在inotify事件后不能打开/dev/input/js文件描述符吗

I'm creating a Linux module for a game library that let's you hotplug multiple joysticks, it uses inotify to watch /dev/input.

我正在为一个游戏库创建一个Linux模块，让我们热插多个操纵杆，它使用inotify来监视/dev/input.

I am testing it with 3 joysticks:

我正在用3个操纵杆测试它:

First I connect 2 joysticks.
首先我连接两个操纵杆。
Then I start the application, the joysticks work and I don't get a error.
然后我启动应用程序，操纵杆工作，我不会出错。
After that I connect the third joystick, perror gives: /dev/input/js1: Permission denied.
在那之后，我连接第三个操纵杆，perror给出:/dev/input/js1:权限被拒绝。
When I check ls -l /proc/<pid-of-process>/fd it lists /dev/input/js0 and /dev/input/js2.
当我检查ls -l /proc/ /fd时，它会列出/dev/input/js0和/dev/ input/js2。

All the joysticks work fine when I run it as root.

当我以root身份运行时，所有的操纵杆都可以正常工作。

This is how it's initialized:

这是初始化的方式:

static void createGamepad(char *locName){
    char dirName[30];
    int fd;

    snprintf(dirName, 30, "/dev/input/%s", locName);

    fd = open(dirName, O_RDONLY | O_NONBLOCK, 0);
    if(fd < 0){
        perror(dirName);
    }
}

struct dirent *dir;
DIR *d;
int i, notifyfd, watch;

// Attach notifications to check if a device connects/disconnects
notifyfd = inotify_init();

watch = inotify_add_watch(notifyfd, "/dev/input", IN_CREATE | IN_DELETE);

d = opendir("/dev/input");

i = 0;
while((dir = readdir(d)) != NULL){
    if(*dir->d_name == 'j' && *(dir->d_name + 1) == 's'){
        createGamepad(dir->d_name, i);
        i++;
    }
}

closedir(d);

After that inotify handles it like this in the while(1) loop:

之后，inotify在while(1)循环中是这样处理的:

static bool canReadINotify(){
    fd_set set;
    struct timeval timeout;

    FD_ZERO(&set);
    FD_SET(notifyfd, &set);
    timeout.tv_sec = 0;
    timeout.tv_usec = 0;

    return select(notifyfd + 1, &set, NULL, NULL, &timeout) > 0 && 
        FD_ISSET(notifyfd, &set);
 }

// Inside the event loop
struct inotify_event ne;

while(canReadINotify()){
    if(read(notifyfd, &ne, sizeof(struct inotify_event) + 16) >= 0){
        if(*ne.name != 'j' || *(ne.name + 1) != 's'){
            continue;
        }

        if(ne.mask & IN_CREATE){
            createGamepad(ne.name);
        }
    }
}

Is it even possible with inotify or should I use udev? And if it's possible, how can I solve this?

是否可以使用inotify或者我应该使用udev?如果可能的话，我怎么解决这个问题呢?

1 个解决方案

#1

It is very likely a race condition. You see, you get the inotify event when the device node is created (by udev using a mknod() call), but the access permissions are set by udev using a separate chown() call, just a tiny bit later.

这很可能是一种竞争状态。您看到，在创建设备节点时(通过使用mknod()调用的udev创建)，您将获得inotify事件，但是访问权限是由udev使用一个单独的chown()调用设置的，稍后再进行。

See systemd src/udev/udev-node.c, node_permissions_apply(). In this particular case, /dev/input/jsX is not a symlink, but the actual device node; at least with systemd the device node access mode gets set sometime later, after the actual node is created.

看到systemd src / udev / udev-node。c,node_permissions_apply()。在本例中，/dev/input/jsX不是符号链接，而是实际的设备节点;至少在systemd中，设备节点访问模式将在创建实际节点之后的某个时候设置。

One robust solution would be to modify your createGamepad() function, so that instead of failing completely at fd == -1 && errno == EACCES, you instead retry after a short while; at least a few times, say for up to a second or two.

一个健壮的解决方案是修改createGamepad()函数，这样就不会在fd = -1 && & errno == EACCES上完全失败，而是在短时间后重新尝试;至少有几次，比如说一两秒钟。

However, ninjalj pointed out a better suggestion: use also the access permissions change as a trigger to check the device node. This is trivially accomplished, by using IN_CREATE | IN_DELETE | IN_ATTRIBUTE in the inotify_add_watch() function!

不过，ninjalj给出了一个更好的建议:使用访问权限更改作为触发器来检查设备节点。通过在inotify_add_watch()函数中使用IN_CREATE | IN_DELETE IN_ATTRIBUTE来实现这一点。

(You'll also want to ignore open()==-1, errno==EACCES errors in createGamepad(), as they are likely caused by this race condition, and the following IN_ATTRIBUTE inotify event will yield access to the same device.)

(您还需要忽略createGamepad()中的open()==-1, errno==EACCES错误()，因为它们可能是由该竞态条件引起的，下面的IN_ATTRIBUTE inotify事件将允许访问相同的设备。)

Prior to ninjalj's comment, I'd personally have used an array of input devices, and another for "possible" input devices that can/need to be retried after a short timeout to decide whether they are available or not, but I think his suggestion is much better.

在ninjalj发表评论之前，我个人会使用一系列的输入设备，以及另一个“可能的”输入设备，这些设备可以/需要在短时间后重新尝试，以决定是否可用，但我认为他的建议要好得多。

Need/want an example?

需要/想要举个例子吗?

#1