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.
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.1‐2008 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:
STDIN_FILENO Standard input value, stdin. Its value is 0.
STDOUT_FILENO Standard output value, stdout. Its value is 1.
STDERR_FILENO Standard error value, stderr. Its value is 2.
The stderr stream is expected to be open for reading and writing.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 STDIN(3P)