bytes for 16-bit compilations and 16KB for 32-bit compilations.
Aligning data objects
In 16-bit compilations, the compiler align data objects on 16-bit word boundaries. This improves the execution speed of code running on CPUs with a 16-bit data bus (like the 80286). The default alignment for 32-bit compilations is on 32-bit boundaries. The default alignment of structure members depends on the memory model used for the compilation. Change the alignment within structures via the -a compiler option (see the section "-a Specify structure alignment" later in this chapter), so that structure members are aligned on different boundaries. This option is useful for defining a struct that maps onto a hardware device or a predefined data element.
You can control alignment only within structures; the compiler still aligns all other data objects on word or dword boundaries.
Struct member alignment can also be specified with the #pragma pack preprocessor directive. Refer to Digital Mars C++ Language Implementation for more information.
The C++ compiler does not generate structs with a size of 0 bytes if there are no nonstatic data members; the minimum size of a struct is 1 byte. This prevents new() from returning 0 when it tries to allocate an instance of a struct.
Warning: Each file referencing a structure must be compiled with the same type of alignment. If two files compiled with different alignment reference the same structure, any erroneous results could be hard to debug.
You can set this option from the IDDE in the Compiler subpage on the Build page of the IDDE's Project Settings dialog box.
Compiling for debugging
Programs compiled with Digital Mars C and C++ can be debugged using many third-party debuggers, as well as the Digital Mars C and C++ debuggers.
There are two basic types of debugger that can be used with Digital Mars C and C++ programs: fully symbolic debuggers (those that work with symbolic information, like the Digital Mars debuggers or Microsoft Corporation's CodeView), and partially symbolic debuggers (those that require only source code line numbers and map file information, such as Microsoft's Symdeb).
Compatibility with third-party C debuggers
Fully symbolic debuggers that are compatible with the data format used by Microsoft CodeView should give acceptable results when used to debug C programs compiled with Digital Mars C++, provided they can handle both C and Pascal function calling conventions. Some information on local variables may not be available due to the automatic register assignments carried out by Digital Mars compilers. (Digital Mars C and C++ make intelligent use of registers within functions, automatically assigning frequently used variables to registers. This register assignment uses any available register and not simply SI and DI.) Some debuggers cannot handle variables in registers other than SI and DI and consequently cannot display or track them.
The Digital Mars C and C++ debuggers can display, modify, and track all the local variables within a function.
Compatibility with third-party C++ debuggers
Although some other third-party C debuggers can partially debug C++ programs, they cannot obtain or handle all the information needed to fully debug C++ code. The main difficulties are these:
- C++ function names are displayed by the debugger in their mangled form, as used internally by the compiler.
- Expanding a derived class shows only those variables added in the derived class. Base-class variables cannot be seen.
- Pointers to a base class always are shown as such, even if they really point to a derived class.
- Inline functions can cause some debuggers severe problems and must be avoided.
- C debuggers have no facilities for examining class definitions or hierarchies.
To fully overcome these problems, use a C++ debugger. The Digital Mars C++ debugger provides full debugging facilities for C++ programmers. It lets class definitions be fully expanded, C++ scope rules be understood, pointer conversions be done automatically, overloaded operators be presented correctly, and function prototypes be examined.
Overview of compiler debugging options
If the compiler always generates full debugging information for all variables, then the object files would be very large, and the linker would have to remove a great deal of redundant information. You can control the level of debugging information the compiler provides with the -g option (see the section "-g Generate debugging information" later in this chapter). The compiler can produce five levels of debugging information:
- Full symbolic debugging and line number information (-g option) for all variables used in a module. Programs can be debugged at the source level with full debugging information.
- Symbolic information only (-gs option). This provides all the symbolic information but does not generate any line number information. This level conserves space and allows the debugger to work on larger programs but prevents debugging at the source level; programs must be debugged at the object code level.
- Symbolic information for global structs and typedefs, whether they are referenced or not, as well as for all variables referenced (-gh option).
- Symbolic information for each class that the generated code references, as well as for all variables referenced. (-gf option).
- Line numbers only (-gl option). This option is mainly for use with partially symbolic debuggers. It emits no symbolic information, except what is normally placed in the linker map file. This level provides full line number information. This allows debugging at the source code level, but only global symbols are available; local and static symbols cannot be observed.
Compiling with any level of debugging information significantly increases the size of the object file. The size of the executable file also increases, often doubling.
Debugging options
Linking for debugging
When compiling for debugging, the compiler automatically passes the correct information to the linker. If you use a separate link step, pass the appropriate options to the linker explicitly.
These are the options for Digital Mars OPTLINK or Microsoft LINK for all levels of debugging information:
- /m (create map file)
- /co (link for CodeView)
- To use Microsoft LINK, specify the -Llinker compiler option. linker is which linker to run. Only Microsoft LINK 5.1 or later versions correctly handle the symbolic information.
Note: When creating debugging information, OPTLINK uses CodeView Version 4 format if any object module in the link step contains CV4 records. In this case, it throws away any CV3 records it encounters. OPTLINK creates an executable with CV3 debugging format only if every debug record is in CV3 format. To retain CV3 records in objects, run OPTLINK with the /CO:3 or /NOCVPACK options, and then run Microsoft's CVPACK.
See the User's Guide and Reference for more information on using the Digital Mars debuggers.
Notes on debugging with CodeView
Unlike the Digital Mars C++ debugger, which is statement oriented, the Microsoft CodeView debugger is a line oriented source debugger. At the source code level, the smallest unit of code CodeView can handle is a line, not a statement. To use CodeView most effectively with Digital Mars C++, follow these rules:
- CodeView cannot handle inline functions.
- Make each statement in the source code occupy a separate line. Also, CodeView does not break down complex macros, so if you suspect a macro of containing a bug, write the macro as a regular source statement.
- Compile source modules with the -g option and link with the /co (CodeView) and /noi (do-not-ignore case) options.
- An apparent limitation causes CodeView for Windows to halt unexpectedly if you attempt to debug executables that contain large tables, such as applications built with MFC and compiled with Digital Mars C++. One workaround is to find the table that exceeds the maximum size, rebuild the objects that contribute to it without debugging information, and then re-link. Or, relink the with OPTLINK using the /NODEBUG and /DEBUG options (no need to recompile).
Refer to Microsoft's documentation for information on CodeView.
Debugging aids in the run-time library
There are a number of features in the run-time library that help detect common run-time errors:
If a NULL pointer is passed to printf where a pointer to a string is expected, printf uses the string "null" instead. For example:
printf("String '%s'\n", NULL);
prints:
String '(null)'
If a NULL near pointer is de-referenced, the string that is at location 0 in the data segment is "ERROR: NULL pointer." For example:
char buf[100]; strcpy(buf, NULL); printf("String '%s'\n", buf);
prints (for code compiled with the Tiny, Small, and Medium models):
String 'ERROR: NULL pointer'
If you optimize the code being debugged, remember that optimization moves variables and sections of code and creates only one scope per function. Avoid using the same variable names in nested scopes.
Specifying linker and assembler options
The compiler options described below let you run third party linkers and assemblers on the dmc command line, or pass options to the linker or assembler.
Controlling segmentation
When writing large 16 bit Windows programs, you sometimes need to place functions that frequently call each other into the same segment. This optimization helps speed execution. This section explains how to segment your code. Use these methods independently or in any combination.
- Use the -NTcsname compiler option, which sets the code segment for a file to be csname.
- Use the -NS compiler option, which creates a new code segment each time the compiler encounters a global far function. The name of the segment is the name of the function, with _TEXT appended.
- For Compact and Large model compilations, use the -GT compiler option, which places arrays larger than a specified number of bytes in far data segments instead of DGROUP.
- Use the #pragma:
#pragma SC cseg csnamewhich places what follows in the segment csname. The -NS option overrides the #pragma for global far functions.
Preventing data segment overflow
Many large programs run out of space in the default data segment DGROUP. The solution is to move some data out of DGROUP into other segments. Here are some methods you can use:
- Compile with the -R option, which causes switch statement tables to be placed in the code segment rather than in DGROUP.
- Declare literal strings you want to place in the code segment with the type modifier __cs.
- Compile with the -Ns option, which places strings that are part of expressions into the code segment rather than in DGROUP.
- Declare statically allocated data as far; for example: int __far array[100];
For information on declaring data as far and placing data in the code segment, see Choosing a Memory Model.
Data declared as far should be data that is relatively large and is accessed relatively infrequently, as more overhead is required to access those data.
If another module should reference far static data, declare it as far, as in:
extern int __far array[];
Otherwise, the compiler assumes that the data is in the default data segment DGROUP. Digital Mars C++ differs from some other compilers in this respect- extern data is not assumed to be in DGROUP. Although our implementation involves more work on the programmer's part (putting __far in the right places), the control this offers makes it very worthwhile for high-performance applications. Also, most applications need less than 64KB of static data, and so need not pay a penalty for assuming that extern data is not accessible via DS.
Virtual function tables (vtbls)
The compiler generates a table of pointers to functions for each class that has virtual functions or is derived from a class with virtual functions. In the small data models (Tiny, Small, and Medium), it creates these tables in DGROUP by default. For the Compact and Large data models, it writes these tables to the code segment by default.
To further minimize use of code space in Compact and Large model programs, place vtbls in far data segments with -NV.
Segmentation options
Use these options to control how the compiler segments the program:
Code generation options
These options control how the compiler generates code.
Compiling for Windows
See Windows Prolog/Epilog Code Generation.