In Unix and Unix-like operating systems (and, to some extent, Windows), as well as certain programming language interfaces, the standard streams are preconnected input and output channels between a computer program and its environment (typically a text terminal) when it begins execution. The three I/O connections are called standard input (stdin), standard output (stdout) and standard error (stderr).
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In most operating systems predating Unix, programs had to explicitly connect to the appropriate input and output data. On many of those systems, this could be an intimidating programming challenge created by OS-specific intricacies such as obtaining control environment settings, accessing a local file table, determining the intended data set, and handling the correct case of a card reader, magnetic tape drive, disk drive, line printer, card punch, or interactive terminal.
Unix provided several groundbreaking advances, one of which was to provide abstract devices: it removed the need for a program to know or care what kind of devices it was communicating with. Older operating systems forced upon the programmer a record structure and, frequently non-orthogonal data semantics and device control. Unix eliminated this complexity with the concept of a data stream: an ordered sequence of data bytes which can be read until the end of file. A program may also write bytes as desired and need not (and can't easily) declare how many there will be, or how they will be grouped.
Another Unix breakthrough was to automatically associate input and output by default—the program (and programmer) did absolutely nothing to establish input and output for a typical input-process-output program (unless it chose a different paradigm). In contrast, previous operating systems usually required some—often complex—job control language to establish connections, or the equivalent burden had to be orchestrated by the program.
Since Unix provided standard streams, the Unix C runtime environment was obligated to support it as well. As a result, most C runtime environments (and C's descendants), regardless of the operating system, provide equivalent functionality.
Standard input is data (often text) going into a program. The program requests data transfers by use of the read operation. Not all programs require input. For example, the dir or ls program (which displays file names contained in a directory) performs its operation without any stream data input.
Unless redirected, input is expected from the keyboard which started the program.
The file descriptor for standard input is 0 (zero); the POSIX <unistd.h> definition is STDIN_FILENO; the corresponding <stdio.h> variable is FILE* stdin; similarly, the <iostream> variable is std::cin.
Standard output is the stream where a program writes its output data. The program requests data transfer with the write operation. Not all programs generate output. For example the file rename command (variously called mv, move, ren) is silent on success.
Unless redirected, standard output is the text terminal which initiated the program.
The file descriptor for standard output is 1 (one); the POSIX <unistd.h> definition is STDOUT_FILENO; the corresponding <stdio.h> variable is FILE* stdout; similarly, the <iostream> variable is std::cout.
Standard error is another output stream typically used by programs to output error messages or diagnostics. It is a stream independent of standard output and can be redirected separately. The usual destination is the text terminal which started the program to provide the best chance of being seen even if standard output is redirected (so not readily observed). For example, output of a program in a pipeline is redirected to input of the next program, but errors from each program still go directly to the text terminal.
It is acceptable—and normal—for standard output and standard error to be directed to the same destination, such as the text terminal. Messages appear in the same order as the program writes them, unless buffering is involved. (For example, a common situation is when the standard error stream is unbuffered but the standard output stream is line-buffered; in this case, text written to standard error later may appear on the terminal earlier, if the standard output stream's buffer is not yet full.)
The file descriptor for standard error is 2; the POSIX <unistd.h> definition is STDERR_FILENO; the corresponding <stdio.h> variable is FILE* stderr. The C++ <iostream> standard header provides two variables associated with this stream: std::cerr and std::clog, the former being unbuffered and the latter using the same buffering mechanism as all other C++ streams.
Most shells allow both standard output and standard error to be redirected to the same file using
&> filename
Bourne-style shells allow standard error to be redirected to the same destination that standard output is directed to using
2>&1
Fortran has the equivalent of Unix file descriptors: UNIT=5 for stdin, UNIT=6 for stdout and UNIT=0 for stderr.
! FORTRAN 77 example PROGRAM MAIN READ(UNIT=5,*)NUMBER WRITE(UNIT=6,'(F5.3)')' NUMBER IS: ',NUMBER END
ALGOL 60 was criticized for having no standard file access.
ALGOL 68's input and output facilities were collectively referred to as the transput. Koster coordinated the definition of the transput standard. The model included three standard channels: stand in, stand out, and stand back.
Example:
# ALGOL 68 example # main:( REAL number; getf(stand in,($g$,number)); printf(($"Number is: "g(6,4)"OR "$,number)); # OR # putf(stand out,($" Number is: "g(6,4)"!"$,number)); newline(stand out) )
| Input: | Output: |
|---|---|
3.14159 |
Number is: +3.142 OR Number is: +3.142! |
In the C programming language the standard input, output, and error streams are attached to the existing Unix file descriptors 0, 1 and 2 respectively. In a POSIX environment the <unistd.h> definitions STDIN_FILENO, STDOUT_FILENO or STDERR_FILENO should be used instead rather than magic numbers. File pointers stdin, stdout, and stderr are also provided.
In Java, the standard streams are referred to by System.in (for stdin), System.out (for stdout), and System.err (for stderr).
public static void main(String args[]) { try { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); String s = br.readLine(); double number = Double.parseDouble(s); System.out.println("Number is:" + number); } catch (Exception e) { System.err.println("Error:" + e.getMessage()); } }
Or you can use the Scanner class of package java.util.
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
while(sc.hasNextLine()) {
String line = sc.nextLine();
double number = Double.parseDouble(line);
System.out.println("Number is: " + number);
}
}
In C# and other .NET languages, the standard streams are referred to by System.Console.In (for stdin), System.Console.Out (for stdout) and System.Console.Error (for stderr). Basic read and write capabilities for the stdin and stdout streams are also accessible directly through the class System.Console (e.g. System.Console.WriteLine() can be used instead of System.Console.Out.WriteLine()).
System.Console.In, System.Console.Out and System.Console.Error are System.IO.TextReader (stdin) and System.IO.TextWriter (stdout, stderr) objects, which only allow access to the underlying standard streams on a text basis. Full binary access to the standard streams must be performed through the System.IO.Stream objects returned by System.Console.OpenStandardInput(), System.Console.OpenStandardOutput() and System.Console.OpenStandardError() respectively.
// C# example public static int Main(string[] args) { try { string s = System.Console.In.ReadLine(); double number = double.Parse(s); System.Console.Out.WriteLine("Number is: {0:F3}", number); return 0; // If Parse() threw an exception } catch (System.ArgumentNullException) { System.Console.Error.WriteLine("No number was entered!"); } catch (System.FormatException) { System.Console.Error.WriteLine("The specified value is not a valid number!"); } catch (System.OverflowException) { System.Console.Error.WriteLine("The specified number is too big!"); } return -1; }
' Visual Basic .NET example Public Function Main() As Integer Dim number As Double Dim s As String Try s = System.Console.In.ReadLine() number = CDbl(s) System.Console.Out.WriteLine("Number is: {0:F3}", number) Return 0 Catch e As System.InvalidCastException ' if CDbl() threw an exception System.Console.Error.WriteLine("No number was entered!") Return 1 End Try End Function
When applying the System.Diagnostics.Process class one can use the instance properties StandardInput, StandardOutput, and StandardError of that class to access the standard streams of the process.
Graphical user interfaces (GUIs) rarely make use of the standard streams. Consequently, redirecting GUI programs or constructing a GUI pipeline is neither practical nor useful. The nearest analogy is probably cutting (or copying) from one application and pasting into another. Since manual user operations are required, moving large numbers of pastes is not especially efficient. One notable exception is the dwm tiling window manager, which displays data directed through stdin on a status bar.
Some GUI programs, primarily on Unix, still write debug information to standard error. Others may take files to operate from standard in (for example many Unix media players do so).
Popular Windows programs that open a separate console window in addition to their GUI windows are the emulators pSX and DOSBox.
GTK-server can use stdin as communication interface with an interpreted program to realize a GUI.
The Common Lisp Interface Manager paradigm "presents" GUI elements sent to an extended output stream..
iostream library uses c prefix instead of std for naming standard streams. In addition to cin, cout and cerr, it offers the clog standard stream, similar to cerr, but buffered.