Linux下監控文件系統
Linux的后台程序通常在機器沒有問題的情況下，需要長期運行（比如說數個月，甚至是數年）。但是，程序的配置文件有時候是需要定期作調整。為了不影響程序對外服務（不重啟），動態加載配置文件是一種非常常見的需求。通過監控某個文件的創建、刪除和修改等事件，可以很方便做出對應的動作（比如說reload）。
1. Linux下監控文件系統的常用方法
監控配置文件或配置文件目錄的變化，一種可行的方法是程序啟動的時候記錄下文件（或目錄）的修改時間，周期性檢查（比如說一秒一次）文件是否已經被修改，來決定是否需要重新加載配置文件。
另一種更為優雅的辦法是使用Linux系統從內核層面支持的系統API dnotify、inotify或者fanotify。inotify API提供一個文件描述符，可以在該文件描述符上注冊對指定的文件或者目錄的文件系統事件（文件刪除、文件修改和文件創建），然后通過read系統調用讀取該文件描述法上的事件。
2. 使用stat或fstat監控Linux文件系統
通過周期性地獲取被監控文件的狀態，stat和fstat可以幫助用戶監控指定文件的狀態。
int stat(const char *path, struct stat *buf);
int fstat(int fd, struct stat *buf);
struct stat {
dev_t st_dev; /* ID of device containing file */
ino_t st_ino; /* inode number */
mode_t st_mode; /* protection */
nlink_t st_nlink; /* number of hard links */
uid_t st_uid; /* user ID of owner */
gid_t st_gid; /* group ID of owner */
dev_t st_rdev; /* device ID (if special file) */
off_t st_size; /* total size, in bytes */
blksize_t st_blksize; /* blocksize for file system I/O */
blkcnt_t st_blocks; /* number of 512B blocks allocated */
time_t st_atime; /* time of last access */
time_t st_mtime; /* time of last modification */
time_t st_ctime; /* time of last status change */
};
文件狀態結構體struct stat的st_mtime字段記錄了path對應的文件的最后修改時間，只用周期性地檢查st_mtime的值，就可以監控文件是否被修改了。
開啟一個單獨的線程，並在線程中注冊一個回調函數，當文件改變了以后，就調用注冊的回調函數重新加載監控的目標文件。一個簡單的例子如下：
#include <iostream>
#include <string>
#include <thread>
#include <functional>
#include <ev.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
typedef std::function<void (std::string &)> Notifier;
class ConfWatcher {
public:
explicit ConfWatcher(const std::string &conf_path, Notifier notifier)
: conf_path_(conf_path), notifier_(notifier), wait_sec_(1), stop_(false)
{ }
bool Init() {
if (access(conf_path_.c_str(), F_OK) != 0) {
std::cout << "conf file " << conf_path_ << " doesn't exist" << std::endl;
return false;
}
if (!update_last_mod_time()) {
return false;
}
thread_ = std::thread([this] () {
std::cout << "starting..." << std::endl;
time_t now = 0;
while (!stop_) {
now = time(nullptr);
time_t delay = now - last_detect_time_;
if (delay < wait_sec_) {
//std::cout << "sleep " << wait_sec_ - delay << " seconds" << std::endl;
sleep(wait_sec_ - delay);
}
time_t mod_time = last_mod_time_;
update_last_mod_time();
if (mod_time != last_mod_time_) {
std::cout << "target file " << conf_path_ <<
" has been modified, so reload it" << std::endl;
notifier_(conf_path_);
}
}
std::cout << "stopping..." << std::endl;
});
return true;
}
bool Stop() { stop_ = true; }
bool Wait() { thread_.join(); }
private:
bool update_last_mod_time() {
struct stat f_stat = {0};
if (stat(conf_path_.c_str(), &f_stat) == -1) {
std::cout << "update_last_mod_time() failed on conf_file "
<< conf_path_ << " ,error: " << strerror(errno) << std::endl;
return false;
}
last_mod_time_ = f_stat.st_mtime;
last_detect_time_ = time(nullptr);
return true;
}
private:
std::string conf_path_;
int wait_sec_;
bool stop_;
time_t last_mod_time_;
time_t last_detect_time_;
std::thread thread_;
Notifier notifier_;
};
int main(int argc, char **argv) {
if (argc != 2) {
std::cout << "Usage: " << argv[0] << " target-file-path" << std::endl;
return -1;
}
auto notifier = [] (const std::string &conf_file_path) {
std::cout << "watched file " << conf_file_path
<< " has been modified, maybe you need reload it" << std::endl;
};
ConfWatcher watcher(argv[1], notifier);
if (!watcher.Init()) {
return -1;
}
watcher.Wait();
return 0;
}
編譯：g++ -std=c++11 -lpthread main.cpp
3. dnotify監控Linux文件系統
dnofigy是Linux kernel 2.4.0開始支持的一個系統API。它提供了非常有限的方式來和內核交互，以便獲取指定的目錄下文件的修改事件。dnotify的文件監控室通過fcntl的F_NOTIFY選項來實現的。因此它需要使用一個用open系統調用返回的一個文件描述符。可監控的事件有：文件讀取、文件創建、文件內容修改、文件屬性修改和文件重命名。dnotify的事件通常是通過SIGIO信號來得到通知的。
dnotify有以下缺陷，因此使用起來不是很方便：無法監控單個文件；被監控文件夾所在的文件系統分區在進程退出前無法卸載；文件事件發生后，需要調用stat系統調用來檢查到底是那個文件被修改了。
4. 使用inotify監控Linux文件系統
inotify是Linux 2.6.13引入的一個inode監控系統。它的API提供了監控單個文件或者目錄的機制，當監控目錄時，它可以返回目錄本身的事件和目錄下文件的事件。因此inotify可以完全替代dnotify。
inotigy可以監控以下事件（在此只列出部分，更詳細信息可以man 7 inotify查看）：
IN_ACCESS File was accessed (read) (*).
IN_ATTRIB Metadata changed, e.g., permissions, timestamps, extended attributes, link count (since Linux 2.6.25), UID, GID, etc. (*).
IN_CLOSE_WRITE File opened for writing was closed (*).
IN_CLOSE_NOWRITE File not opened for writing was closed (*).
IN_CREATE File/directory created in watched directory (*).
IN_DELETE File/directory deleted from watched directory (*).
IN_DELETE_SELF Watched file/directory was itself deleted.
IN_MODIFY File was modified (*).
IN_MOVE_SELF Watched file/directory was itself moved.
IN_MOVED_FROM Generated for the directory containing the old filename when a file is renamed (*).
IN_MOVED_TO Generated for the directory containing the new filename when a file is renamed (*).
IN_OPEN File was opened (*).
inotify的接口inotify_init()返回一個文件描述符，然后調用inotify_add_watch()來添加監控的目錄或文件及其對應的感興趣的事件。對於不感興趣的文件或目錄，可以使用inotify_rm_watch()來取消關注。注冊了文件監控后，可以使用select、poll或者epoll來檢測inotify_init()返回的文件描述符是否有可讀事件，然后使用read系統調用將可讀事件讀取到struct inotify_event里。
struct inotify_event {
int wd; /* Watch descriptor */
uint32_t mask; /* Mask of events */
uint32_t cookie; /* Unique cookie associating related events (for rename(2)) */
uint32_t len; /* Size of name field */
char name[]; /* Optional null-terminated name */
};
正常情況下，read可以讀取到若干個struct inotify_event，然后逐個遍歷就能知道是哪個文件（或者目錄）上發生了什么事件。下面是一個簡單的使用方法：
//#pragma once
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/inotify.h>
#include <string>
#include <unordered_map>
#include <vector>
#include <iostream>
#include <thread>
#include <ev.h>
struct fs_io {
struct ev_io io;
void * data;
};
class FsMonitor {
public:
~FsMonitor() {
if (fd_ != -1) {
close(fd_);
}
}
// register file/directory path and event mask.
bool Init(const std::vector<std::pair<std::string,uint32_t>> &item_list);
bool Run();
bool Stop();
private:
static void HandleEvents(struct ev_loop *loop, struct ev_io *watcher, int revents);
bool RemoveItem(const std::string &item_name);
bool ModifyItem(const std::string &item_name, uint32_t mask);
private:
std::unordered_map<std::string,int> item_wd_;
int fd_;
struct ev_loop *loop_;
struct fs_io watcher_;
std::thread thread_;
};
#define EVENT_SIZE (sizeof (struct inotify_event))
#define BUF_LEN (1024 * (EVENT_SIZE + 16))
bool FsMonitor::Init(const std::vector<std::pair<std::string,uint32_t>> &item_list)
{
fd_ = inotify_init1(IN_NONBLOCK);
if (fd_ < 0) {
perror("inotify_init1");
return false;
}
for (auto item : item_list) {
int wd = inotify_add_watch(fd_, item.first.c_str(), item.second);
if (wd == -1) {
std::cout << "inotify_add_watch() item " << item.first << " failed:" << strerror(errno) << std::endl;
return false;
}
item_wd_.insert(std::pair<std::string,int>(item.first, wd));
}
loop_ = EV_DEFAULT;
watcher_.data = this;
ev_io_init (&watcher_.io, HandleEvents, fd_, EV_READ);
ev_io_start (loop_, &watcher_.io);
return true;
}
bool FsMonitor::Run()
{
thread_ = std::thread([this] () {
ev_run(loop_, 0);
});
return true;
}
bool FsMonitor::Stop()
{
ev_break(loop_, EVBREAK_ALL);
return true;
}
void FsMonitor::HandleEvents(struct ev_loop *loop, struct ev_io *watcher, int revents)
{
struct fs_io * fs_watcher = (struct fs_io *)watcher;
FsMonitor * fs_monitor = (FsMonitor *)fs_watcher->data;
int length, i = 0;
char buffer[BUF_LEN];
length = read(fs_monitor->fd_, buffer, BUF_LEN);
if (length < 0) {
perror("read");
return;
}
while (i < length) {
struct inotify_event *event = (struct inotify_event *) &buffer[i];
if (event->len) {
if (event->mask & IN_CREATE) {
if (event->mask & IN_ISDIR) {
std::cout <<"The directory " << event->name <<" was created." << std::endl;
}
else {
std::cout <<"The file " << event->name << " was created." << std::endl;
}
}
else if (event->mask & IN_DELETE) {
if (event->mask & IN_ISDIR) {
std::cout <<"The directory " << event->name << " was deleted." << std::endl;
}
else {
std::cout <<"The file " << event->name << " was deleted." << std::endl;
}
}
else if (event->mask & IN_MODIFY) {
if (event->mask & IN_ISDIR) {
std::cout <<"The directory " << event->name << " was modified." << std::endl;
}
else {
std::cout <<"The file " << event->name << " was modified." << std::endl;
}
}
}
i += EVENT_SIZE + event->len;
}
}
bool FsMonitor::RemoveItem(const std::string &item_name) {
auto find_item = item_wd_.find(item_name);
if (find_item == item_wd_.end()) {
std::cout << "no item " << item_name << " to remove" << std::endl;
return false;
}
int wd = find_item->second;
inotify_rm_watch(fd_, wd);
return true;
}
bool FsMonitor::ModifyItem(const std::string &item_name, uint32_t mask) {
// remove and add
return true;
}
int main(int argc, char **argv) {
FsMonitor fs_monitor;
std::vector<std::pair<std::string,uint32_t>> item_list;
item_list.emplace_back(std::pair<std::string,uint32_t>("/tmp", IN_MODIFY | IN_CREATE | IN_DELETE));
if (fs_monitor.Init(item_list) == false) {
return -1;
}
std::cout << "Starting..." << std::endl;
fs_monitor.Run();
while (true) {
usleep(1000 * 1000);
}
std::cout << "End..." << std::endl;
return 0;
}
5. fanotify
fanotify需要root權限才能使用，應用場景比較少，這里就不介紹啦。有興趣的可以查看給出的參考文章。
References: