What happens when you launch a D application ?

[Mike via Digitalmars-d-learn]

On Saturday, 23 May 2015 at 10:57:22 UTC, Suliman wrote:
>>Every D program is started as if it were a C program.
> Why is so necessary?

It's not actually necessary.  You could implement the `_start` 
function in your D program.  Here's a D program without any C 
runtime, D runtime, or main.

long __d_sys_write(long arg1, in void* arg2, long arg3)
{
     ulong result;

     asm
     {
         "syscall"
         : "=a" result
         : "a" 1,
         "D" arg1,
         "S" arg2,
         "m" arg2,
         "d" arg3
         : "memory", "cc", "rcx", "r11";
     }

     return result;
}

extern(C) void __d_sys_exit(long arg1)
{
     asm
     {
         "syscall"
         :
         : "a" 60,
         "D" arg1,
         : "memory", "cc", "rcx", "r11";

     }
}

extern(C) void _start()
{
     // you could put this in a main function and
     // call main() from here.
     auto text = "Hello, World!\n";
     __d_sys_write(1, text.ptr, text.length);

     __d_sys_exit(0);
}

Compile with:
gdc -fno-emit-moduleinfo -nostartfiles test.d

This produces a 733 byte "Hello World" binary.  
-fno-emit-moduleinfo will prevent the compiler from generating a 
lot of auxiliary code that's not needed for this program.  
-nostartfiles will prevent the C runtime from being linked in.

This is a nice little experiment, but once you start trying to 
use structs, classes, arrays, exceptions, or any other feature of 
D, you will need more and more of the D runtime.

There may also be some things generated by GDC that get called 
implicitly by the C runtime's initialization procedures (e.g. 
_init, _fini, etc..) but I'm not that knowledgeable about the 
compiler's codegen.

>
> What about C++ and other languages? Does they have more then 
> one main?

C++ is a superset of C, so in my experience, C++ only needs C 
main.  However, C++ has static constructors that get called by 
some of the initialization procedures in the C runtime before 
actually calling main.  I wonder if C++ free functions mangled in 
C++...I'm not sure.

>
> Why it's more than one main is needed? Why D apps can't start 
> with single main?

I don't think the current implementation is necessary.  There are 
probably many different ways to implement D program 
initialization.  I don't see why D runtime couldn't just 
implement _start, omit the C runtime altogether, and do 
everything in D, but I trust the compiler implementers have their 
reasons.

If you're interested in digging deeper, you'll probably have to 
ask questions on D.GNU to get the attention of the compiler 
implementers.

Mike