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STDIN(3)		 BSD Library Functions Manual		      STDIN(3)

NAME
     stdin, stdout, stderr -- standard I/O streams

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	inter-
     face, that	is, the	functions like read(2) and lseek(2).  The integer file
     descriptors associated with the streams stdin, stdout, and	stderr are 0,
     1,	and 2, respectively. The preprocessor symbols STDIN_FILENO, STD-
     OUT_FILENO, and STDERR_FILENO are defined with these values in
     <unistd.h>.

     Note that mixing use of FILEs and raw file	descriptors can	produce	unex-
     pected results and	should generally be avoided.  (For the masochistic
     among you:	POSIX.1, section 8.2.3,	describes in detail how	this interac-
     tion 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 ex-
     ample, that after an exec,	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 non-portable.	 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 nor-
     mal program termination.

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

CONSIDERATIONS
     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 termi-
     nal, 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 us-
     ing calls like tcsetattr(3); see also stty(1), and	termios(3).

CONFORMING TO
     The stdin,	stdout,	and stderr macros conform to ANSI X3.159-1989
     ("ANSI C89"), and this standard also stipulates that these	three streams
     shall be open at program startup.

Linux 2.0			March 24, 1998			     Linux 2.0

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | CONSIDERATIONS | CONFORMING TO

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