Linux file descriptor the difference between open file descriptors and open files

http://blog.itpub.net/15480802/viewspace-734062

What is an open file

An open file is a regular file, a directory, a block special file, a character special file, an executing text referen CE, a library, a stream or a network file.

What is file descriptor

A file descriptor is a data structure used by a program to get a handle on a file. The most commonly known is:

0 in

1 for standard out

2 for standard error

Relationship between the two

Even if a file is opened, there may be no file descriptor, such as current working directories, memory mapped files and executable text files

Lsof can find out the number of files opened by a process,

[Email protected] ~]# Lsof | grep ' 4448 '

Oracle 4448 Oracle CWD DIR 3,2 4096 1913941/u01/app/oracle/product/10.2.0/db_1/dbs

Oracle 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/oracle

Oracle 4448 Oracle MEM REG 3,2 95148 2174926/lib/libnsl-2.3.4.so

Oracle 4448 Oracle MEM REG 3,2 106397 2174894/lib/ld-2.3.4.so

Oracle 4448 Oracle MEM REG 3,2 1454546 2501884/lib/tls/libc-2.3.4.so

:

[Email protected] ~]# Lsof | grep ' 4448 ' | Wc-l

42

The following methods are used to query the number of file descriptors used by the process

[Email protected] ~]# ls-l/proc/4448/fd/

Total 18

LR-X------1 Oracle Oinstall 5 15:04 0-/dev/null

LR-X------1 Oracle Oinstall 5 15:04 1-/dev/null

LRWX------1 Oracle Oinstall 5 15:04-/u01/app/oracle/product/10.2.0/db_1/rdbms/audit/ora_4446.aud

LR-X------1 Oracle Oinstall 5 15:04-/dev/zero

LR-X------1 Oracle Oinstall 5 15:04-/dev/zero

:

[Email protected] ~]# Ls-l/proc/4448/fd/| Wc-l

19

Process 4448 opens a total of 42 files, occupying only 19 file descriptors;

To view the settings for a file descriptor

$cat/proc/sys/fs/file-max

65536

--This parameter can be modified dynamically

Calculates the number of file descriptors currently being used

$cat/PROC/SYS/FS/FILE-NR

1380 180 65536

|   | |_ Max No. of file descriptors allowed on the system

| |

| |__ Total Free Allocated file descriptors

|

|__ Total Allocated file descriptors

To compute the number of file descriptors currently being used:

1380-180 = 1200

Other useful parameters

Nr_open = Maximum number of OPEN files per process

Nr_file = Total number of files The can is open in the system at any time

File-max = Kernel parameter refers to maximum number of file descriptors allowed per system

File-nr = refers to the number of the current file descriptors used at any moment.

LSOF = Gives The number of open files.

Ulimit–a View the current user's shell limit

If some resources are set too small, the system will not be able to provide services properly

http://space.itpub.net/15480802/viewspace-720722 about the Nofile/nproc test

References Open files/open File descriptors [ID 787780.1]

Linux file descriptor the difference between open file descriptors and open files