（转）浅析dev目录下设备文件mknod节点

init
=>handle_device_fd 收到uevent信息
=>handle_device_event
=>
static void handle_device_event(struct uevent *uevent)
{
    ...
    /* are we block or char? where should we live? */
    if(!strncmp(uevent->path, "/block", 6)) {
        block = 1;
        base = "/dev/block/";//根据uevent路径改变该节点路径
        mkdir(base, 0755);
    } else {
        block = 0;
            /* this should probably be configurable somehow */
        if(!strncmp(uevent->path, "/class/graphics/", 16)) {
            base = "/dev/graphics/";//根据uevent路径改变该uevent需要创建节点的路径
            mkdir(base, 0755);
        } else if (!strncmp(uevent->path, "/class/oncrpc/", 14)) {
            base = "/dev/oncrpc/";
            mkdir(base, 0755);
        } else if (!strncmp(uevent->path, "/class/adsp/", 12)) {
            base = "/dev/adsp/";
            mkdir(base, 0755);
        } else if(!strncmp(uevent->path, "/class/input/", 13)) {
            base = "/dev/input/";//根据uevent路径改变该uevent需要创建节点的路径
            mkdir(base, 0755);
        } else if(!strncmp(uevent->path, "/class/sensors/", 15)) {
            base = "/dev/sensors/";
            mkdir(base, 0755);
        } else if(!strncmp(uevent->path, "/class/mtd/", 11)) {
            base = "/dev/mtd/";//根据uevent路径改变该uevent需要创建节点的路径
            mkdir(base, 0755);
        } else if(!strncmp(uevent->path, "/class/misc/", 12) &&
                    !strncmp(name, "log_", 4)) {
            base = "/dev/log/";//根据uevent路径改变该uevent需要创建节点的路径
            mkdir(base, 0755);
            name += 4;
        } else if(!strncmp(uevent->path, "/class/sound/", 13)) {
            base = "/dev/snd/";
            mkdir(base, 0755);
        } else
            base = "/dev/";
    }
    
    snprintf(devpath, sizeof(devpath), "%s%s", base, name);

    if(!strcmp(uevent->action, "add")) {
        make_device(devpath, block, uevent->major, uevent->minor);//创建节点文件文件devpath
        return;
    }
    ...
}

static void make_device(const char *path, int block, int major, int minor)
{
    unsigned uid;
    unsigned gid;
    mode_t mode;
    dev_t dev;

    if(major > 255 || minor > 255)
        return;

    mode = get_device_perm(path, &uid, &gid) | (block ? S_IFBLK : S_IFCHR);//获取将要创建的节点是否需要重设它的mode数值
    dev = (major << 8) | minor;
    mknod(path, mode, dev);
    chown(path, uid, gid);
}
get_device_perm函数会比较path路径是否和下面的devperms[]数组中的inode路径相一致,如果一致,那么返回devperms[]数组中指定的uid,gid和mode数值,这样make_device就会向/dev这个tmpfs'内存虚拟物理盘'创建inode节点,同时改变该inode的uid和gid.[luther.gliethtt]
于是在devperms[]数组中我们就可以方便的控制生成的/dev文件的uid,gid和mode了.[luther.gliethtt]

static struct perms_ devperms[] = {
    { "/dev/null", 0666, AID_ROOT, AID_ROOT, 0 },
    { "/dev/zero", 0666, AID_ROOT, AID_ROOT, 0 },
    { "/dev/full", 0666, AID_ROOT, AID_ROOT, 0 },
    { "/dev/ptmx", 0666, AID_ROOT, AID_ROOT, 0 },
    { "/dev/tty", 0666, AID_ROOT, AID_ROOT, 0 },
    { "/dev/random", 0666, AID_ROOT, AID_ROOT, 0 },
    { "/dev/urandom", 0666, AID_ROOT, AID_ROOT, 0 },
    { "/dev/ashmem", 0666, AID_ROOT, AID_ROOT, 0 },
    { "/dev/binder", 0666, AID_ROOT, AID_ROOT, 0 },
    { "/dev/m2d", 0666, AID_ROOT, AID_ROOT, 0 },
    { "/dev/bmm", 0666, AID_ROOT, AID_ROOT, 0 },
    { "/dev/innodev", 0666, AID_ROOT, AID_ROOT, 0 },
    { "/dev/innodev0", 0666, AID_ROOT, AID_ROOT, 0 },
    { "/dev/innodev1", 0666, AID_ROOT, AID_ROOT, 0 },
    { "/dev/innodev2", 0666, AID_ROOT, AID_ROOT, 0 },


    /* logger should be world writable (for logging) but not readable */
    { "/dev/log/", 0662, AID_ROOT, AID_LOG, 1 },

        /* these should not be world writable */
    { "/dev/android_adb", 0660, AID_ADB, AID_ADB, 0 },
    { "/dev/android_adb_enable", 0660, AID_ADB, AID_ADB, 0 },
    { "/dev/ttyMSM0", 0660, AID_BLUETOOTH, AID_BLUETOOTH, 0 },
    { "/dev/alarm", 0664, AID_SYSTEM, AID_RADIO, 0 },
    { "/dev/tty0", 0666, AID_ROOT, AID_SYSTEM, 0 },
    { "/dev/graphics/", 0666, AID_ROOT, AID_GRAPHICS, 1 },
    { "/dev/hw3d", 0660, AID_SYSTEM, AID_GRAPHICS, 0 },
    { "/dev/input/", 0660, AID_ROOT, AID_INPUT, 1 },
    { "/dev/eac", 0660, AID_ROOT, AID_AUDIO, 0 },
    { "/dev/cam", 0660, AID_ROOT, AID_CAMERA, 0 },
    { "/dev/video0", 0660, AID_ROOT, AID_CAMERA, 0 },
    { "/dev/mved", 0660, AID_SYSTEM, AID_CAMERA, 0 },
    { "/dev/pmem", 0660, AID_SYSTEM, AID_GRAPHICS, 0 },
    { "/dev/pmem_gpu", 0660, AID_SYSTEM, AID_GRAPHICS, 1 },
    { "/dev/pmem_adsp", 0660, AID_SYSTEM, AID_AUDIO, 1 },
    { "/dev/pmem_camera", 0660, AID_SYSTEM, AID_CAMERA, 1 },
    { "/dev/oncrpc/", 0660, AID_ROOT, AID_SYSTEM, 1 },
    { "/dev/adsp/", 0660, AID_SYSTEM, AID_AUDIO, 1 },
    { "/dev/sensors/", 0666, AID_SYSTEM, AID_SYSTEM, 1 },
    { "/dev/mt9t013", 0660, AID_SYSTEM, AID_SYSTEM, 0 },
    { "/dev/akm8976_daemon",0640, AID_COMPASS, AID_SYSTEM, 0 },
    { "/dev/akm8976_aot", 0640, AID_COMPASS, AID_SYSTEM, 0 },
    { "/dev/akm8976_pffd", 0640, AID_COMPASS, AID_SYSTEM, 0 },
    { "/dev/msm_pcm_out", 0660, AID_SYSTEM, AID_AUDIO, 1 },
    { "/dev/msm_pcm_in", 0660, AID_SYSTEM, AID_AUDIO, 1 },
    { "/dev/msm_pcm_ctl", 0660, AID_SYSTEM, AID_AUDIO, 1 },
    { "/dev/msm_mp3", 0660, AID_SYSTEM, AID_AUDIO, 1 },
    { "/dev/smd0", 0640, AID_RADIO, AID_RADIO, 0 },
    { "/dev/qmi", 0640, AID_RADIO, AID_RADIO, 0 },
    { "/dev/qmi0", 0640, AID_RADIO, AID_RADIO, 0 },
    { "/dev/qmi1", 0640, AID_RADIO, AID_RADIO, 0 },
    { "/dev/qmi2", 0640, AID_RADIO, AID_RADIO, 0 },
    { "/dev/htc-acoustic", 0640, AID_RADIO, AID_RADIO, 0 },
    { "/dev/snd/", 0666, AID_ROOT, AID_AUDIO, 1 },
    { NULL, 0, 0, 0, 0 },
};
转自： http://blog.chinaunix.net/u1/38994/showart.php?id=1348852