基于redis ae实现 Linux中的文件系统监控机制(inotify)

（英文部分为转的。代码是个人代码）

1 What’s inotify

The inotify API provides a mechanism for monitoring file system events. Inotify can be used to monitor individual files, or to monitor directories. When a directory is
monitored, inotify will return events for the directory itself, and for files inside the directory.

2 How to use inotify

2.1 system calls used with this API

The following system calls are used with this API: inotify_init(orinotify_init1),inotify_add_watch, inotify_rm_watch, read,
and close.

2.1.1 inotify_init

It creates an inotify instance and returns a file descriptor referring to the inotify instance. The more recent inotify_init1 is like inotify_init,
but provides some extra functionality.

2.1.2 inotify_add_watch

It manipulates the "watch list" associated with an inotify instance. Each item ("watch") in the watch list specifies the pathname of a file or directory, along with some set of events that the kernel should
monitor for the file referred to by that pathname.

inotify_add_watch either creates a new watch item, or modifies an existing watch. Each watch has a unique "watch descriptor", an integer returned by inotify_add_watch when
the watch is created.

2.1.3 inotify_rm_watch

It removes an item from an inotify watch list.

When all file descriptors referring to an inotify instance have been closed, the underlying object and its resources are freed for reuse by the kernel; all associated watches are automatically freed.

2.1.4 read

To determine what events have occurred, an application reads from the inotify file descriptor.
If no events have so far occurred, then, assuming a blocking file descriptor, read will block until at least one event occurs (unless interrupted
by a signal, in which case the call fails with the error EINTR; see signal(7)).

Each successful read returns a buffer containing one or more of the following structures:

struct inotify_event {

int wd; /* Watch descriptor */

uint32_t mask; /* Mask of events */

uint32_t cookie; /* Unique cookie associating related events (for rename ) */

uint32_t len; /* Size of name field */

char name[]; /* Optional null-terminated name */

};

2.2 Inotify events

The inotify_add_watch mask argument and the mask field of the inotify_event structure returned
when reading an inotify file descriptor are both bit masks identifying inotify events. The following bits can be specified in mask when calling inotify_add_watch and
may be returned in the mask field returned by read:

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 File moved out of watched directory (*).

IN_MOVED_TO File moved into watched directory (*).

IN_OPEN File was opened (*).

……

2.3 The flow for the systme calls used with inotify





3 代码演示样例

#include <sys/inotify.h>

static aeEventLoop *loop;
/* 全局的notify watch item */
static const char *wds[10];

void sync_file_thread(void*args)
{
int inotify_fd, wd;
int poll_num;
const char *dir, *file;
dir = "/data/wcl/redis_proxy";
file = "binlog_1";

loop =aeCreateEventLoop(1024);
/* 创建inotify instance */
inotify_fd = inotify_init1(IN_NONBLOCK);
if (inotify_fd == -1)
{
perror("Unable to create inotify instance\n");
exit(-1);
}
printf("inotify_fd [%d]\n",inotify_fd);

/* 监控文件夹以下的新建文件事件 */
wd = inotify_add_watch(inotify_fd, dir, IN_CREATE);
wds[wd] = dir;

/* 监控文件夹以下文件的改动事件 */
wd = inotify_add_watch(inotify_fd, file, IN_MODIFY);
wds[wd] = file;

/* 监听inotify的可读事件, 有可读事件, 则表示监控的文件系统有事件产生 */
if (aeCreateFileEvent(loop,inotify_fd,AE_READABLE,handle_inotify_events,NULL))
{
exit(0);
}
aeMain(loop);
aeDeleteEventLoop(loop);
}

void handle_inotify_events(struct aeEventLoop *eventLoop, int fd, void *clientData, int mask)
{
char buf[4096], *ptr;
ssize_t len;
struct inotify_event *event;

len = read(fd, buf, sizeof(buf));
if (len == -1 && errno != EAGAIN)
{
perror("read error");
exit(-1);
}

if (len <= 0)
{
return;
}

for (ptr = buf; ptr < buf + len; ptr += sizeof(struct inotify_event) + event->len)
{
event = (struct inotify_event *)ptr;

if(event->mask & IN_CREATE)
{
/* new binlog files created */
if(event->len)
{
printf("New File created: %s\n", event->name);
}
}

if(event->mask & IN_MODIFY)
{
/* existing files are modified */
printf("File is modified: %s\n", wds[event->wd]);
}
}

}

这里我仅仅监控了创建和改动的两个事件，然后依据文件名称字和一些其它的相应机制进行对文件推断改动等。