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The standard streams for input, output, and error

The standard streams are preconnected input and output channels between a computer program and its environment (typically a text terminal) when it begins execution. These standard connections are provided in Unix and Unix-like operating systems, C and C++ runtime environments, and their successors. The three I/O connections are called standard input, standard output and standard error.

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 text terminal which started the program.

The file descriptor for standard input is 0 (zero); the corresponding variable is FILE* stdin; similarly, the 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 corresponding variable is FILE* stdout; similarly, the 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 corresponding variable is FILE* stderr. The C++ 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.

Fortran had the equivalent of Unix file descriptors, UNIT=5 for stdin, and UNIT=6 for stdout.

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. This standard included: stand in, stand out, stand error and stand back.

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)
)

3.14159

Number is: +3.142 OR Number is: +3.142!

In the C programming language stdin, stdout and stderr streams were attached to the existing Unix file descriptors 0, 1 and 2 respectively.

In C++, the standard streams are referred to by cin, cout and cerr objects (see iostream).

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 analog 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 uses its stdin to display status information.

Some GUI programs, primarily on Unix, still write debug information to standard error.

• Redirection (Unix)
• Pipeline (Unix)
• Stream (computer)
• Input/output
• C file input/output

• 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, ISBN 1-57398-013-7, 1977

• Standard Output Definition - by The Linux Information Project (LINFO)