Stdin

Standard input, or stdin, is a standard data stream used for reading input by a program. It is one of the three standard I/O streams defined by many operating systems and programming environments, alongside standard output (stdout) and standard error (stderr). By default, stdin is connected to the keyboard in interactive programs but it can be redirected from files, devices, or other programs.

In C and C++ with the standard library, stdin is a FILE* object defined in stdio.h. It is normally buffered, and input functions such as getchar, fgets, and scanf read from it. In POSIX terms, the standard input corresponds to file descriptor 0; the read(2) system call reads from this descriptor. The actual source can be a terminal, a file, or a pipe.

Shells and many programming languages support redirecting or piping data into stdin. In command lines, you can supply input from a file using program < input.txt, or pipe output from one command into another using command1 | command2. If there is no redirection, programs running in a terminal usually read from the keyboard.

Examples by language: In Python, input() reads a line from stdin. In Java, System.in provides an InputStream for stdin (often used with a Scanner). In JavaScript (Node.js), process.stdin is a stream used to read data. In C, scanf and fgets read from stdin; in shells, builtins like read read from stdin.

Notes: Stdin is not a file by itself; it is a stream abstraction that may refer to various input sources. It is central to scripting and batch processing, enabling non-interactive input handling and data pipelines.