Creating a Linux Daemon (service) in Delphi
With the introduction of the Linux target for Delphi, a wide range of possibilities are opened up to Delphi developers, to create Linux server applications. Unfortunately there are currently a limited number of project types available from the RAD Studio IDE, and those do not include creating a service (or Daemon as it’s called in the Linux world).
*note* In this post I am assuming you are already configured to deploy an application to Linux and understand how to launch it from the Linux command line. If this is not the case, see my earlier posts covering these subjects:
- http://chapmanworld.com/2017/02/28/embarcadero-delphi-linux-bootcamp/
- http://chapmanworld.com/2016/12/29/configure-delphi-and-redhat-or-ubuntu-for-linux-development/
Under the Linux operating system, a Daemon is simply an executable which continues running while ignoring the standard input and output streams. That-is, they do not accept input from the keyboard, and they do not present data to the screen (except in special circumstances). So an application which simply continues to run in the background without using the standard input and output, is a Daemon.
One other property of a Daemon application is that it runs in the background. Applications run from the terminal window in Linux are provided with the standard input and output streams, and therefore the terminal becomes unusable until the application terminates. To demonstrate, create a new command line application and set it’s source code to read as follows…
program Project1;
{$APPTYPE CONSOLE} {$R *.res} uses
System.SysUtils; begin
try
while true do sleep(100);
except
on E: Exception do
Writeln(E.ClassName, ': ', E.Message);
end;
end.
If you run this application from the command line, you’ll find that it enters into an infinite loop (the while loop), and you do not regain control of the terminal window until you issue [CTRL] + [C] to abort the application.
Lets now turn this application into a Daemon. Alter your source code to read as follows…
program Project1;
{$APPTYPE CONSOLE} {$R *.res} uses
System.SysUtils,
Posix.Unistd; begin
try
if fork()<>0 then begin
exit;
end;
while true do sleep(100);
except
on E: Exception do
Writeln(E.ClassName, ': ', E.Message);
end;
end.
Now, when you attempt to run this application from the command line, it will immediately exit. It will appear as though the application did nothing. However, issue the instruction:
ps -e
and you should see something like this:
You’ll notice that “Project1” is actually still running, it’s simply been moved to the background.
As a side note, you can kill the application by making a note of it’s process ID (in my case 16898 from the screenshot) and issuing the appropriate kill instruction:
kill 16898
You could now replace the infinite while loop with whatever application logic you need within your daemon.
How does this work?
The key to this working is the “fork()” method, which can be found in the unit “Posix.Unistd”. The fork method is a part of the standard posix API, who’s documentation may be found here: https://linux.die.net/man/3/fork
What fork does is to create a copy of which-ever process calls it, in our case Project1.
So at the moment fork is called, our application execution either continues in the originating process, or it continues in the copy process.
The return value of the call to fork tells us which instance of the process we are executing in. A return value of zero tells us that we are executing in the child process (the copy), and any other value tells us that we are executing in the parent process (the originator). In fact, when the return of fork is not zero, it is the process ID of the newly created child process. We simply call ‘exit’ to exit the application when we determine that we are in the parent process, allowing the child copy to proceed on to the infinite while loop.
Conclusion
At this point you are able to create daemon applications. You may wish to research the Linux (posix) API’s in a little more detail to see how to handle ‘signals’ which may be used to gracefully terminate your application, rather than using the brute force termination of a kill instruction. You might also want some way to communicate with your daemon process. There are many ways to communicate with the process, but possibly the most familiar to a Delphi developer would be to use TCP/IP. Turning your service into a TCP/IP server would allow you to send data and instructions to it.
Regardless of what you decide to do with your daemon, I hope you found this brief instructional useful.
http://chapmanworld.com/2017/04/05/creating-a-linux-daemon-service-in-delphi/
Creating a Linux Daemon (service) in Delphi的更多相关文章
- Linux daemon与service 学习笔记
service 常驻在内存中的进程,且可以提供一些系统或网络功能,就是服务. daemon service的提供需要进程的运行,所以实现service的程序我们称为daemon. eg: 实现 ...
- [转] Linux Daemon Writing HOWTO
Linux Daemon Writing HOWTO Devin Watson v1.0, May 2004 This document shows how to write a daemon in ...
- Linux命令service - 系统服务管理(转)
用途说明 service命令用于对系统服务进行管理,比如启动(start).停止(stop).重启(restart).查看状态(status)等.相关的命令还包括chkconfig.ntsysv等,c ...
- Azure的CentOS上安装LIS (Linux Integration Service)
Azure上虚拟化技术都是采用的Hyper-v,每台Linux虚拟机都安装了LIS(Linux Integration Service).LIS的功能是为VM提供各种虚拟设备的驱动.所以LIS直接影响 ...
- 创建 SysV 风格的 linux daemon 程序
本文介绍如何使用 C 语言创建 Linux 系统中 SysV 风格的 daemon 程序.注意:这是一种旧式的 daemon 程序写法,进入 systemd 时代后是不需要通过这样的方式创建 daem ...
- 在 CentOS7 上将自定义的 jar 包注册为 linux 服务 service
在 CentOS7 上将自定义的 jar 包注册为 linux 服务 service 1.在 /etc/rc.d/init.d/ 目录下创建一个名字和服务名完全相同的 shell 脚本文件 joyup ...
- 将Apache加入到linux系统service
将Apache加入到linux系统service 将apache加入到linux系统服务,用service命令来控制apache的启动和停止. 本文由乌合之众瞎写http://www.cnblogs. ...
- 时隔两年最近再次折腾opensuse 的一些笔记 - opensuse linux java service shell
时隔两年最近再次折腾opensuse 的一些笔记 - opensuse linux java service shell opensuse 一些常用命令: service xxx start/s ...
- Linux: service network/Network/NetworkManager
Linux:service network/Network/NetworkManager start 这三种有什么不同? 1.network service的制御网络接口配置信息改动后,网络服务必须从 ...
随机推荐
- 关于JDBC连接数据库时出现的Public Key Retrieval is not allowed错误
问题描述 最近在学习MyBatis框架,参考官方的文档通过配置文件的方式已经实现了通过Configuration配置文件和mapper映射文件访问mysql8数据库,于是想试试不使用XML文件去构建S ...
- R 语言文件读写
1. working directory:工作目录 > getwd() > setwd("C:/data") # 设定当前工作目录 2. 读取格式化的 table &g ...
- 华为软件开发云(DevCloud):免费可商用的项目管理工具
在软件开发技术和理念层出不穷的今天,如何更快的适应变化的环境,更好的满足客户的需求,已经成为决定从小到大各种规模企业能否活下去的关键. 天下武功唯快不破,在当今大环境中更是如此,微服务,敏捷开发,新的 ...
- 自己动手编写一个VS插件(六)
作者:朱金灿 来源:http://blog.csdn.net/clever101 在上篇中我们已经实现了创建和显示一个工具栏出来,它的效果图是这样的: 现在我们实现一些简单功能,具体就是单击按钮弹出一 ...
- iOS 注册或登录页面(UILable,UITextField,UIButton)
注册或登录页面 例如下面的附图 1,为了在这里展示UITextField文本框关联的键盘设置.在这里,"password"和"判定password"关联键盘被设 ...
- 深刻认识OpenStack
OpenStack 1 OpenStack简单介绍 Openstack archite ...
- 在WPF窗体中重绘
原文:在WPF窗体中重绘 写这篇主要是为了验证任何元素自身都具备绘图功能. 在默认Window中重写OnRender方法 protected override void OnRender(Draw ...
- DDD实战3 领域层的设计
1.新建一个解决方案文件夹 取名Product 2.在Product解决方案文件夹下面创建一个.net core 类库项目 取名Product.Domain,引用项目DDD.Base项目 3.在类库下 ...
- 微信小程序支付结果 c#后台回调
又为大家带来简单的c#后台支付结果回调方法,首先还是要去微信官网下载模板(WxPayAPI),将模板(WxPayAPI)添加到服务器上,然后在打开WxPayAPI项目中的example文件下的 Nat ...
- 作为学术用的 matlab
1. 可重复实验 程序实现的所谓随机,并非完全的随机,而是由某一算法(或者再需要一个种子值)生成出来的. randn('state', 0) % set state so that example c ...