File Descriptor


A file descriptor is a unique identifier, typically an integer, used by an operating system to reference an open file or input/output resource, such as a file, socket, or pipe. File descriptors act as handles to access and manage the underlying resources within the operating system. They allow for efficient data transmission while providing an abstracted interface to application programs for managing different types of resources.


The phonetic pronunciation of “File Descriptor” is: /ˈfaɪl dɪˈskrɪptər/

Key Takeaways

  1. A file descriptor is a non-negative integer that acts as an identifier for an open file or input/output resource in an operating system; it’s used to access files, I/O devices, sockets, or pipes.
  2. File descriptors are created through system calls such as open, socket, and pipe. They can be manipulated using I/O operations like read, write, and close, and are released automatically when a process terminates.
  3. Standard file descriptors are assigned the values 0, 1, and 2 for standard input (stdin), standard output (stdout), and standard error (stderr) respectively. These are pre-allocated and available to all processes by default.


The technology term “file descriptor” is important because it refers to a crucial component in modern operating systems that helps manage access and communication with files and other I/O resources.

File descriptors serve as unique, non-negative integers assigned by the operating system to represent open files, sockets, pipes, or other input/output objects within a process, allowing efficient indexing and manipulation of these resources.

This abstraction simplifies the underlying complexities of file and I/O management for developers and ensures a standard way to operate across various devices and file systems.

In essence, file descriptors play a vital role in enabling seamless interaction between applications and the underlying system resources while maintaining the integrity and organization of data traffic.


A file descriptor is an essential element in the realm of computer programming, often utilized for maintaining efficient communication between various components of an application and the operating system. The primary purpose of a file descriptor is to serve as an abstract handle or reference point that uniquely identifies an open file, a pipe, or a network socket.

This makes it possible for an application to read data from, write data to, or control files, sockets or other I/O devices. File descriptors have a high degree of importance since they meticulously manage the flow of data within the software and its interaction with the underlying system, ultimately allowing for more streamlined and effective computing processes.

File descriptors are especially useful in scenarios where multiple open files or connections are being managed simultaneously by an application, avoiding issues such as data leakage and resource contention. As an example, in the UNIX operating system, the first three file descriptors are reserved for standard input (stdin), standard output (stdout), and standard error (stderr), with any additional files using incremental file descriptor numbers as they are created.

Another noteworthy aspect of file descriptors is their ability to facilitate complex operations like duplicating or redirecting I/O streams, which proves invaluable for applications that involve data interchanging between different processes. Overall, file descriptors are integral to ensuring optimal resource management and seamless data exchange within a computing system.

Examples of File Descriptor

File descriptors are integral elements in various operating systems that help in managing the interactions between the OS and active files or resources. Here are three real-world examples of how file descriptors are used in technology:

Unix/Linux Operating Systems: In Unix-based operating systems like Linux, a file descriptor is a non-negative integer representing an open file or a resource. When a file is opened in read or write mode, a file descriptor is generated and assigned to it. For instance, by default, the numbers 0, 1, and 2 are assigned to standard input (stdin), standard output (stdout), and standard error (stderr), respectively. Programmers and system administrators often work with file descriptors when creating scripts or managing file I/O (input/output) operations.

Database Systems: Many database systems use file descriptors to handle the active file connections they manage. When a database system opens a data file, log file, or any other resource, it assigns a file descriptor to efficiently perform read and write operations, track file positions, and manage file locking mechanisms. This helps maintain the interactions between the database application and the underlying operating system, ensuring that data reads and writes are handled efficiently and reliably.

Web Servers: Web servers like Apache and Nginx use file descriptors extensively to manage connections between the server and the clients. When a client makes a request to the webserver, a new file descriptor is created for each established connection. The server utilizes these descriptors to read and write data to and from the clients, manage connections efficiently, and instantly detect problems with open connections. This is especially critical during high-traffic periods, as the server needs a reliable way to manage multiple concurrent connections.

File Descriptor FAQ

What is a File Descriptor?

A File Descriptor is a non-negative integer that serves as an abstract indicator for accessing a file or I/O resource, such as a file, socket, or pipe. In most operating systems, file descriptors provide a uniform interface for different input/output operations.

How do File Descriptors work?

When a program opens a file or creates a socket, it requests the operating system to create an entry in a table to manage the resource. The operating system associates a unique file descriptor number with this entry. The program then uses this number for further I/O operations, and the operating system translates these operations using the file descriptor to perform actions on the actual resource.

What are the standard File Descriptors in Unix-like systems?

In Unix-like systems, there are three standard file descriptors by default: standard input (stdin), standard output (stdout), and standard error (stderr), which have file descriptor numbers 0, 1, and 2, respectively. Programs read input from stdin, write output to stdout, and write error messages to stderr.

What is the process of closing a File Descriptor?

To close a file descriptor, the “close()” system call is used in most programming languages. It informs the operating system to release the associated resource and remove the file descriptor entry from the table. Closing a file descriptor is essential to prevent resource leaks and to free up system resources for future use.

How do you duplicate a File Descriptor?

Duplicating a file descriptor can be done using the “dup()” or “dup2()” system calls. When duplicated, a new file descriptor is created, pointing to the same file or resource as the original file descriptor. The new file descriptor can be used independently, and any changes to file offsets or flags will not affect the other one. This is useful for scenarios such as redirecting the output of a program to a file instead of the screen.

Related Technology Terms

  • File Descriptor Table
  • File System Operations
  • File I/O (Input/Output)
  • File Handles
  • Process Control Block (PCB)

Sources for More Information


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