# OpenWRT procd启动过程

## Overview

我们都知道 linux kernel 启动完成以后，会启动 /sbin/initd 作为第一个用户空间的进程。同样地，OpenWrt 也遵循这个规则，不同的是 OpenWrt 的主服务器进程是 procd，而 procd 是系统启动过程由 initd 切换过来的。 

## Checkout source

使用以下命令克隆 procd 源码到本地：

```bash
# git clone git://git.openwrt.org/project/procd.git
```

## Initd process

initd 进程是在内核启动完成后启动的第一个用户空间进程，它的主程序是 init.c，主要做了以下的一些工作：

1. mount filesystem, proc & sysfs & cgroup & tmpfs & devpts  
2. create essential temporary directory, /tmp/run & /tmp/lock & /tmp/state  
3. set PATH environment, PATH="/usr/sbin:/sbin:/usr/bin:/bin"  
4. set debug level by finding key is "init\_debug" from /proc/cmdline  
5. register timeout callback, which call watchdog  
6. fork one child process to execute "/sbin/kmodloader /etc/modules-boot.d/", which will load kernel modules, and parent process continue  
7. process management: procd and preinit. For procd, fork a child process to run as hotplug daemon with command "/sbin/procd -h /etc/hotplug-preinit.json", the parent process register plugd\_proc to uloop\_process. For preinit, fork a child process to execute "/bin/sh /etc/preinit", and the parent process register preinit\_proc to uloop\_process  
8. run loop  

注意：这里的第7点，用到了 uloop 的进程管理。对于调用者，只需要关心 struct uloop\_process 的 pid 和 cb 成员，其中，cb 是回调，当子进程退出时，父进程会收到 SIGCHLD 信号而调用。通过 uloop\_process\_add 把 struct uloop\_process 添加到 uloop 事件循环中。

## Initd to Procd

我们透过简化代码分析 initd 进程是如何切换到 procd 进程：

```c
/* global various */
static struct uloop_process preinit_proc;

void preinit(void)
{
    char *init[] = { "/bin/sh", "/etc/preinit", NULL };

    preinit_proc.cb = spawn_procd;
    preinit_proc.pid = fork();
    if (!preinit_proc.pid) {
        execvp(init[0], init);
        ERROR("Failed to start preinit\n");
        exit(-1);
    }
    if (preinit_proc.pid <= 0) {
        ERROR("Failed to start new preinit instance\n");
        return;
    }
    uloop_process_add(&preinit_proc);
}
```

根据上面提到 uloop 的进程管理可知，一旦执行完 `/bin/sh /etc/preinit`，则会执行 spawn\_procd 回调。spawn\_procd 的代码段：

```c
static void spawn_procd(struct uloop_process *proc, int ret)
{
    char *argv[] = { "/sbin/procd", NULL};
    struct stat s;

    if (plugd_proc.pid > 0)
        kill(plugd_proc.pid, SIGKILL);

    if (!stat("/tmp/sysupgrade", &s))
        while (true)
            sleep(1);

    execvp(argv[0], argv);
}
```

spawn\_procd 作为 initd 进程中 uloop\_run 的回调函数，其 pid = 1(initd)，通过执行 execvp 拉起一进程取代原进程，这里是 initd 进程被 /sbin/procd 进程取代。至此，procd 成为 OpenWrt 系统的主服务进程(pid = 1)。

## Procd process

TODO


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://kysonlok.gitbook.io/blog/openwrt/how_procd_work.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.