Android如何生成设备节点

http://buaadallas.blog.51cto.com/399160/394350

 

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 },};