Standard text streams

Names specified here
Name	Description	Source	Availability
`stderr`	Output stream for errors	`<stdio.h>`	C89	C90	C95	C99	C11
`stdin`	Standard input stream	`<stdio.h>`	C89	C90	C95	C99	C11
`stdout`	Standard output stream	`<stdio.h>`	C89	C90	C95	C99	C11

When a program starts executing, three text streams are already open for use. stdin is standard input, which by default is usually taken from the console or terminal that a program is executed in. stdout is standard output, which by default is usually directed to the console or terminal. stderr is standard error output, which by default is also usually directed to the console or terminal. All three are of type FILE *, and are declared in <stdio.h>.

Several functions read from stdin implicitly:

Functions reading from `stdin`
Name	Description			Source	Availability
`getchar()`	Input character from standard input		(·)	`<stdio.h>`	C89	C90	C95	C99	C11
`getwchar()`	Input wide character from standard input		(·)	`<wchar.h>`			C95	C99	C11
`scanf()`	Input formatted text		(·)	`<stdio.h>`	C89	C90	C95	C99	C11
`scanf_s()`	Input formatted text	?	(·)	`<stdio.h>`					C11
`vscanf()`	Input formatted text		(·)	`<stdio.h>`				C99	C11
`vscanf_s()`	Input formatted text	?	(·)	`<stdio.h>`					C11
`vwscanf()`	Input formatted text		(·)	`<wchar.h>`				C99	C11
`vwscanf_s()`	Input formatted text	?	(·)	`<wchar.h>`					C11
`wscanf()`	Input formatted text		(·)	`<wchar.h>`			C95	C99	C11
`wscanf_s()`	Input formatted text	?	(·)	`<wchar.h>`					C11

Several functions write to stdout implicitly:

Functions writing to `stdout`
Name	Description			Source	Availability
`printf()`	Print formatted text		(·)	`<stdio.h>`	C89	C90	C95	C99	C11
`printf_s()`	Print formatted text	?	(·)	`<stdio.h>`					C11
`putchar()`	Output character to standard output		(·)	`<stdio.h>`	C89	C90	C95	C99	C11
`putwchar()`	Output wide character to standard output		(·)	`<wchar.h>`			C95	C99	C11
`vprintf()`	Print formatted text		(·)	`<stdio.h>`	C89	C90	C95	C99	C11
`vprintf_s()`	Print formatted text	?	(·)	`<stdio.h>`					C11
`vwprintf()`	Print formatted text		(·)	`<wchar.h>`			C95	C99	C11
`vwprintf_s()`	Print formatted text	?	(·)	`<wchar.h>`					C11
`wprintf()`	Print formatted text		(·)	`<wchar.h>`			C95	C99	C11
`wprintf_s()`	Print formatted text	?	(·)	`<wchar.h>`					C11

The second output stream stderr allows programs to report error messages to the console even when the standard output has be redirected to another destination, such as a file or another program.

Functions writing to `stderr`
Name	Description				Source	Availability
`abort_handler_s()`	Report constraint violation and abort	?		(·)	`<stdlib.h>`					C11
`assert()`	Test assertion		M	(·)	`<assert.h>`	C89	C90	C95	C99	C11
`perror()`	Report last error			(·)	`<stdio.h>`	C89	C90	C95	C99	C11

Initially, stdin, stdout and stderr are unoriented. You should not use wide-oriented functions on stderr, because it would then be an error to use the standard functions that use it implicitly, which are byte-oriented and pertain to error handling. It's not a good idea to have an error in your error handler.