Standard streams

This article is about standard I/O file descriptors. For System V streams, seeSTREAMS.

Incomputer programming,standard streamsare preconnected input and outputcommunication channelsbetween a computer program and its environment when it begins execution. The threeinput/output(I/O) connections are calledstandard input(stdin),standard output(stdout) andstandard error(stderr). Originally I/O happened via a physically connectedsystem console(input via keyboard, output via monitor), but standard streams abstract this. When a command is executed via an interactiveshell,the streams are typically connected to thetext terminalon which the shell is running, but can be changed withredirectionor apipeline.More generally, achild processinherits the standard streams of itsparent process.

Application

The standard streams for input, output, and error in a common default configuration

Users generally know standard streams as input and output channels that handle data coming from an input device, or that write data from the application. The data may be text with any encoding, orbinary data. When a program is run as adaemon,its standard error stream is redirected into a log file, typically for error analysis purposes.

Streams may be used to chain applications, meaning that the output stream of one program can be redirected to be the input stream to another application. In many operating systems this is expressed by listing the application names, separated by the vertical bar character, for this reason often called thepipelinecharacter. A well-known example is the use of apaginationapplication, such asmore,providing the user control over the display of the output stream on the display.

Background

In most operating systems predatingUnix,programs had to explicitly connect to the appropriate input and output devices. OS-specific intricacies caused this to be a tedious programming task. On many systems it was necessary to obtain control of environment settings, access a local file table, determine the intended data set, and handle hardware correctly in the case of apunch card reader,magnetic tape drive,disk drive,line printer,card punch, or interactive terminal.

One of Unix's several groundbreaking advances wasabstract devices,which 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 frequentlynon-orthogonaldata 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 theend of file.A program may also write bytes as desired and need not, and cannot easily declare their count or grouping.

Another Unix breakthrough was to automatically associate input and output to terminal keyboard and terminal display, respectively, 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 languageto establish connections, or the equivalent burden had to be orchestrated by the program.

Since Unix provided standard streams, the UnixCruntime environment was obliged to support it as well. As a result, most C runtime environments (andC's descendants), regardless of the operating system, provide equivalent functionality.

Standard input (stdin)

Standard input is a stream from which a program reads its input data. The program requests data transfers by use of thereadoperation. Not all programs require stream input. For example, thedirandlsprograms (which display file names contained in a directory) may takecommand-line arguments,but perform their operations without any stream data input.

Unlessredirected,standard input is inherited from the parent process. In the case of an interactive shell, that is usually associated with the input device of aterminal(orpseudo terminal) which is ultimately linked to a user'skeyboard.

OnPOSIXsystems, thefile descriptorfor standard input is 0 (zero); thePOSIX<unistd.h>definition isSTDIN_FILENO;the corresponding C<stdio.h>abstraction is provided via theFILE* stdinglobal variable. Similarly, the global C++std::cinvariable of type<iostream>provides an abstraction viaC++ streams.Similar abstractions exist in the standard I/O libraries of practically everyprogramming language.

Standard output (stdout)

Standard output is a stream to which a program writes its output data. The program requests data transfer with thewriteoperation. Not all programs generate output. For example, thefile renamecommand (variously calledmv,move,orren) is silent on success.

Unlessredirected,standard output is inherited from the parent process. In the case of an interactive shell, that is usually thetext terminalwhich initiated the program.

Thefile descriptorfor standard output is 1 (one); thePOSIX<unistd.h>definition isSTDOUT_FILENO;the corresponding C<stdio.h>variable isFILE* stdout;similarly, the C++<iostream>variable isstd::cout.

Standard error (stderr)

Standard error is another output stream typically used by programs to outputerror messagesor diagnostics. It is a stream independent of standard output and can be redirected separately.

This solves thesemi-predicate problem,allowing output and errors to be distinguished, and is analogous to a function returning a pair of values – seeSemipredicate problem § Multivalued return.The usual destination is thetext terminalwhich started the program to provide the best chance of being seen even ifstandard outputis redirected (so not readily observed). For example, output of a program in apipelineis redirected to input of the next program or a text file, but errors from each program still go directly to the text terminal so they can be reviewed by the user in real time.

It is acceptable and normal to directstandard outputandstandard errorto the same destination, such as the text terminal. Messages appear in the same order as the program writes them, unlessbufferingis involved. For example, in common situations 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 buffer is not yet full.

Thefile descriptorfor standard error is defined byPOSIXas 2 (two); the<unistd.h>header file provides the symbolSTDERR_FILENO;the corresponding C<stdio.h>variable isFILE* stderr.The C++<iostream>standard header provides two variables associated with this stream:std::cerrandstd::clog,the former being unbuffered and the latter using the same buffering mechanism as all other C++ streams.

Bourne-style shells allowstandard errorto be redirected to the same destination that standard output is directed to using

2>&1

csh-style shells allowstandard errorto be redirected to the same destination that standard output is directed to using

>&

Standard error was added to Unix in the 1970s after several wasted phototypesetting runs ended with error messages being typeset instead of displayed on the user's terminal.

Timeline

1950s: Fortran

Fortranhas the equivalent of Unix file descriptors: By convention, many Fortran implementations use unit numbersUNIT=5for stdin,UNIT=6for stdout andUNIT=0for stderr. In Fortran-2003, the intrinsicISO_FORTRAN_ENVmodule was standardized to include the named constantsINPUT_UNIT,OUTPUT_UNIT,andERROR_UNITto portably specify the unit numbers.

!FORTRAN 77 example
PROGRAMMAIN
INTEGERNUMBER
READ(UNIT=5,*)NUMBER
WRITE(UNIT=6,'(A,I3)')' NUMBER IS: ',NUMBER
END

!Fortran 2003 example
programmain
useiso_fortran_env
implicit none
integer::number
read(unit=INPUT_UNIT,*)number
write(unit=OUTPUT_UNIT,'(a,i3)')'Number is: ',number
end program

1960: ALGOL 60

ALGOL 60was criticized for having no standard file access.

1968: ALGOL 68

ALGOL 68's input and output facilities were collectively referred to as the transput.Kostercoordinated the definition of thetransputstandard. The model included three standard channels:stand in,stand out,andstand 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!

1970s: C and Unix

In theC programming language,the standard input, output, and error streams are attached to the existing Unix file descriptors 0, 1 and 2 respectively.In aPOSIXenvironment the<unistd.h>definitionsSTDIN_FILENO,STDOUT_FILENOorSTDERR_FILENOshould be used instead rather thanmagic numbers.File pointersstdin,stdout,andstderrare also provided.

Ken Thompson(designer and implementer of the original Unix operating system) modifiedsortinVersion 5 Unixto accept "-" as representing standard input, which spread to other utilities and became a part of the operating system as aspecial fileinVersion 8.Diagnostics were part of standard output throughVersion 6,after whichDennis M. Ritchiecreated the concept of standard error.

1995: Java

InJava,the standard streams are referred to bySystem.in(for stdin),System.out(for stdout), andSystem.err(for stderr).

publicstaticvoidmain(Stringargs[]){
try{
BufferedReaderbr=
newBufferedReader(newInputStreamReader(System.in));
Strings=br.readLine();
doublenumber=Double.parseDouble(s);
System.out.println("Number is:"+number);
}catch(Exceptione){
System.err.println("Error:"+e.getMessage());
}
}

2000s:.NET

InC#and other.NETlanguages, the standard streams are referred to bySystem.Console.In(for stdin),System.Console.Out(for stdout) andSystem.Console.Error(for stderr).Basic read and write capabilities for the stdin and stdout streams are also accessible directly through the classSystem.Console(e.g.System.Console.WriteLine()can be used instead ofSystem.Console.Out.WriteLine()).

System.Console.In,System.Console.OutandSystem.Console.ErrorareSystem.IO.TextReader(stdin) andSystem.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 theSystem.IO.Streamobjects returned bySystem.Console.OpenStandardInput(),System.Console.OpenStandardOutput()andSystem.Console.OpenStandardError()respectively.

// C# example
publicstaticintMain(string[]args)
{
try{
strings=System.Console.In.ReadLine();
doublenumber=double.Parse(s);
System.Console.Out.WriteLine("Number is: {0:F3}",number);
return0;

// If Parse() threw an exception
}catch(ArgumentNullException){
System.Console.Error.WriteLine("No number was entered!");
}catch(FormatException){
System.Console.Error.WriteLine("The specified value is not a valid number!");
}catch(OverflowException){
System.Console.Error.WriteLine("The specified number is too big!");
}

return-1;
}

' Visual Basic.NET example

PublicFunctionMain()AsInteger
Try
DimsAsString=System.Console.[In].ReadLine()
DimnumberAsDouble=Double.Parse(s)
System.Console.Out.WriteLine("Number is: {0:F3}",number)
Return0

' If Parse() threw an exception
CatchexAsSystem.ArgumentNullException
System.Console.[Error].WriteLine("No number was entered!")
Catchex2AsSystem.FormatException
System.Console.[Error].WriteLine("The specified value is not a valid number!")
Catchex3AsSystem.OverflowException
System.Console.[Error].WriteLine("The specified number is too big!")
EndTry

Return-1
EndFunction

When applying theSystem.Diagnostics.Processclassone can use the instancepropertiesStandardInput,StandardOutput,andStandardErrorof that class to access the standard streams of the process.

2000 -: Python (2 or 3)

The following example, written inPython,shows how to redirect the standard input both to the standard output and to a text file.

#!/usr/bin/env python
importsys
# Save the current stdout so that we can revert sys.stdout
# after we complete our redirection
stdin_fileno=sys.stdin
stdout_fileno=sys.stdout
# Redirect sys.stdout to the file
sys.stdout=open("myfile.txt","w")
ctr=0
forinpsinstdin_fileno:
ctrs=str(ctr)
# Prints to the redirected stdout ()
sys.stdout.write(ctrs+") this is to the redirected --->"+inps+"\n")
# Prints to the actual saved stdout handler
stdout_fileno.write(ctrs+") this is to the actual --->"+inps+"\n")
ctr=ctr+1
# Close the file
sys.stdout.close()
# Restore sys.stdout to our old saved file handler
sys.stdout=stdout_fileno

GUIs

Graphical user interfaces(GUIs) do not always make use of the standard streams; they do when GUIs are wrappers of underlying scripts and/or console programs, for instance theSynapticpackage manager GUI, which wraps apt commands in Debian and/or Ubuntu. GUIs created with scripting tools like Zenity and KDialog byKDEprojectmake use of stdin, stdout, and stderr, and are based on simple scripts rather than a complete GUI programmed and compiled in C/C++ usingQt,GTK,or other equivalent proprietary widget framework.

TheServices menu,as implemented onNeXTSTEPandMac OS X,is also analogous to standard streams. On these operating systems, graphical applications can provide functionality through a system-wide menu that operates on the currentselectionin the GUI, no matter in what application.

Some GUI programs, primarily on Unix, still write debug information to standard error. Others (such as many Unix media players) may read files from standard input. Popular Windows programs that open a separate console window in addition to their GUI windows are the emulatorspSXandDOSBox.

GTK-servercan use stdin as a communication interface with an interpreted program to realize a GUI.

TheCommon Lisp Interface Managerparadigm "presents" GUI elements sent to an extended output stream.

See also

• Redirection (computing)
• Stream (computing)
• Input/output
• C file input/output
• SYSINandSYSOUT
• Standard streams in the Files-11 file system

Sources

• "Standard Streams",The GNU C Library
• KRONOS 2.1 Reference Manual,Control Data Corporation, Part Number 60407000, 1974
• NOS Version 1 Applications Programmer's Instant,Control Data Corporation, Part Number 60436000, 1978
• Level 68 Introduction to Programming on MULTICSArchived2021-02-25 at theWayback Machine,Honeywell Corporation, 1981
• Evolution of the MVS Operating System,IBM Corporation, 1981
• Lions' Commentary on UNIX Sixth Edition,John Lions,ISBN1-57398-013-7,1977
• Console Class,.NET Framework Class Library,Microsoft Corporation, 2008

External links

• Standard Input Definition- by The Linux Information Project
• Standard Output Definition- by The Linux Information Project
• Standard Error Definition- by The Linux Information Project