创建后台程序



在Unix和其他多任务计算机操作系统中，后台程序是指，作为后台进程运行的计算机，而不是由交互用户直接控制。

后台程序概念的主要好处是，后台程序可以直接启动，而不需要将其发送到精确的用户或者shell的后台（然而，这不适用于Nuttx），其状态可以在运行的时候，通过shell查询。也可以终止。

Step 1: 创建一个小的标准应用

根据 FirstOnboard Application Tutorial (Hello Sky)教程（见PX4源码开发人员文档（二）），这是一个基本程序（简化）：

..
__EXPORT int px4_daemon_app_main(int argc, char *argv[]);
..
int px4_daemon_app_main(int argc, char *argv[])
{
    while (true) {
        warnx("Hello Daemon!\n");
        sleep(1);
    }
    return 0;
}

..
__EXPORT int px4_daemon_app_main(int argc, char *argv[]);
..
int px4_daemon_app_main(int argc, char *argv[])
{
	while (true) {
		warnx("Hello Daemon!\n");
		sleep(1);
	}
	return 0;
}

这个应用的问题非常明显，如果不使用&启动，将会阻塞shell（Nuttx，并不如此，并且会出于small footprint和可靠性的原因，支持CTRL-Z / fg / bg）。为了回避这个问题，下面部分将应用转换为一个后台程序。

Step 2: 创建后台进程管理函数

主函数由后台进程管理函数替代，旧的主函数的内容现在位于后台任务/进程中

#include <systemlib/systemlib.h>

..
__EXPORT int px4_daemon_app_main(int argc, char *argv[]);
..
int mavlink_thread_main(int argc, char *argv[]);
..
int mavlink_thread_main(int argc, char *argv[])
{
    while (true) {
        warnx("Hello Daemon!\n");
        sleep(1);
        if (thread_should_exit) break;
    }

    return 0;
}
..
int px4_daemon_app_main(int argc, char *argv[])
{
    if (argc < 1)
        usage("missing command");

    if (!strcmp(argv[1], "start")) {

        if (thread_running) {
            warnx("daemon already running\n");
            /* this is not an error */
            exit(0);
        }

        thread_should_exit = false;
        daemon_task = task_spawn_cmd("daemon",
                         SCHED_RR,
                         SCHED_PRIORITY_DEFAULT,
                         4096,
                         px4_daemon_thread_main,
                         (argv) ? (const char **)&argv[2] : (const char **)NULL);
        thread_running = true;
        exit(0);
    }

    usage("unrecognized command");
    exit(1);
}

#include <systemlib/systemlib.h>
 
..
__EXPORT int px4_daemon_app_main(int argc, char *argv[]);
..
int mavlink_thread_main(int argc, char *argv[]);
..
int mavlink_thread_main(int argc, char *argv[])
{
	while (true) {
		warnx("Hello Daemon!\n");
		sleep(1);
		if (thread_should_exit) break;
	}
 
	return 0;
}
..
int px4_daemon_app_main(int argc, char *argv[])
{
	if (argc < 1)
		usage("missing command");
 
	if (!strcmp(argv[1], "start")) {
 
		if (thread_running) {
			warnx("daemon already running\n");
			/* this is not an error */
			exit(0);
		}
 
		thread_should_exit = false;
		daemon_task = task_spawn_cmd("daemon",
					     SCHED_RR,
					     SCHED_PRIORITY_DEFAULT,
					     4096,
					     px4_daemon_thread_main,
					     (argv) ? (const char **)&argv[2] : (const char **)NULL);
		thread_running = true;
		exit(0);
	}
 
	usage("unrecognized command");
	exit(1);
}

这将会启动一个新的任务，具有4096字节的堆栈，并传递非后台程序的具体指令行选项到后台主函数。典型的调用如下所示：

px4_daemon_app start

px4_daemon_app start

上面的代码没有报告状态，并且没有对多次调用后台进程进行保护。

Step 3: 添加停止/状态指令以及安全保护

具有合适的启动/停止/状态建立和附加安全保护的完整px4_daemon_app代码如下：

/**
 * @file px4_daemon_app.c
 * daemon application example for PX4 autopilot
 *
 * @author Example User <[email protected]>
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <px4_config.h>
#include <nuttx/sched.h>

#include <systemlib/systemlib.h>
#include <systemlib/err.h>

static bool thread_should_exit = false;     /**< daemon exit flag */
static bool thread_running = false;     /**< daemon status flag */
static int daemon_task;             /**< Handle of daemon task / thread */

/**
 * daemon management function.
 */
__EXPORT int px4_daemon_app_main(int argc, char *argv[]);

/**
 * Mainloop of daemon.
 */
int px4_daemon_thread_main(int argc, char *argv[]);

/**
 * Print the correct usage.
 */
static void usage(const char *reason);

static void
usage(const char *reason)
{
    if (reason) {
        warnx("%s\n", reason);
    }

    warnx("usage: daemon {start|stop|status} [-p <additional params>]\n\n");
}

/**
 * The daemon app only briefly exists to start
 * the background job. The stack size assigned in the
 * Makefile does only apply to this management task.
 *
 * The actual stack size should be set in the call
 * to task_create().
 */
int px4_daemon_app_main(int argc, char *argv[])
{
    if (argc < 2) {
        usage("missing command");
        return 1;
    }

    if (!strcmp(argv[1], "start")) {

        if (thread_running) {
            warnx("daemon already running\n");
            /* this is not an error */
            return 0;
        }

        thread_should_exit = false;
        daemon_task = px4_task_spawn_cmd("daemon",
                         SCHED_DEFAULT,
                         SCHED_PRIORITY_DEFAULT,
                         2000,
                         px4_daemon_thread_main,
                         (argv) ? (char *const *)&argv[2] : (char *const *)NULL);
        return 0;
    }

    if (!strcmp(argv[1], "stop")) {
        thread_should_exit = true;
        return 0;
    }

    if (!strcmp(argv[1], "status")) {
        if (thread_running) {
            warnx("\trunning\n");

        } else {
            warnx("\tnot started\n");
        }

        return 0;
    }

    usage("unrecognized command");
    return 1;
}

int px4_daemon_thread_main(int argc, char *argv[])
{

    warnx("[daemon] starting\n");

    thread_running = true;

    while (!thread_should_exit) {
        warnx("Hello daemon!\n");
        sleep(10);
    }

    warnx("[daemon] exiting.\n");

    thread_running = false;

    return 0;
}

/**
 * @file px4_daemon_app.c
 * daemon application example for PX4 autopilot
 *
 * @author Example User <[email protected]>
 */
 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
 
#include <px4_config.h>
#include <nuttx/sched.h>
 
#include <systemlib/systemlib.h>
#include <systemlib/err.h>
 
static bool thread_should_exit = false;		/**< daemon exit flag */
static bool thread_running = false;		/**< daemon status flag */
static int daemon_task;				/**< Handle of daemon task / thread */
 
/**
 * daemon management function.
 */
__EXPORT int px4_daemon_app_main(int argc, char *argv[]);
 
/**
 * Mainloop of daemon.
 */
int px4_daemon_thread_main(int argc, char *argv[]);
 
/**
 * Print the correct usage.
 */
static void usage(const char *reason);
 
static void
usage(const char *reason)
{
	if (reason) {
		warnx("%s\n", reason);
	}
 
	warnx("usage: daemon {start|stop|status} [-p <additional params>]\n\n");
}
 
/**
 * The daemon app only briefly exists to start
 * the background job. The stack size assigned in the
 * Makefile does only apply to this management task.
 *
 * The actual stack size should be set in the call
 * to task_create().
 */
int px4_daemon_app_main(int argc, char *argv[])
{
	if (argc < 2) {
		usage("missing command");
		return 1;
	}
 
	if (!strcmp(argv[1], "start")) {
 
		if (thread_running) {
			warnx("daemon already running\n");
			/* this is not an error */
			return 0;
		}
 
		thread_should_exit = false;
		daemon_task = px4_task_spawn_cmd("daemon",
						 SCHED_DEFAULT,
						 SCHED_PRIORITY_DEFAULT,
						 2000,
						 px4_daemon_thread_main,
						 (argv) ? (char *const *)&argv[2] : (char *const *)NULL);
		return 0;
	}
 
	if (!strcmp(argv[1], "stop")) {
		thread_should_exit = true;
		return 0;
	}
 
	if (!strcmp(argv[1], "status")) {
		if (thread_running) {
			warnx("\trunning\n");
 
		} else {
			warnx("\tnot started\n");
		}
 
		return 0;
	}
 
	usage("unrecognized command");
	return 1;
}
 
int px4_daemon_thread_main(int argc, char *argv[])
{
 
	warnx("[daemon] starting\n");
 
	thread_running = true;
 
	while (!thread_should_exit) {
		warnx("Hello daemon!\n");
		sleep(10);
	}
 
	warnx("[daemon] exiting.\n");
 
	thread_running = false;
 
	return 0;
}

代码测试将会产生如下的输出：

nsh> px4_daemon_app start
[daemon] starting
Hello Daemon!

nsh> px4_daemon_app start
[daemon] starting
Hello Daemon!

为了使用这一APP，只需在Firmware/makefiles/config_px4fmu_default.mk中，取消对这一示例部分的注释。