How to call libc library in C#

env.md

$uname -a
Linux 4b44f62b3201 5.10.104-linuxkit #1 SMP Wed Mar 9 19:05:23 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
$ mono -V
Mono JIT compiler version 6.12.0.179 (tarball Tue May 17 21:33:45 UTC 2022)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
        TLS:           __thread
        SIGSEGV:       altstack
        Notifications: epoll
        Architecture:  amd64
        Disabled:      none
        Misc:          softdebug
        Interpreter:   yes
        LLVM:          yes(610)
        Suspend:       hybrid
        GC:            sgen (concurrent by default)
$ mono Main.exe
return from fork() is 92
pid is 89
end!
return from fork() is 0
pid is 92
end!

Main.cs

using System;
using System.Runtime.InteropServices;

namespace PInvokeSamples
{
    public static class Program
    {
        [DllImport("libc.so.6")]
        private static extern int fork();
        [DllImport("libc.so.6")]
        private static extern int getpid();

        public static void Main(string[] args)
        {
            int pid, ret;
            if((ret = fork()) == -1){
                Console.WriteLine("fork() failed.");
                return ;
            }
            // Invoke the function and get the process ID.
            pid = getpid();
            Console.WriteLine("return from fork() is " + ret);
            Console.WriteLine("pid is " + pid);
            Console.WriteLine("end!");
        }
    }
}

ref.md

https://docs.microsoft.com/ja-jp/dotnet/standard/native-interop/pinvoke