Runtime internals

List of compiler hooks

Runtime Hooks

D program startup sequence

This section provides a general overview of a typical D program's startup sequence. Each operating system and compiler implements this procedure slightly differently, but the fundamental procedure is the same.

Symbols

The startup sequence is essentially an orchestration of the following symbols:

• "C main" - In DMD and LDC C main seems to be provided by the compiler, while in GDC it is provided by the D Runtime.

extern(C) void main(int argc, char **argv)

• "D main". This is the `main` function provided by the user in nearly every D program. It is D-mangled making it distinguishable from C main above.

int main(char[][] args)

• _Dmain is an alias provided by the compiler to refer to D main.

int _Dmain(char[][] args)

• rt_init() is the function called before D main to initialize the D Runtime. The D runtime initialization procedure is quite complex and out of scope for this document.

extern(C) int rt_init()

• rt_term() is the function called after D main to unwind the initialization of the D Runtime.

extern(C) int rt_term()

• _d_run_main orchestrates the calling sequence of rt_init(), D main, and rt_term(). mainFunc is a pointer to D main.

int _d_run_main(int argc, char **argv, void* mainFunc)

Sequence

The following pseudocode illustrates the calling sequence of the symbols above.

Every D program is started as if it were a C program. On Linux, the operating system calls _start which is defined the C Runtime. After performing C program initialization, the C Runtime calls C main just like a typical C program.

// 1.  _start calls C main
extern(C) void main(int argc, char **argv)
{
    // 2. C main calls _d_run_main passing program arguments and a pointer
    // to D main
    return _d_run_main(argc, argv, &_Dmain);    
}

alias extern(C) int function(char[][] args) MainFunc;
int _d_run_main(int argc, char **argv, MainFunc mainFunc)
{
    // 3. D some operating-system-specific logic to change argc and argv
    // to char[][] args
    char[][] args;
    
    // 4. Do D runtime initialization
    int result = EXIT_FAILURE;
    if(rt_init()
    {
        // 5. call D main
        result = mainFunc(args);
    }
    
    // 6.  unwind runtime initialization
    if (!rt_term())
    {
        result = (result == EXIT_SUCCESS) ? EXIT_FAILURE : result;
    }
    
    // 7. Return to C main
    return result;
}