In
computer programming
Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programming involves tasks such as anal ...
, standard streams are interconnected input and output
communication channel
A communication channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used for inform ...
s between a computer program and its environment when it begins execution. The three
input/output
In computing, input/output (I/O, or informally io or IO) is the communication between an information processing system, such as a computer, and the outside world, possibly a human or another information processing system. Inputs are the signals ...
(I/O) connections are called standard input (stdin), standard output (stdout) and standard error (stderr). Originally I/O happened via a physically connected
system console
One meaning of system console, computer console, root console, operator's console, or simply console is the text entry and display device for system administration messages, particularly those from the BIOS or boot loader, the kernel, from the i ...
(input via keyboard, output via monitor), but standard streams abstract this. When a command is executed via an interactive
shell, the streams are typically connected to the
text terminal on which the shell is running, but can be changed with
redirection or a
pipeline. More generally, a
child process
A child process in computing is a process created by another process (the parent process). This technique pertains to multitasking operating systems, and is sometimes called a subprocess or traditionally a subtask.
There are two major procedure ...
inherits the standard streams of its
parent process
In computing, a parent process is a process that has created one or more child processes.
Unix-like systems
In Unix-like operating systems, every process except (the swapper) is created when another process executes the fork() system call. Th ...
.
Application
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, or
binary data
Binary data is data whose unit can take on only two possible states. These are often labelled as 0 and 1 in accordance with the binary numeral system and Boolean algebra.
Binary data occurs in many different technical and scientific fields, wher ...
.
In many modern systems, the standard error stream of a program 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 the
pipeline character. A well-known example is the use of a
pagination
Pagination, also known as paging, is the process of dividing a document into discrete pages, either electronic pages or printed pages.
In reference to books produced without a computer, pagination can mean the consecutive page numbering to indi ...
application, such as
more, providing the user control over the display of the output stream on the display.
Background
In most operating systems predating
Unix
Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
, 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 a
punch card reader
A computer punched card reader or just computer card reader is a computer input device used to read computer programs in either source or executable form and data from punched cards. A computer card punch is a computer output device that punche ...
,
magnetic tape drive,
disk drive,
line printer
A line printer prints one entire line of text before advancing to another line. Most early line printers were
impact printers.
Line printers are mostly associated with unit record equipment and the early days of digital computing, but the ...
, card punch, or interactive terminal.
One of Unix's several groundbreaking advances was ''abstract 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 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 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 language
Job Control Language (JCL) is a name for scripting languages used on IBM mainframe operating systems to instruct the system on how to run a batch job or start a subsystem.
More specifically, the purpose of JCL is to say which programs to run, ...
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 obliged 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 (stdin)
Standard input is a stream from which a program reads its input data. The program requests data transfers by use of the ''read'' operation. Not all programs require stream input. For example, the ''
dir'' and ''
ls'' programs (which display file names contained in a directory) may take
command-line arguments, but perform their operations without any stream data input.
Unless
redirected, standard input is inherited from the parent process. In the case of an interactive shell, that is usually associated with the
keyboard.
The
file descriptor
In Unix and Unix-like computer operating systems, a file descriptor (FD, less frequently fildes) is a process-unique identifier ( handle) for a file or other input/output resource, such as a pipe or network socket.
File descriptors typically ...
for standard input is 0 (zero); the
POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inte ...
''
'' definition is STDIN_FILENO
; the corresponding C '''' variable is FILE* stdin
; similarly, the C++ '''' variable is std::cin
.
Standard output (stdout)
Standard output is a stream to which 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
Move may refer to:
People
* Daniil Move (born 1985), a Russian auto racing driver
Brands and enterprises
* Move (company), an online real estate company
* Move (electronics store), a defunct Australian electronics retailer
* Daihatsu Move
...
'', or ''ren Ren or REN may refer to:
Abbreviations
* Orenburg Tsentralny Airport, IATA code REN, civil airport in Russia
* Redes Energéticas Nacionais (REN), Portuguese company
* Renanthera, abbreviated as Ren, orchid genus
* Ringer equivalence number ( ...
'') is silent on success.
Unless redirected, standard output is inherited from the parent process. In the case of an interactive shell, that is usually the text terminal which initiated the program.
The file descriptor
In Unix and Unix-like computer operating systems, a file descriptor (FD, less frequently fildes) is a process-unique identifier ( handle) for a file or other input/output resource, such as a pipe or network socket.
File descriptors typically ...
for standard output is 1 (one); the POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inte ...
'''' definition is STDOUT_FILENO
; the corresponding C '''' variable is FILE* stdout
; similarly, the C++ '''' variable is std::cout
.
Standard error (stderr)
Standard error is another output stream typically used by programs to output error message
An error message is information displayed when an unforeseen occurs, usually on a computer or other device. On modern operating systems with graphical user interfaces, error messages are often displayed using dialog boxes. Error messages are us ...
s or diagnostics. It is a stream independent of standard output and can be redirected separately.
This solves the semi-predicate problem, allowing output and errors to be distinguished, and is analogous to a function returning a pair of values – see Semi-predicate problem: Multi valued return. 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 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 direct ''standard output'' and ''standard error'' 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, 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.
The file descriptor
In Unix and Unix-like computer operating systems, a file descriptor (FD, less frequently fildes) is a process-unique identifier ( handle) for a file or other input/output resource, such as a pipe or network socket.
File descriptors typically ...
for standard error is defined by POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inte ...
as 2 (two); the ''<unistd.h>'' header file provides the symbol STDERR_FILENO
; the corresponding C ''<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.
Bourne-style shells allow ''standard error'' to be redirected to the same destination that standard output is directed to using
2>&1
csh-style shells allow ''standard error'' to 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
Fortran has the equivalent of Unix file descriptors: By convention, many Fortran implementations use unit numbers UNIT=5
for stdin, UNIT=6
for stdout and UNIT=0
for stderr. In Fortran-2003, the intrinsic ISO_FORTRAN_ENV
module was standardized to include the named constants INPUT_UNIT
, OUTPUT_UNIT
, and ERROR_UNIT
to portably specify the unit numbers.
! FORTRAN 77 example
PROGRAM MAIN
INTEGER NUMBER
READ(UNIT=5,*) NUMBER
WRITE(UNIT=6,'(A,I3)') ' NUMBER IS: ',NUMBER
END
! Fortran 2003 example
program main
use iso_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 60
ALGOL 60 (short for ''Algorithmic Language 1960'') is a member of the ALGOL family of computer programming languages. It followed on from ALGOL 58 which had introduced code blocks and the begin and end pairs for delimiting them, representing a k ...
was criticized for having no standard file access.
1968: ALGOL 68
ALGOL 68
ALGOL 68 (short for ''Algorithmic Language 1968'') is an imperative programming language that was conceived as a successor to the ALGOL 60 programming language, designed with the goal of a much wider scope of application and more rigorously ...
'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
.
1970s: C and Unix
In the C programming language
''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the language, as well as ...
, the standard input, output, and error streams are attached to the existing Unix file descriptors 0, 1 and 2 respectively. In a POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inte ...
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.
Ken Thompson
Kenneth Lane Thompson (born February 4, 1943) is an American pioneer of computer science. Thompson worked at Bell Labs for most of his career where he designed and implemented the original Unix operating system. He also invented the B programmi ...
(designer and implementer of the original Unix operating system) modified sort in Version 5 Unix to accept "-" as representing standard input, which spread to other utilities and became a part of the operating system as a special file
In Unix-like operating systems, a device file or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These special files al ...
in Version 8. Diagnostics were part of standard output through Version 6, after which Dennis M. Ritchie
Dennis MacAlistair Ritchie (September 9, 1941 – October 12, 2011) was an American computer scientist. He is most well-known for creating the C programming language and, with long-time colleague Ken Thompson, the Unix operating system and B ...
created the concept of standard error.
1995: Java
In Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
, the standard streams are referred to by (for stdin), (for stdout), and (for stderr).
public static void main(String args[])
2000s: .NET
In C Sharp (programming language), C# and other .NET Framework, .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)
' Visual Basic .NET example
Public Function Main() As Integer
Try
Dim s As String = System.Console.[In].ReadLine()
Dim number As Double = Double.Parse(s)
System.Console.Out.WriteLine("Number is: ", number)
Return 0
' If Parse() threw an exception
Catch ex As System.ArgumentNullException
System.Console. rrorWriteLine("No number was entered!")
Catch ex2 As System.FormatException
System.Console. rrorWriteLine("The specified value is not a valid number!")
Catch ex3 As System.OverflowException
System.Console. rrorWriteLine("The specified number is too big!")
End Try
Return -1
End Function
When applying the System.Diagnostics.Process
class one can use the instance properties
Property is the ownership of land, resources, improvements or other tangible objects, or intellectual property.
Property may also refer to:
Mathematics
* Property (mathematics)
Philosophy and science
* Property (philosophy), in philosophy an ...
StandardInput
, StandardOutput
, and StandardError
of that class to access the standard streams of the process.
2000 - : Python (2 or 3)
The following example shows how to redirect the standard input both to the standard output
and to a text file.
#!/usr/bin/env python
import sys
# 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
for inps in stdin_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 interface
The GUI ( "UI" by itself is still usually pronounced . or ), graphical user interface, is a form of user interface that allows User (computing), users to Human–computer interaction, interact with electronic devices through graphical icon (comp ...
s (GUIs) don't always make use of the standard streams; they do when GUIs are wrappers of underlying scripts and/or console programs, for instance the Synaptic package manager GUI, which wraps apt commands in Debian and/or Ubuntu. GUIs created with scripting tools like Zenity and KDialog by KDE project make use of stdin, stdout, and stderr, and are based on simple scripts rather than a complete GUI programmed and compiled in C/C++ using Qt, GTK, or other equivalent proprietary widget framework.
The Services menu, as implemented on NeXTSTEP
NeXTSTEP is a discontinued object-oriented, multitasking operating system based on the Mach kernel and the UNIX-derived BSD. It was developed by NeXT Computer in the late 1980s and early 1990s and was initially used for its range of proprieta ...
and Mac OS X
macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and lapt ...
, is also analogous to standard streams. On these operating systems, graphical applications can provide functionality through a system-wide menu that operates on the current selection
Selection may refer to:
Science
* Selection (biology), also called natural selection, selection in evolution
** Sex selection, in genetics
** Mate selection, in mating
** Sexual selection in humans, in human sexuality
** Human mating strat ...
in 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 emulators pSX and DOSBox
DOSBox is a free and open-source emulator which runs software for MS-DOS compatible disk operating systems—primarily video games. It was first released in 2002, when DOS technology was becoming obsolete. Its adoption for running DOS games i ...
.
GTK-server
GTK-server is an open-source project released under the GNU General Public License. The GTK-server project aims to bring Graphical User Interface programming to any interpreted language using the GIMP Tool Kit (GTK) or XForms.
Philosophy
The GTK-s ...
can use stdin as a 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.
See also
* Redirection (computing)
In computing, redirection is a form of interprocess communication, and is a function common to most command-line interpreters, including the various Unix shells that can redirect standard streams to user-specified locations.
In Unix-like oper ...
* Stream (computing)
In computer science, a stream is a sequence of data elements made available over time. A stream can be thought of as items on a conveyor belt being processed one at a time rather than in large batches.
Streams are processed differently from ...
* Input/output
In computing, input/output (I/O, or informally io or IO) is the communication between an information processing system, such as a computer, and the outside world, possibly a human or another information processing system. Inputs are the signals ...
* C file input/output
* SYSIN
This article discusses support programs included in or available for OS/360 and successors. IBM categorizes some of these programs as utilities
and others as service aids; the boundaries are not always consistent or obvious. Many, but not all, of ...
and SYSOUT
This article discusses support programs included in or available for OS/360 and successors. IBM categorizes some of these programs as utilities
and others as service aids; the boundaries are not always consistent or obvious. Many, but not all, of ...
* Standard streams in OpenVMS
References
Sources
*
Standard Streams
* ''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 MULTICS
Honeywell Corporation, 1981
Evolution of the MVS Operating System
IBM Corporation, 1981
* ''Lions' Commentary on UNIX Sixth Edition'', John Lions, , 1977
Console Class, .NET Framework Class Library
Microsoft Corporation, 2008
External links
- by The Linux Information Project
- by The Linux Information Project
- by The Linux Information Project
{{DEFAULTSORT:Standard Streams
Unix