在Linux守护进程中使用用户空间驱动程序

Question

我正在尝试编写一个守护程序，该守护程序监视USB GPIO设备(Velleman VM167(的状态，然后对更改的状态采取行动。

我找到了一个用户空间驱动程序(https://github.com/rahlskog/VM167)并设置了一些/etc/udev规则和ldconfig路径，这样我就可以运行测试，它可以按预期工作。

如果我编译以下内容，它将按预期工作：

#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, exitingn");
//fflush(stdout);
//return -1;
}
int ver = VersionDLL();
syslog(LOG_INFO, "DLL Version: %d.%d.%d.%dn", (ver>>24), (ver>>16)&0xFF, (ver>>8)&0xFF, ver&0xFF);
}

系统日志：

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

然而，当我将其添加到守护程序模板中时，我再也看不到USB卡了。。。

#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, exitingn");
//fflush(stdout);
//return -1;
}
int ver = VersionDLL();
syslog(LOG_INFO, "DLL Version: %d.%d.%d.%dn", (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);
}

系统日志：

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

Answer 1

在每个循环开始时调用OpenDevices()。当然，你需要在循环结束时关闭它们。由于您的真实(注释掉的(代码具有早期返回，因此处理此问题的正确C++方法是在OpenDevices()和CloseDevices()周围添加RAII包装，以确保打开的设备也关闭。

示例：

// A wrapper that open devices on creation and closes them on destruction, 
// like when it goes out of scope.
struct DeviceCtx {
DeviceCtx() : devices(OpenDevices()) {}
~DeviceCtx() { CloseDevices(); }
operator int() const { return devices; }  // implicit conversion to `int`
int devices;
};
void do_heartbeat() { // 
syslog(LOG_INFO, "Loop");
DeviceCtx devices;                 // calls OpenDevices
if (devices == 0) {
syslog(LOG_ERR, "Driver error condition, exiting");
return;                        // CloseDevices() will be called
} else if (devices == -1) {
syslog(LOG_ERR, "No devices found on system, exitingn");
return;                        // CloseDevices() will be called
}
int ver = VersionDLL();
syslog(LOG_INFO, "DLL Version: %d.%d.%d.%dn", (ver >> 24),
(ver >> 16) & 0xFF, (ver >> 8) & 0xFF, ver & 0xFF);
}   // `devices` goes out of scope and calls CloseDevices()

Answer 2

Whelp，似乎我忘记了关闭每个循环的设备。这就解决了。

CloseDevices();