linux开发---17.【C】busybox中inotifyd解析

1. inotifyd
这次看的是busybox源码下的./miscutils/inotifyd.c

使用说明:
inotifyd PROG FILE1[:MASK]...
If PROG is -, events are sent to stdout.
Events:
    a    File is accessed
    c    File is modified
    e    Metadata changed
    w    Writable file is closed
    0    Unwritable file is closed
    r    File is opened
    D    File is deleted
    M    File is moved
    u    Backing fs is unmounted
    o    Event queue overflowed
    x    File can't be watched anymore
If watching a directory:
    m    Subfile is moved into dir
    y    Subfile is moved out of dir
    n    Subfile is created
    d    Subfile is deleted

使用方法:
./_install/busybox inotifyd - /tmp/test/1.c:r

a. 监测文件
cong@msi:/tmp/test$ cat 1.c //-->terminal1
就会打印:
cong@msi:/work/busybox$ ./_install/busybox inotifyd - /tmp/test/1.c:r //-->terminal2先运行
r /tmp/test/1.c

b. 监测目录
cong@msi:/tmp/test$ touch 4.c //-->terminal1
cong@msi:/work/busybox$ ./_install/busybox  inotifyd - /tmp/test/:n //-->terminal2 先运行
n    /tmp/test/    4.c

cong@msi:/tmp/test$ rm 4.c //-->terminal1
cong@msi:/work/busybox$ ./_install/busybox  inotifyd - /tmp/test/:d //-->terminal2 先运行
d    /tmp/test/    4.c

2. 源码
我把./miscutils/inotifyd.c改成了一个独立的小程序

cong@msi:/work/test/busytest/14inotify$ cat inotify.c
#include "./utils.h"
#include <sys/inotify.h>
#include <poll.h>
#include <sys/ioctl.h> //FIONREAD

//static const char mask_names[] ALIGN1 =
static const char mask_names[] =
    "a"    // 0x00000001    File was accessed
    "c"    // 0x00000002    File was modified
    "e"    // 0x00000004    Metadata changed
    "w"    // 0x00000008    Writable file was closed
    "0"    // 0x00000010    Unwritable file closed
    "r"    // 0x00000020    File was opened
    "m"    // 0x00000040    File was moved from X
    "y"    // 0x00000080    File was moved to Y
    "n"    // 0x00000100    Subfile was created
    "d"    // 0x00000200    Subfile was deleted
    "D"    // 0x00000400    Self was deleted
    "M"    // 0x00000800    Self was moved
    "\0"    // 0x00001000 (unused)                   //-->\0之后就没有用了,mask=0x0fff也是12位的
    // Kernel events, always reported:
    "u"    // 0x00002000   Backing fs was unmounted
    "o"    // 0x00004000   Event queued overflowed
    "x"    // 0x00008000   File is no longer watched (usually deleted)
;
enum {
    MASK_BITS = sizeof(mask_names) - 1
};

int main(int argc, char **argv)
{
    int n;
    unsigned mask;
    struct pollfd pfd;
    char **watches; // names of files being watched
    const char *args[5];
    char eventbuf[1024];

    //argv[1]结果显示的地方,-代表输出到stdout; argv[2]是要监测的文件或目录
    if (!argv[1] || !argv[2])
    {
        printf("usage: inotify - [FILE]:[MASK]\n");
        return -1;
    }

    argv++;

    watches = argv;
    args[0] = *argv;
    args[4] = NULL;
    argc -= 2; // number of files we watch

    //a. 创建 inotify实例
    pfd.fd = inotify_init();
    if (pfd.fd < 0)
        dbmsg("no kernel support");

    //这个地方很经典,要详细分析
    while (*++argv) {                       //假设参数="/tmp/test/1.c:r"
        char *path = *argv;                 //此处path="/tmp/test/1.c:r"
        char *masks = strchr(path, ':');    //查找:此时masks=:r
        mask = 0x0fff;
        if (masks) {
            *masks = '\0';                 //masks=\0r,那么path="/tmp/test/1.c"
            mask = 0;
            while (*++masks) {             //因为是++masks,所以*masks='r'
                const char *found;
                found = memchr(mask_names, *masks, MASK_BITS);  //从mask_names[] ="acew0rmyndDm\0uox";中找到r的地址
                if (found)
                    mask |= (1 << (found - mask_names));   //found-mask_names就是r在mask_names中的第几项,然后移位就是mask
            }
        }
        //b.添加一个watch
        n = inotify_add_watch(pfd.fd, path, mask);
        if (n < 0)
            dbmsg("add watch (%s) failed", path);
    }
    //c.poll结构体初始化
    pfd.events = POLLIN;
    while (1) {
        int len;
        void *buf;
        struct inotify_event *ie;
 again:
        n = poll(&pfd, 1, -1);                 //d.调用poll轮询监控watch上有没有事件发生
        if (n < 0 && errno == EINTR)           //被信号中断,返回继续等待
            goto again;
        if (n <= 0)                            //真正有error了,就break退出
            break;
        ioctl(pfd.fd, FIONREAD, &len);         //调用ioctl 命令的FIONREAD 来得到当前队列的长度
        ie = buf = (len <= sizeof(eventbuf)) ? eventbuf : malloc(len);
        len = read(pfd.fd, buf, len);
        //e.下面就处理buffer
        while (len > 0) {
            int i;
            // cache relevant events mask
            unsigned m = ie->mask & ((1 << MASK_BITS) - 1);
            if (m) {
                char events[MASK_BITS + 1];
                char *s = events;
                for (i = 0; i < MASK_BITS; ++i, m >>= 1) {
                    if ((m & 1) && (mask_names[i] != '\0'))
                        *s++ = mask_names[i];
                }
                *s = '\0';
                if ( strncmp(args[0],"-", 1) == 0) {   //如果argv[1]=='-'时就输出到终端
                    printf(ie->len ? "%s\t%s\t%s\n" : "%s\t%s\n", events, watches[ie->wd], ie->name);
                    fflush(stdout);
                } else {
                    args[1] = events;
                    args[2] = watches[ie->wd];
                    args[3] = ie->len ? ie->name : NULL;
                    //spawn_and_wait((char **)args);
                }
                // we are done if all files got final x event
                if (ie->mask & 0x8000) {
                    if (--argc <= 0)
                        goto done;
                    inotify_rm_watch(pfd.fd, ie->wd);
                }
            }
            // next event
            i = sizeof(struct inotify_event) + ie->len;
            len -= i;
            ie = (void*)((char*)ie + i);
        }
        if (eventbuf != buf)
            free(buf);
    }
 done:
    return 0;
}

2.
使用inotify函数时需要以下几步
a. pfd.fd = inotify_init();                         //创建 inotify实例,这儿也可以直接用int fd,但为了后面poll方便就放在了pfd中
b. inotify_add_watch(pfd.fd, path, mask); //添加一个watch
c. pfd.events = POLLIN;                         //初始化poll
d. poll(&pfd, 1, -1);                               //调用poll轮询监控watch上有没有事件发生
e. 解析poll的结果                                    //如果有事件发生就解析事件
14inotify.rar (下载后解压为14inotify.tar.gz)