Linux守护进程使用VM167 USB GPIO用户空间驱动无法识别设备问题

我正在编写Linux守护进程，用于监控Velleman VM167型号USB GPIO设备的状态，状态变更时执行对应操作。我已完成该设备的用户空间驱动部署、/etc/udev规则配置、ldconfig路径设置，编写的简化测试程序编译后运行正常，可成功识别设备并获取驱动版本信息，迁移到标准守护进程模板后，仅能正常获取驱动版本号，无法识别设备。

正常运行的测试代码 #

#include <iterator>
#include <cstdlib>
#include <cstring>
#include <sstream>
#include <iostream>
#include <stdlib.h>
#include <string>
#include <sys/stat.h>
#include <syslog.h>
#include <unistd.h>
#include <vector>
#include "VM167.h"

int test_openDevices();
void test_readWriteDigital(int card);
void test_readAnalog(int card);
void test_PWM(int card);

int main(void)
{
   syslog(LOG_INFO, "Loop");
   int devices;
        devices = OpenDevices();
        if (devices == 0)
        {
                syslog(LOG_ERR, "Driver error condition, exiting");
                //fflush(stdout);
                //return -1;
        }
        else if (devices == -1)
        {
                syslog(LOG_ERR, "No devices found on system, exiting\n");
                //fflush(stdout);
                //return -1;
        }
        int ver = VersionDLL();
        syslog(LOG_INFO, "DLL Version: %d.%d.%d.%d\n", (ver>>24), (ver>>16)&0xFF, (ver>>8)&0xFF, ver&0xFF);
}

测试程序syslog输出 #

Nov  4 01:12:12 testserver console: Loop
Nov  4 01:12:12 testserver console: DLL Version: 0.1.0.19

故障守护进程代码 #

#include <dirent.h>
#include <iterator>
#include <cstdlib>
#include <cstring>
#include <sstream>
#include <iostream>
#include <stdlib.h>
#include <string>
#include <sys/stat.h>
#include <syslog.h>
#include <unistd.h>
#include <vector>
#include "VM167.h"

int test_openDevices();
void test_readWriteDigital(int card);
void test_readAnalog(int card);
void test_PWM(int card);

void do_heartbeat()
{
   // TODO: implement processing code to be performed on each heartbeat
   syslog(LOG_INFO, "Loop");
   int devices;
        devices = OpenDevices();
        if (devices == 0)
        {
                syslog(LOG_ERR, "Driver error condition, exiting");
                //fflush(stdout);
                //return -1;
        }
        else if (devices == -1)
        {
                syslog(LOG_ERR, "No devices found on system, exiting\n");
                //fflush(stdout);
                //return -1;
        }
        int ver = VersionDLL();
        syslog(LOG_INFO, "DLL Version: %d.%d.%d.%d\n", (ver>>24), (ver>>16)&0xFF, (ver>>8)&0xFF, ver&0xFF);
}

// For security purposes, we don't allow any arguments to be passed into the daemon
int main(void)
{
   // Define variables
   pid_t pid, sid;

   // Fork the current process
   pid = fork();
   // The parent process continues with a process ID greater than 0
   if(pid > 0)
   {
      exit(EXIT_SUCCESS);
   }
   // A process ID lower than 0 indicates a failure in either process
   else if(pid < 0)
   {
      exit(EXIT_FAILURE);
   }
   // The parent process has now terminated, and the forked child process will continue
   // (the pid of the child process was 0)

   // Since the child process is a daemon, the umask needs to be set so files and logs can be written
   umask(0);

   // Open system logs for the child process
   openlog("Daemon-GPIO", LOG_NOWAIT | LOG_PID, LOG_USER);
   syslog(LOG_NOTICE, "Successfully started Daemon-GPIO");

   // Generate a session ID for the child process
   sid = setsid();
   // Ensure a valid SID for the child process
   if(sid < 0)
   {
      // Log failure and exit
      syslog(LOG_ERR, "Could not generate session ID for child process");

      // If a new session ID could not be generated, we must terminate the child process
      // or it will be orphaned
      exit(EXIT_FAILURE);
   }

   // Change the current working directory to a directory guaranteed to exist
   if((chdir("/")) < 0)
   {
      // Log failure and exit
      syslog(LOG_ERR, "Could not change working directory to /");

      // If our guaranteed directory does not exist, terminate the child process to ensure
      // the daemon has not been hijacked
      exit(EXIT_FAILURE);
   }

   // A daemon cannot use the terminal, so close standard file descriptors for security reasons
   close(STDIN_FILENO);
   close(STDOUT_FILENO);
   close(STDERR_FILENO);

   // Daemon-specific intialization should go here
   const int SLEEP_INTERVAL = 5;

   // Enter daemon loop
   while(1)
   {
      syslog(LOG_INFO, "Starting helper loop");
      // Execute daemon heartbeat, where your recurring activity occurs
      do_heartbeat();

      // Sleep for a period of time
      sleep(SLEEP_INTERVAL);
   }

   // Close system logs for the child process
   syslog(LOG_NOTICE, "Stopping Daemon-GPIO");
   closelog();

   // Terminate the child process when the daemon completes
   exit(EXIT_SUCCESS);
}

守护进程syslog输出 #

Nov  4 00:18:10 testserver Daemon-GPIO[29638]: Loop
Nov  4 00:18:10 testserver Daemon-GPIO[29638]: No devices found on system, exiting
Nov  4 00:18:10 testserver Daemon-GPIO[29638]: DLL Version: 0.1.0.19

按照以下优先级排查修复：

• 调整设备打开逻辑：不要在心跳循环中反复调用OpenDevices()，将设备打开操作移到守护进程初始化阶段（关闭标准文件描述符之后、进入循环之前）仅执行一次，心跳循环只保留设备状态读取逻辑，反复开关设备会导致内核USB驱动资源占用异常，引发识别失败。
• 排查权限问题：确认守护进程的运行用户归属的用户组，与udev规则中分配VM167设备权限的用户组一致，测试时可以临时将udev规则中设备的MODE改为0666验证是否为权限问题。
• 处理标准描述符关闭的影响：VM167驱动依赖的libusb在标准输入输出描述符被直接关闭的场景下可能出现异常，将关闭描述符的逻辑替换为重定向到/dev/null：

  int fd = open("/dev/null", O_RDWR);
  dup2(fd, STDIN_FILENO);
  dup2(fd, STDOUT_FILENO);
  dup2(fd, STDERR_FILENO);
  if (fd > 2) close(fd);
  

• 调整初始化顺序：libusb的上下文无法跨fork继承，可将OpenDevices()的初始化逻辑移到fork操作之前的父进程中执行，再将设备句柄传递到子进程使用。

内容的提问来源于stack exchange，提问作者Dangerarea