stdin - Man Page

standard I/O streams


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <stdio.h>

extern FILE *stderr, *stdin, *stdout;


The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard.

A file with associated buffering is called a stream and is declared to be a pointer to a defined type FILE. The fopen() function shall create certain descriptive data for a stream and return a pointer to designate the stream in all further transactions. Normally, there are three open streams with constant pointers declared in the <stdio.h> header and associated with the standard open files.

At program start-up, three streams shall be predefined and need not be opened explicitly: standard input (for reading conventional input), standard output (for writing conventional output), and standard error (for writing diagnostic output). When opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device.

The following symbolic values in <unistd.h> define the file descriptors that shall be associated with the C-language stdin, stdout, and stderr when the application is started:


Standard input value, stdin. Its value is 0.


Standard output value, stdout. Its value is 1.


Standard error value, stderr. Its value is 2.

The stderr stream is expected to be open for reading and writing.

Return Value



No errors are defined.

The following sections are informative.



Application Usage




Future Directions


See Also

fclose(), feof(), ferror(), fileno(), fopen(), fprintf(), fread(), fscanf(), fseek(), getc(), gets(), popen(), putc(), puts(), read(), setbuf(), setvbuf(), tmpfile(), ungetc(), vfprintf()

The Base Definitions volume of POSIX.1-2017, <stdio.h>, <unistd.h>


2017 IEEE/The Open Group POSIX Programmer's Manual