在调试Android 的输入事件时，经常使用 "getevent -lrt" 命令，来确认驱动上报数据是否正常。从源码的角度来详细的分析一下getevent 这个程序。
首先用ls命令来看一下getevent
c
lrwxr-xr-x 1 root shell 7 2023-11-20 10:08 system/bin/getevent -> toolbox可以看出，getevent 链接的是toolbox，来看一下toolbox的main方法
c
//system\core\toolbox\toolbox.c
int main(int argc, char** argv) {
// Let's assume that none of this code handles broken pipes. At least ls,
// ps, and top were broken (though I'd previously added this fix locally
// to top). We exit rather than use SIG_IGN because tools like top will
// just keep on writing to nowhere forever if we don't stop them.
signal(SIGPIPE, SIGPIPE_handler);
char* cmd = strrchr(argv[0], '/');
char* name = cmd ? (cmd + 1) : argv[0];
for (size_t i = 0; tools[i].name; i++) {
if (!strcmp(tools[i].name, name)) {//name为getevent
return tools[i].func(argc, argv);//1
}
}
printf("%s: no such tool\n", argv[0]);
return 127;
}tools是一个数组
c
//system\core\toolbox\toolbox.c
static struct {
const char* name;
int (*func)(int, char**);
} tools[] = {
#define TOOL(name) { #name, name##_main },
#include "tools.h"
#undef TOOL
{ 0, 0 },
};而在tools.h里面定义了 TOOL(getevent)，将name替换为getevent，即注释1处调用getevent_main方法。接下来重点看一下getevent_main方法
c
//system\core\toolbox\getevent.c
static struct pollfd *ufds;
static char **device_names;
static int nfds;
int getevent_main(int argc, char *argv[])
{
//省略
const char *device_path = "/dev/input";
//省略
nfds = 1;
ufds = calloc(1, sizeof(ufds[0]));
ufds[0].fd = inotify_init();//1
ufds[0].events = POLLIN;
if(device) {//getevent命令后面可以指定device,如getevent dev/input/event0
//省略
} else {
if(!print_flags_set)
print_flags |= PRINT_DEVICE_ERRORS | PRINT_DEVICE | PRINT_DEVICE_NAME;
print_device = 1;
res = inotify_add_watch(ufds[0].fd, device_path, IN_DELETE | IN_CREATE);//2
res = scan_dir(device_path, print_flags); //3
}
//省略
}注释1处进行inotify初始化，注释2处使用inotify监听/dev/input目录下是否有设备增加或者移除，注释3处扫描/dev/input目录。此时/dev/input目录下是已经存在的设备。来看一下scan_dir扫描操作
c
//system\core\toolbox\getevent.c
static int scan_dir(const char *dirname, int print_flags)
{
char devname[PATH_MAX];
char *filename;
DIR *dir;
struct dirent *de;
dir = opendir(dirname);
if(dir == NULL)
return -1;
strcpy(devname, dirname);
filename = devname + strlen(devname);
*filename++ = '/';
while((de = readdir(dir))) {
if(de->d_name[0] == '.' &&
(de->d_name[1] == '\0' ||
(de->d_name[1] == '.' && de->d_name[2] == '\0')))
continue;
strcpy(filename, de->d_name);
open_device(devname, print_flags);//1
}
closedir(dir);
return 0;
}注释1处，得到/dev/input目录下的设备名后，打开设备。注意该方法是处于循环中，会依次打开/dev/input目录下所有满足要求的设备。
c
//system\core\toolbox\getevent.c
static int open_device(const char *device, int print_flags)
{
//省略
fd = open(device, O_RDONLY | O_CLOEXEC);//打开设备
//调用ioctl获取信息
/*加到ufds数组中*/
ufds[nfds].fd = fd;
ufds[nfds].events = POLLIN;
device_names[nfds] = strdup(device);
nfds++;
return 0;
}open_device方法主要功能是打开设备，然后将打开设备的fd添加进ufds数组中。
已有的设备扫描完成后，在getevent_main方法内，接着会进入一个循环
c
//system\core\toolbox\getevent.c
int getevent_main(int argc, char *argv[])
{
//省略
while(1) {
//int pollres =
poll(ufds, nfds, -1);//1
//printf("poll %d, returned %d\n", nfds, pollres);
if(ufds[0].revents & POLLIN) {//2
read_notify(device_path, ufds[0].fd, print_flags);
}
for(i = 1; i < nfds; i++) {
if(ufds[i].revents) {
if(ufds[i].revents & POLLIN) {//3
res = read(ufds[i].fd, &event, sizeof(event));
if(res < (int)sizeof(event)) {
fprintf(stderr, "could not get event\n");
return 1;
}
if(get_time) {
printf("[%8ld.%06ld] ", event.time.tv_sec, event.time.tv_usec);
}
if(print_device)
printf("%s: ", device_names[i]);
print_event(event.type, event.code, event.value, print_flags);//4
if(sync_rate && event.type == 0 && event.code == 0) {
int64_t now = event.time.tv_sec * 1000000LL + event.time.tv_usec;
if(last_sync_time)
printf(" rate %lld", 1000000LL / (now - last_sync_time));
last_sync_time = now;
}
printf("%s", newline);
if(event_count && --event_count == 0)
return 0;
}
}
}
}
return 0;
}注释1处使用poll来监测ufds数组中的每项是否有数据产生，没数据时休眠，有数据产生就会被唤醒。
由于ufds[0]的fd放入的是inotify_init得到的fd，而inotify是用来监测/dev/input 下是否有设备增加或者删除。注释2处，如果ufds[0]的数据是POLLIN，则表示此时是有设备增加或者删除，调用read_notify方法处理。而注释3处表示是某个设备有输入事件了，则调用read读取事件并使用print_event打印出来。
c
static int read_notify(const char *dirname, int nfd, int print_flags)
{
int res;
char devname[PATH_MAX];
char *filename;
char event_buf[512];
int event_size;
int event_pos = 0;
struct inotify_event *event;
res = read(nfd, event_buf, sizeof(event_buf));//读出数据
strcpy(devname, dirname);
filename = devname + strlen(devname);
*filename++ = '/';
while(res >= (int)sizeof(*event)) {
event = (struct inotify_event *)(event_buf + event_pos);
//printf("%d: %08x \"%s\"\n", event->wd, event->mask, event->len ? event->name : "");
if(event->len) {
strcpy(filename, event->name);
if(event->mask & IN_CREATE) {//设备增加
open_device(devname, print_flags);
}
else {
close_device(devname, print_flags);
}
}
event_size = sizeof(*event) + event->len;
res -= event_size;
event_pos += event_size;
}
return 0;
}可以看出，在read_notify方法内，如果有设备增加，也是调用open_device来进行处理。open_device在前面分析过，主要是打开设备并将fd添加到ufds数组中。
总结
getevent 使用 inotify和poll机制来监测有无输入设备的增加和删除，以及哪个设备有数据产生。具体的工作流程如下图
