If you are working with input and output operations such as network sockets or pipes, or if you’re developing applications using the POSIX application programming interface, then understanding what a file descriptor (FD) is can help you in your work. A file descriptor is an integer that helps identify an open file within a process. It can be considered as an index table of open files.
Definition of File Descriptor
A file descriptor is a small, non-negative integer that is generated by the operating system kernel when it encounters an open call. It is an important part of the POSIX application programming interface and provides a low-level interface for input and output operations. A file descriptor is the Unix abstraction for an open input/output stream, which can be a file, network connection, pipe, terminal, and others.
Functions of File Descriptor
When working with files in Unix, the file descriptor is passed as a parameter to system calls such as read, write, and close. The file descriptor is used to keep track of currently open files within a process. It also enables communication between different processes using pipes, as each pipe has a pair of file descriptors representing its input and output ends.
Uses of File Descriptor
File descriptors are widely used within the Unix architecture for input and output operations. An operating system kernel creates a file descriptor for each open file during system calls. The file descriptors are stored in a file descriptor table and are represented by integers that make it easy for the kernel to manage them within the operating system’s architecture.
The standard input, output, and error files are represented by file descriptor 0, 1, and 2 respectively. It simplifies communication between different processes, as it allows processes to communicate using pipes over the network connection.
FAQs
What is the difference between a file descriptor and a file pointer?
A file pointer is a high-level abstraction that points to a specific location within a file and helps to process data in a structured way. A file descriptor, on the other hand, is a low-level abstraction that acts as an index table of open files allowing operating system kernel to manage input and output operations.
Are file descriptors platform dependent?
Yes, file descriptors are platform dependent. Different operating systems might use different file descriptor values or not use file descriptors at all.
Can a file descriptor represent more than one file?
No, file descriptors are for a specific file only, and each file has its unique file descriptor.
Wrapping Up
A file descriptor is a crucial part of the Unix architecture, and understanding how it works can help in developing applications that use input and output streams. It is the index table of open files and allows communication between processes using pipes, network connections, and other input and output resources.
