D programs generally require:

1. The D runtime library to be linked in, because many features of the core language require runtime library support.
2. The main() function to be written in D, to ensure that the required runtime library support is properly initialized.

To link D functions and libraries into C programs, it's necessary to only require the C runtime library to be linked in. This is accomplished by defining a subset of D that fits this requirement, called BetterC.

Better C

When BetterC is enabled, the predefined Conditional Compilation D_BetterC can be used for conditional compilation.

An entire program can be written in BetterC by supplying a C main() function:

extern(C) void main()
{
    import core.stdc.stdio : printf;
    printf("Hello betterC\n");
}

> dmd -betterC hello.d && ./hello
Hello betterC

Limiting a program to this subset of runtime features is useful when targeting constrained environments where the use of such features is not practical or possible.

BetterC makes embedding D libraries in existing larger projects easier by:

1. Simplifying the process of integration at the build-system level
2. Removing the need to ensure that Druntime is properly initialized on calls to the library, for situations when an initialization step is not performed or would be difficult to insert before the library is used.
3. Mixing memory management strategies (GC + manual memory management) can be tricky, hence removing D's GC from the equation may be worthwhile sometimes.

Note: BetterC and ImportC are very different. ImportC is an actual C compiler. BetterC is a subset of D that relies only on the existence of the C Standard library.

Retained Features

Nearly the full language remains available. Highlights include:

1. Unrestricted use of compile-time features
2. Full metaprogramming facilities
3. Nested functions, nested structs, delegates and lambdas
4. Member functions, constructors, destructors, operating overloading, etc.
5. The full module system
6. Array slicing, and array bounds checking
7. RAII (yes, it can work without exceptions)
8. scope(exit)
9. Memory safety protections
10. Interfacing to C++
11. COM classes and C++ classes
12. assert failures are directed to the C runtime library
13. switch with strings
14. final switch
15. unittest
16. printf format validation

Running unittests in -betterC

While testing can be done without the -betterC flag, it is sometimes desirable to run the testsuite in -betterC too. unittest blocks can be listed with the getUnitTests trait:

unittest
{
   assert(0);
}

extern(C) void main()
{
    static foreach(u; __traits(getUnitTests, __traits(parent, main)))
        u();
}

> dmd -betterC -unittest -run test.d
dmd runpezoXK
foo.d:3: Assertion `0' failed.

However, in -betterC, assert expressions don't use Druntime's assert and are directed to assert from the C runtime library instead.

Unavailable Features

D features not available with BetterC:

1. Garbage Collection
2. TypeInfo and ModuleInfo
3. Classes
4. Built-in threading (e.g. core.thread)
5. Dynamic arrays (though slices of static arrays and pointers work)
6. Associative arrays
7. Exceptions
8. synchronized and core.sync
9. Static module constructors or destructors

simd, Vector Extensions, importc, ImportC