fdinfo 的说明

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我是靠谱客的博主无限酒窝，最近开发中收集的这篇文章主要介绍fdinfo 的说明，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

在/proc/pid/fdinfo里有关于进程打开文件的一些权限&位置信息，按照manpage是这样描述的:

/proc/[pid]/fdinfo/ (since kernel 2.6.22)
This is a subdirectory containing one entry for each file which the process has open, named by its file descriptor. The
contents of each file can be read to obtain information about the corresponding file descriptor, for example:

                  $ cat /proc/12015/fdinfo/4
                  pos:    1000
                  flags: 01002002

The pos field is a decimal number showing the current file offset. The flags field is an octal number that displays the
file access mode and file status flags (see open(2)).

The files in this directory are readable only by the owner of the process.

就是说pos:表明当前该进程对该文件的读写位置；flags是open文件时的权限,比如O_RDWR|O_TRUNC等等。下面举栗说明：

1）某未知进程打开一个log文件

[root@xx /proc/108490/fdinfo]#ls -l ../fd
total 0
...
l-wx------ 1 xx xx 64 Aug 9 21:39 5 -> /data1/home/xx/workspace/trunk/bin/log/lobby.log

2)查看fd==5

pos: 4114656
flags: 0100001

根据说明，pos是当前读写位置，一般写日志文件肯定都是写到文件尾的，所以目测这个pos可以代表文件的大小，验证一下

File: '/data1/home/xx/workspace/trunk/bin/log/lobby.log'
Size: 4114656     Blocks: 8048       IO Block: 4096   regular file
Device: 804h/2052d   Inode: 6448732095 Links: 1
Access: (0666/-rw-rw-rw-) Uid: (30020/ xx)   Gid: (30020/ xx)
Access: 2018-08-09 21:08:35.375586664 +0800
Modify: 2018-09-10 20:37:35.277841439 +0800
Change: 2018-09-10 20:37:35.277841439 +0800

确实文件size与pos一致。

3）下面对比flag，0100001

#define O_RDONLY         00
#define O_WRONLY         01
#define O_RDWR             02
#define O_CREAT           0100   /* not fcntl */
#define O_EXCL           0200   /* not fcntl */
#define O_NOCTTY       0400   /* not fcntl */
#define O_TRUNC          01000   /* not fcntl */
#define O_APPEND      02000
#define O_NONBLOCK      04000
#define O_NDELAY   O_NONBLOCK
#define O_SYNC       010000
#define FASYNC       020000   /* fcntl, for BSD compatibility */
#define O_DIRECT   040000   /* direct disk access hint */
#define O_LARGEFILE   0100000
#define O_DIRECTORY   0200000   /* must be a directory */
#define O_NOFOLLOW   0400000 /* don't follow links */
#define O_NOATIME   01000000

比较一下，说明是open("xxxxx" , O_LARGEFILE|O_WRONLY);这样子打开的--

比如，另外打开一个文件O_WRONLY|O_APPEND|O_SYNC|O_NONBLOCK

然后查看对应的flags:

04116001

一般在64位下会默认加上O_LARGEFILE 所以实际上是

O_LARGEFILE|O_WRONLY|O_APPEND|O_SYNC|O_NONBLOCK=116001

最上面的04不知道是怎么搞上去的，这个仿佛适合O_SYNC标记有关，此处存疑。