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
