stdin, stdout, stderr

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Input/output library

I/O manipulators

Print functions (C++23)

C-style I/O

Buffers

basic_streambuf

basic_filebuf

basic_stringbuf

basic_spanbuf (C++23)

strstreambuf (C++98/26*)

basic_syncbuf (C++20)

Streams

Abstractions

ios_base

basic_ios

basic_istream

basic_ostream

basic_iostream

File I/O

basic_ifstream

basic_ofstream

basic_fstream

String I/O

basic_istringstream

basic_ostringstream

basic_stringstream

Array I/O

basic_ispanstream (C++23)

basic_ospanstream (C++23)

basic_spanstream (C++23)

istrstream (C++98/26*)

ostrstream (C++98/26*)

strstream (C++98/26*)

Synchronized Output

basic_osyncstream (C++20)

Types

streamoff

streamsize

fpos

Error category interface

iostream_category (C++11)

io_errc (C++11)

 

C-style I/O

Types and objects
FILE fpos_t	stdinstdoutstderr
Functions
File access fopen freopen fclose fflush fwide setbuf setvbuf Direct input/output fread fwrite Unformatted input/output fgetcgetc fgets fputcputc fputs getchar gets (until C++14) putchar puts ungetc fgetwcgetwc fgetws fputwcputwc fputws getwchar putwchar ungetwc Formatted input scanffscanfsscanf vscanfvfscanfvsscanf (C++11)(C++11)(C++11)     wscanffwscanfswscanf vwscanfvfwscanfvswscanf (C++11)(C++11)(C++11)	Formatted output printffprintfsprintfsnprintf (C++11) vprintfvfprintfvsprintfvsnprintf (C++11)     wprintffwprintfswprintf vwprintfvfwprintfvswprintf File positioning ftell fgetpos fseek fsetpos rewind Error handling clearerr feof ferror perror Operations on files remove rename tmpfile tmpnam

 

Defined in header <cstdio>
#define stdin  /* implementation-defined */	(1)
#define stdout /* implementation-defined */	(2)
#define stderr /* implementation-defined */	(3)

Three text streams are predefined. These streams are implicitly opened and unoriented at program startup.

1) Associated with the standard input stream, used for reading conventional input. At program startup, the stream is fully buffered if and only if the stream can be determined not to refer to an interactive device.

2) Associated with the standard output stream, used for writing conventional output. At program startup, the stream is fully buffered if and only if the stream can be determined not to refer to an interactive device.

3) Associated with the standard error stream, used for writing diagnostic output. At program startup, the stream is not fully buffered.

What constitutes an interactive device is implementation-defined.

These macros are expanded to expressions of type std::FILE*.

Notes

Although not mandated by POSIX, the UNIX convention is that stdin and stdout are line-buffered if associated with a terminal and stderr is unbuffered.

These macros may be expanded to modifiable lvalues. If any of these std::FILE* lvalue is modified, subsequent operations on the corresponding stream result in unspecified or undefined behavior.

Example

This example shows a function similar to std::printf.

Run this code

#include <concepts>
#include <cstdio>
#include <type_traits>
 
template<typename T>
concept IsPrintable = std::integral<T> or std::floating_point<T> or std::is_pointer_v<T>;
 
int my_printf(char const* const format, IsPrintable auto const ... arguments)
{
    return std::fprintf(stdout, format, arguments...);
}
 
int main(int argv, char*[])
{
    my_printf("Strings and chars:\t%s %c\n", "hello", 'x');
    my_printf("Rounding:\t\t%f %.0f %.32f\n", 1.5, 1.5, 1.3);
    my_printf("Padding:\t\t%05.2f %.2f %5.2f\n", 1.5, 1.5, 1.5);
    my_printf("Scientific:\t\t%E %e\n", 1.5, 1.5);
    my_printf("Hexadecimal:\t\t%a %A 0x%X\n", 1.5, 1.5, &argv);
}

Possible output:

Strings and chars:  hello x
Rounding:           1.500000 2 1.30000000000000004440892098500626
Padding:            01.50 1.50  1.50
Scientific:         1.500000E+00 1.500000e+00
Hexadecimal:        0x1.8p+0 0X1.8P+0 0x2CFB41BC

See also

cinwcin	reads from the standard C input stream stdin (global object)
coutwcout	writes to the standard C output stream stdout (global object)
cerrwcerr	writes to the standard C error stream stderr, unbuffered (global object)
clogwclog	writes to the standard C error stream stderr (global object)
printffprintfsprintfsnprintf (C++11)	prints formatted output to stdout, a file stream or a buffer (function)
FILE	object type, capable of holding all information needed to control a C I/O stream (typedef)
C documentation for stdin, stdout, stderr