File descriptors list

Gilles 'SO- stop being evil' Gilles 'SO- stop being evil' k gold badges silver badges bronze badges. Hi thanks for giving a good explanation Gilles. Its producing an output with some color coding, I'll just have to look at the manual. For visual inspection, use ls -l. For automated treatment, use readlink to extract the link target. Just use ls -l but i'll experiment with readlink. Thanks again for the additional info.

How can this be the count of files I have open if ulimit -n shows me ? ThorstenStaerk All settings of setrlimit the system call underlying the ulimit shell command are per-process.

They affect only the process that makes the call and indirectly the processes that it later forks. Show 4 more comments. Johan Johan 3, 2 2 gold badges 22 22 silver badges 29 29 bronze badges. The 4th is an open file to write in this case. Note: when lsof prints the file descriptors, it appends the following code to indicate the file access mode:.

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Learn more. How to list the open file descriptors and the files they refer to in my current bash session Ask Question. Asked 5 years ago. Active 1 year, 1 month ago. Viewed k times. Is there a way to list the currently open file descriptors? To determine the number of open file descriptors associated with a process use the following. For the same postfix process, There are 13 file descriptors associated with PID postfix.

When we open an existing file or create a new file, the kernel returns a file descriptor to the process. The kernel maintains a table of all open file descriptors, which are in use.

The allotment of file descriptors is generally sequential and they are allotted to the file as the next free file descriptor from the pool of free file descriptors. When we closes the file, the file descriptor gets freed and is available for further allotment. See this image for more details :.

When we want to read or write a file, we identify the file with the file descriptor that was returned by open or create function call, and use it as an argument to either read or write. File descriptor max value can be obtained with ulimit -n. According to Wikipedia we know for sure: a file descriptor is a non-negative integer. The most important thing I think is missing, would be to say:.

We know most famous file descriptors are 0, 1 and 2. We created a process with the id PID. Using the lsof -p we can get the things like this:. The character after the number i.

Note also that file descriptor table that kernel keeps track of is not the same as files table or inodes table. These are separate, as some other answers explained. These are not regular files, but so called character device files. The file descriptor for standard error is 2. If there is no any directory named as mydir then the output of command will be save to file errorfile. I hope you got your answer. File Descriptors FD are non-negative integers 0, 1, 2, FD's are allocated in the sequential order, meaning the lowest possible unallocated integer value.

As an addition to other answers, unix considers everything as a file system. Your keyboard is a file that is read only from the perspective of the kernel. The screen is a write only file. Similarly, folders, input-output devices etc are also considered to be files.

Whenever a file is opened, say when the device drivers[for device files] requests an open , or a process opens an user file the kernel allocates a file descriptor, an integer that specifies the access to that file such it being read only, write only etc.

Any operating system has processes p's running, say p1, p2, p3 and so forth. Each process usually makes an ongoing usage of files. The first file used in the process is file0 , second is file1 , third is file2 , and so forth. Given we often describe processes as "process-tables", and given that tables has rows entries we can say that the file descriptor cell in each entry, uses to represent the whole entry.

In some operating systems, you can run out of file descriptors, but such case is extremely rare, and the average computer user shouldn't worry from that.

File descriptors might be global process A starts in say 0, and ends say in 1 ; Process B starts say in 2, and ends say in 3 and so forth, but as far as I know, usually in modern operating systems, file descriptors are not global, and are actually process-specific process A starts in say 0 and ends say in 5, while process B starts in 0 and ends say in File descriptors are nothing but references for any open resource. As soon as you open a resource the kernel assumes you will be doing some operations on it.

All the communication via your program and the resource happens over an interface and this interface is provided by the file-descriptor. Since a process can open more than one resource, it is possible for a resource to have more than one file-descriptors.

I'm don't know the kernel code, but I'll add my two cents here since I've been thinking about this for some time, and I think it'll be useful. A file descriptor is an implementation of an API for the file you're opening. The kernel creates this file descriptor, stores it in an array, and gives it to you.

Now, think about what I said again, remembering that everything is a file — printers, monitors, HTTP connections etc. File Descriptors are the descriptors to a file. They give links to a file.

With the help of them we can read, write and open a file. How are we doing? Please help us improve Stack Overflow. Take our short survey.