stderr(3) — Linux manual page


STDIN(3)                  Linux Programmer's Manual                 STDIN(3)

NAME         top

       stdin, stdout, stderr - standard I/O streams

SYNOPSIS         top

       #include <stdio.h>

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

DESCRIPTION         top

       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

       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

       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.

CONFORMING TO         top

       The stdin, stdout, and stderr macros conform to C89 and this standard
       also stipulates that these three streams shall be open at program

NOTES         top

       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).

SEE ALSO         top

       csh(1), sh(1), open(2), fopen(3), stdio(3)

COLOPHON         top

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Linux                            2017-09-15                         STDIN(3)