Android如何生成设备节点

在Android中，由于没有mdev和udev，所以它没有办法动态的生成设备节点，那么它是如何做的呢？
我们可以在system/core/init/下的init.c和devices.c中找到答案：
init.c中
int main(int argc, char **argv)
{
...
/* Get the basic filesystem setup we need put
* together in the initramdisk on / and then we'll
* let the rc file figure out the rest.
*/
mkdir("/dev", 0755);
mkdir("/proc", 0755);
mkdir("/sys", 0755);

mount("tmpfs", "/dev", "tmpfs", 0, "mode=0755");
mkdir("/dev/pts", 0755);
mkdir("/dev/socket", 0755);
mount("devpts", "/dev/pts", "devpts", 0, NULL);
mount("proc", "/proc", "proc", 0, NULL);
mount("sysfs", "/sys", "sysfs", 0, NULL);

for(;;) {
...
if (ufds[0].revents == POLLIN)
handle_device_fd(device_fd);

if (ufds[1].revents == POLLIN)
handle_property_set_fd(property_set_fd);
if (ufds[3].revents == POLLIN)
handle_keychord(keychord_fd);
}

return 0;
}


我们再来看看handle_device_fd()，该函数定义在devices.c中
void handle_device_fd(int fd)
{
...
handle_device_event(&uevent);
handle_firmware_event(&uevent);
}
}


而handle_device_event定义如下：
static void handle_device_event(struct uevent *uevent)
{
...
if(!strcmp(uevent->action, "add")) {
make_device(devpath, block, uevent->major, uevent->minor);
return;
}
...
}


make_device定义如下：
static void make_device(const char *path, int block, int major, int minor)
{
...
mode = get_device_perm(path, &uid, &gid) | (block ? S_IFBLK : S_IFCHR);
dev = (major << 8) | minor;
...
setegid(gid);
mknod(path, mode, dev);
chown(path, uid, -1);
setegid(AID_ROOT);
}


我们看看get_device_perm如下实现：
static mode_t get_device_perm(const char *path, unsigned *uid, unsigned *gid)
{
mode_t perm;

if (get_device_perm_inner(qemu_perms, path, uid, gid, &perm) == 0) {
return perm;
} else if (get_device_perm_inner(devperms, path, uid, gid, &perm) == 0) {
return perm;
} else {
struct listnode *node;
struct perm_node *perm_node;
struct perms_ *dp;

/* Check partners list. */
list_for_each(node, &devperms_partners) {
perm_node = node_to_item(node, struct perm_node, plist);
dp = &perm_node->dp;

if (dp->prefix) {
if (strncmp(path, dp->name, strlen(dp->name)))
continue;
} else {
if (strcmp(path, dp->name))
continue;
}
/* Found perm in partner list. */
*uid = dp->uid;
*gid = dp->gid;
return dp->perm;
}
/* Default if nothing found. */
*uid = 0;
*gid = 0;
return 0600;
}
}


我们最后可以看到在devperms中定义了要生成的设备节点：
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 },

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

/* the msm hw3d client device node is world writable/readable. */
{ "/dev/msm_hw3dc", 0666, AID_ROOT, AID_ROOT, 0 },

/* gpu driver for adreno200 is globally accessible */
{ "/dev/kgsl", 0666, AID_ROOT, AID_ROOT, 0 },

/* these should not be world writable */
{ "/dev/diag", 0660, AID_RADIO, AID_RADIO, 0 },
{ "/dev/diag_arm9", 0660, AID_RADIO, AID_RADIO, 0 },
{ "/dev/android_adb", 0660, AID_ADB, AID_ADB, 0 },
{ "/dev/android_adb_enable", 0660, AID_ADB, AID_ADB, 0 },
{ "/dev/ttyMSM0", 0600, AID_BLUETOOTH, AID_BLUETOOTH, 0 },
{ "/dev/ttyHS0", 0600, AID_BLUETOOTH, AID_BLUETOOTH, 0 },
{ "/dev/uinput", 0660, AID_SYSTEM, AID_BLUETOOTH, 0 },
{ "/dev/alarm", 0664, AID_SYSTEM, AID_RADIO, 0 },
{ "/dev/tty0", 0660, AID_ROOT, AID_SYSTEM, 0 },
{ "/dev/graphics/", 0660, AID_ROOT, AID_GRAPHICS, 1 },
{ "/dev/msm_hw3dm", 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/pmem", 0660, AID_SYSTEM, AID_GRAPHICS, 0 },
{ "/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/snd/", 0660, AID_SYSTEM, AID_AUDIO, 1 },
{ "/dev/mt9t013", 0660, AID_SYSTEM, AID_SYSTEM, 0 },
{ "/dev/msm_camera/", 0660, AID_SYSTEM, AID_SYSTEM, 1 },
{ "/dev/akm8976_daemon",0640, AID_COMPASS, AID_SYSTEM, 0 },
{ "/dev/akm8976_aot", 0640, AID_COMPASS, AID_SYSTEM, 0 },
{ "/dev/akm8973_daemon",0640, AID_COMPASS, AID_SYSTEM, 0 },
{ "/dev/akm8973_aot", 0640, AID_COMPASS, AID_SYSTEM, 0 },
{ "/dev/bma150", 0640, AID_COMPASS, AID_SYSTEM, 0 },
{ "/dev/cm3602", 0640, AID_COMPASS, AID_SYSTEM, 0 },
{ "/dev/akm8976_pffd", 0640, AID_COMPASS, AID_SYSTEM, 0 },
{ "/dev/lightsensor", 0640, AID_SYSTEM, 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_snd", 0660, AID_SYSTEM, AID_AUDIO, 1 },
{ "/dev/msm_mp3", 0660, AID_SYSTEM, AID_AUDIO, 1 },
{ "/dev/audience_a1026", 0660, AID_SYSTEM, AID_AUDIO, 1 },
{ "/dev/tpa2018d1", 0660, AID_SYSTEM, AID_AUDIO, 1 },
{ "/dev/msm_audpre", 0660, AID_SYSTEM, AID_AUDIO, 0 },
{ "/dev/msm_audio_ctl", 0660, AID_SYSTEM, AID_AUDIO, 0 },
{ "/dev/htc-acoustic", 0660, AID_SYSTEM, AID_AUDIO, 0 },
{ "/dev/vdec", 0660, AID_SYSTEM, AID_AUDIO, 0 },
{ "/dev/q6venc", 0660, AID_SYSTEM, AID_AUDIO, 0 },
{ "/dev/snd/dsp", 0660, AID_SYSTEM, AID_AUDIO, 0 },
{ "/dev/snd/dsp1", 0660, AID_SYSTEM, AID_AUDIO, 0 },
{ "/dev/snd/mixer", 0660, AID_SYSTEM, AID_AUDIO, 0 },
{ "/dev/smd0", 0640, AID_RADIO, AID_RADIO, 0 },
{ "/dev/qemu_trace", 0666, AID_SYSTEM, AID_SYSTEM, 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 },
/* CDMA radio interface MUX */
{ "/dev/ts0710mux", 0640, AID_RADIO, AID_RADIO, 1 },
{ "/dev/ppp", 0660, AID_RADIO, AID_VPN, 0 },
{ "/dev/tun", 0640, AID_VPN, AID_VPN, 0 },
{ NULL, 0, 0, 0, 0 },
};