• Kirill (6/6) Oct 21 2021 I am not a compiler expert, but I genuinely would like to know
• Mike Parker (13/19) Oct 22 2021 The entry point for your program is a function `_start`. That's
• Kirill (2/3) Oct 22 2021 Thank you for such a clear explanation Mike and for a quick reply!
• Dukc (8/11) Oct 22 2021 It is possible, in both C and D, to write your own `_start` and
• Kagamin (1/1) Oct 22 2021 Actually C runtime is many megabytes in size.
• Elronnd (7/8) Oct 22 2021 A couple of samples:
• user1234 (9/15) Oct 25 2021 in addition to the other answers, there's also the parameters of
• bauss (8/11) Oct 25 2021 Not just that, but you shouldn't use it at all, rather you can't
• SealabJaster (6/8) Oct 25 2021 Also, all programs running under the D runtime have special

Kirill <kirill.saidov mail.com> writes:

I am not a compiler expert, but I genuinely would like to know 
why we have Dmain.

I've been looking at the generated assembly code recently and 
noticed the _Dmain function. I didn't notice it before. Then 
there is main, where Dmain is called.

Why is that?

Mike Parker <aldacron gmail.com> writes:

On Friday, 22 October 2021 at 05:54:21 UTC, Kirill wrote:
 I am not a compiler expert, but I genuinely would like to know 
 why we have Dmain.

 I've been looking at the generated assembly code recently and 
 noticed the _Dmain function. I didn't notice it before. Then 
 there is main, where Dmain is called.

 Why is that?

The entry point for your program is a function `_start`. That's 
implemented in the C runtime, which all D programs depend on. It 
in turn calls `main`, as it does for C and C++ programs.

The D compiler generates an `extern(C) main` function that hands 
control off to DRuntime for initialization, then in turn calling 
your application `main` (a.k.a. `_DMain`).

See:
https://github.com/dlang/druntime/blob/master/src/rt/dmain2.d#L245

If you declare your main function as `extern(C)`, then the 
compiler does not generate one and you get handed control from 
the C runtime. Which in turn means you have to handle 
initialization of the D runtime yourself.

Kirill <kirill.saidov mail.com> writes:

On Friday, 22 October 2021 at 07:00:25 UTC, Mike Parker wrote:
 [...]

Thank you for such a clear explanation Mike and for a quick reply!

Dukc <ajieskola gmail.com> writes:

On Friday, 22 October 2021 at 07:00:25 UTC, Mike Parker wrote:
 The entry point for your program is a function `_start`. That's 
 implemented in the C runtime, which all D programs depend on. 
 It in turn calls `main`, as it does for C and C++ programs.

It is possible, in both C and D, to write your own `_start` and 
not depend on the C runtime.

This is something you want to do only if necessary though - C 
runtime is so small that you don't want to drop it for any 
realistic desktop or even a smartphone program. The usual reason 
to do this is if there is no C runtime easily available - meaning 
mainly embedded targets.

Kagamin <spam here.lot> writes:

Actually C runtime is many megabytes in size.

Elronnd <elronnd elronnd.net> writes:

On Friday, 22 October 2021 at 09:01:53 UTC, Kagamin wrote:
 Actually C runtime is many megabytes in size.

A couple of samples:

$ wc -c /usr/lib/libc-2.33.so
2150424 /usr/lib/libc-2.33.so

% wc -c /lib/libc.so.7
  1981952 /lib/libc.so.7

I would hardly call two megabytes 'many'.

user1234 <user1234 12.de> writes:

On Friday, 22 October 2021 at 05:54:21 UTC, Kirill wrote:
 I am not a compiler expert, but I genuinely would like to know 
 why we have Dmain.

 I've been looking at the generated assembly code recently and 
 noticed the _Dmain function. I didn't notice it before. Then 
 there is main, where Dmain is called.

 Why is that?

in addition to the other answers, there's also the parameters of 
main:

     int argc, char** argv

becomes

     string[]

so with the D main you're directly in the the D "domain". In 
theory from that point, you don't need to ever use `strlen` for 
example.

bauss <jj_1337 live.dk> writes:

On Monday, 25 October 2021 at 07:49:44 UTC, user1234 wrote:
 so with the D main you're directly in the the D "domain". In 
 theory from that point, you don't need to ever use `strlen` for 
 example.

Not just that, but you shouldn't use it at all, rather you can't 
use it!

You might be able to call it, but it won't work as expected 
because strings are not zero-terminated in D, unless the string 
is a literal.

strlen looks for \0, so using it for a string that isn't 
zero-terminated is asking for trouble.

SealabJaster <sealabjaster gmail.com> writes:

On Monday, 25 October 2021 at 07:49:44 UTC, user1234 wrote:
 in addition to the other answers, there's also the parameters 
 of main:

Also, all programs running under the D runtime have special 
command line arguments(`--DRT` arguments). The Runtime needs to 
be able to parse those arguments at some point (among other 
initilisation), hence why the runtime handles the "true" main for 
us.