Linux的open file 与 file descriptor区别

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1 open file

An open file may be a regular file, a directory, a block special file, a character special file, an executing text reference, a library, a stream or a network file.

2 file descriptor

A file descriptor is a data structure used by a program to get a handle on a file. The most commonly known are:
0 for standard in
1 for standard out
2 for standard error

前后两者关系
一个文件即使被打开，也可能没有文件描述符。
比如current working directories, memory mapped files and executable > text files。

3 区别

lsof 可以查出某个进程打开的文件数目(即 open file)。

root@testbox ~]# lsof | grep '4448'oracle    4448  oracle  cwd       DIR        3,2      4096    1913941 /u01/app/oracle/product/10.2.0/db_1/dbsoracle    4448  oracle  rtd       DIR        3,2      4096          2 /oracle    4448  oracle  txt       REG        3,2  93300099    1915187 /u01/app/oracle/product/10.2.0/db_1/bin/oracleoracle    4448  oracle  mem       REG        3,2     95148    2174926 /lib/libnsl-2.3.4.sooracle    4448  oracle  mem       REG        3,2    106397    2174894 /lib/ld-2.3.4.sooracle    4448  oracle  mem       REG        3,2   1454546    2501884 /lib/tls/libc-2.3.4.so[root@testbox ~]# lsof | grep '4448' | wc -l42                    //打开的文件数目

ls用于查询进程使用的文件描述符数目

[root@testbox ~]# ls -l /proc/4448/fd/total 18lr-x------  1 oracle oinstall 64 Mar  5 15:04 0 -> /dev/nulllr-x------  1 oracle oinstall 64 Mar  5 15:04 1 -> /dev/nulllrwx------  1 oracle oinstall 64 Mar  5 15:04 10 -> /u01/app/oracle/product/10.2.0/db_1/rdbms/audit/ora_4446.audlr-x------  1 oracle oinstall 64 Mar  5 15:04 11 -> /dev/zerolr-x------  1 oracle oinstall 64 Mar  5 15:04 12 -> /dev/zero:[root@testbox ~]# ls -l /proc/4448/fd/ | wc -l19                    //文件描述符数目 + 1 (注意还有一行 “total 18”)

4 查看文件描述符的设置

$cat /proc/sys/fs/file-max65536--该参数可以动态修改

5 计算当前被使用的文件描述符数目

$cat /proc/sys/fs/file-nr           //该文件只能读，不能修改1380    180     65536   |     |       |_ Max no. of file descriptors allowed on the system   |     |          (与file-max一致)   |     |__ Total free allocated file descriptors   |   |__  Total allocated file descriptorsTo compute the number of file descriptors currently being used:1380 - 180 = 1200

参考文献：
[1] http://blog.itpub.net/15480802/viewspace-734062/ 博主：myownstars

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