Ubuntu Manpage: stdin, stdout, stderr

NAME

       stdin, stdout, stderr - standard I/O streams

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <stdio.h>

       extern FILE *stdin;
       extern FILE *stdout;
       extern FILE *stderr;

DESCRIPTION

Under normal circumstances every UNIX program has three streams opened for it when it
starts up, one for input, one for output, and one for printing diagnostic or error
messages. These are typically attached to the user's terminal (see tty(4)) but might
instead refer to files or other devices, depending on what the parent process chose to set
up. (See also the "Redirection" section of sh(1).)

The input stream is referred to as "standard input"; the output stream is referred to as
"standard output"; and the error stream is referred to as "standard error". These terms
are abbreviated to form the symbols used to refer to these files, namely stdin, stdout,
and stderr.

Each of these symbols is a stdio(3) macro of type pointer to FILE, and can be used with
functions like fprintf(3) or fread(3).

Since FILEs are a buffering wrapper around UNIX file descriptors, the same underlying
files may also be accessed using the raw UNIX file interface, that is, the functions like
read(2) and lseek(2).

On program startup, the integer file descriptors associated with the streams stdin,
stdout, and stderr are 0, 1, and 2, respectively. The preprocessor symbols STDIN_FILENO,
STDOUT_FILENO, and STDERR_FILENO are defined with these values in <unistd.h>. (Applying
freopen(3) to one of these streams can change the file descriptor number associated with
the stream.)

Note that mixing use of FILEs and raw file descriptors can produce unexpected results and
should generally be avoided. (For the masochistic among you: POSIX.1, section 8.2.3,
describes in detail how this interaction is supposed to work.) A general rule is that
file descriptors are handled in the kernel, while stdio is just a library. This means for
example, that after an exec(3), the child inherits all open file descriptors, but all old
streams have become inaccessible.

Since the symbols stdin, stdout, and stderr are specified to be macros, assigning to them
is nonportable. The standard streams can be made to refer to different files with help of
the library function freopen(3), specially introduced to make it possible to reassign
stdin, stdout, and stderr. The standard streams are closed by a call to exit(3) and by
normal program termination.

STANDARDS

       C11, POSIX.1-2008.

       The standards also stipulate that these three streams shall be open at program startup.

HISTORY

       C89, POSIX.1-2001.

NOTES

       The stream stderr is unbuffered.  The stream stdout is line-buffered when it points  to  a
       terminal.   Partial  lines  will  not  appear  until  fflush(3) or exit(3) is called, or a
       newline is printed.  This  can  produce  unexpected  results,  especially  with  debugging
       output.   The  buffering mode of the standard streams (or any other stream) can be changed
       using the setbuf(3) or setvbuf(3) call.  Note that in case  stdin  is  associated  with  a
       terminal,  there may also be input buffering in the terminal driver, entirely unrelated to
       stdio buffering.  (Indeed, normally terminal input is line buffered in the kernel.)   This
       kernel input handling can be modified using calls like tcsetattr(3); see also stty(1), and
       termios(3).