[thirdparty/man-pages.git] / man3 / stdin.3

.\" From dholland@burgundy.eecs.harvard.edu Tue Mar 24 18:08:15 1998
.\"
.\" This man page was written in 1998 by David A. Holland
.\" Polished a bit by aeb.
.\"
.\" %%%LICENSE_START(PUBLIC_DOMAIN)
.\" Placed in the Public Domain.
.\" %%%LICENSE_END
.\"
.\" 2005-06-16 mtk, mentioned freopen()
.\" 2007-12-08, mtk, Converted from mdoc to man macros
.\"
.TH STDIN 3 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
stdin, stdout, stderr \- standard I/O streams
.SH SYNOPSIS
.nf
.B #include <stdio.h>
.PP
.BI "extern FILE *" stdin ;
.BI "extern FILE *" stdout ;
.BI "extern FILE *" stderr ;
.fi
.SH 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
.BR 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
.BR sh (1).)
.PP
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
.IR stdin ,
.IR stdout ,
and
.IR stderr .
.PP
Each of these symbols is a
.BR stdio (3)
macro of type pointer to
.IR FILE ,
and can be used with functions like
.BR fprintf (3)
or
.BR fread (3).
.PP
Since
.IR FILE s
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
.BR read (2)
and
.BR lseek (2).
.PP
On program startup, the integer file descriptors
associated with the streams
.IR stdin ,
.IR stdout ,
and
.I stderr
are 0, 1, and 2, respectively.
The preprocessor symbols
.BR STDIN_FILENO ,
.BR STDOUT_FILENO ,
and
.B STDERR_FILENO
are defined with these values in
.IR <unistd.h> .
(Applying
.BR freopen (3)
to one of these streams can change the file descriptor number
associated with the stream.)
.PP
Note that mixing use of
.IR FILE s
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
.BR exec (3),
the child inherits all open file descriptors, but all old streams
have become inaccessible.
.PP
Since the symbols
.IR stdin ,
.IR stdout ,
and
.I 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
.BR freopen (3),
specially introduced to make it possible to reassign
.IR stdin ,
.IR stdout ,
and
.IR stderr .
The standard streams are closed by a call to
.BR exit (3)
and by normal program termination.
.SH CONFORMING TO
The
.IR stdin ,
.IR stdout ,
and
.I stderr
macros conform to C89
and this standard also stipulates that these three
streams shall be open at program startup.
.SH NOTES
The stream
.I stderr
is unbuffered.
The stream
.I stdout
is line-buffered when it points to a terminal.
Partial lines will not
appear until
.BR fflush (3)
or
.BR 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
.BR setbuf (3)
or
.BR setvbuf (3)
call.
Note that in case
.I 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
.BR tcsetattr (3);
see also
.BR stty (1),
and
.BR termios (3).
.SH SEE ALSO
.BR csh (1),
.BR sh (1),
.BR open (2),
.BR fopen (3),
.BR stdio (3)
Commit	Line	Data
	1	.\" From dholland@burgundy.eecs.harvard.edu Tue Mar 24 18:08:15 1998
	2	.\"
	3	.\" This man page was written in 1998 by David A. Holland
	4	.\" Polished a bit by aeb.
	5	.\"
	6	.\" %%%LICENSE_START(PUBLIC_DOMAIN)
	7	.\" Placed in the Public Domain.
	8	.\" %%%LICENSE_END
	9	.\"
	10	.\" 2005-06-16 mtk, mentioned freopen()
	11	.\" 2007-12-08, mtk, Converted from mdoc to man macros
	12	.\"
	13	.TH STDIN 3 2017-09-15 "Linux" "Linux Programmer's Manual"
	14	.SH NAME
	15	stdin, stdout, stderr \- standard I/O streams
	16	.SH SYNOPSIS
	17	.nf
	18	.B #include <stdio.h>
	19	.PP
	20	.BI "extern FILE *" stdin ;
	21	.BI "extern FILE *" stdout ;
	22	.BI "extern FILE *" stderr ;
	23	.fi
	24	.SH DESCRIPTION
	25	Under normal circumstances every UNIX program has three streams opened
	26	for it when it starts up, one for input, one for output, and one for
	27	printing diagnostic or error messages.
	28	These are typically attached to
	29	the user's terminal (see
	30	.BR tty (4))
	31	but might instead refer to files or other devices, depending on what
	32	the parent process chose to set up.
	33	(See also the "Redirection" section of
	34	.BR sh (1).)
	35	.PP
	36	The input stream is referred to as "standard input"; the output stream is
	37	referred to as "standard output"; and the error stream is referred to
	38	as "standard error".
	39	These terms are abbreviated to form the symbols
	40	used to refer to these files, namely
	41	.IR stdin ,
	42	.IR stdout ,
	43	and
	44	.IR stderr .
	45	.PP
	46	Each of these symbols is a
	47	.BR stdio (3)
	48	macro of type pointer to
	49	.IR FILE ,
	50	and can be used with functions like
	51	.BR fprintf (3)
	52	or
	53	.BR fread (3).
	54	.PP
	55	Since
	56	.IR FILE s
	57	are a buffering wrapper around UNIX file descriptors, the
	58	same underlying files may also be accessed using the raw UNIX file
	59	interface, that is, the functions like
	60	.BR read (2)
	61	and
	62	.BR lseek (2).
	63	.PP
	64	On program startup, the integer file descriptors
	65	associated with the streams
	66	.IR stdin ,
	67	.IR stdout ,
	68	and
	69	.I stderr
	70	are 0, 1, and 2, respectively.
	71	The preprocessor symbols
	72	.BR STDIN_FILENO ,
	73	.BR STDOUT_FILENO ,
	74	and
	75	.B STDERR_FILENO
	76	are defined with these values in
	77	.IR <unistd.h> .
	78	(Applying
	79	.BR freopen (3)
	80	to one of these streams can change the file descriptor number
	81	associated with the stream.)
	82	.PP
	83	Note that mixing use of
	84	.IR FILE s
	85	and raw file descriptors can produce
	86	unexpected results and should generally be avoided.
	87	(For the masochistic among you: POSIX.1, section 8.2.3, describes
	88	in detail how this interaction is supposed to work.)
	89	A general rule is that file descriptors are handled in the kernel,
	90	while stdio is just a library.
	91	This means for example, that after an
	92	.BR exec (3),
	93	the child inherits all open file descriptors, but all old streams
	94	have become inaccessible.
	95	.PP
	96	Since the symbols
	97	.IR stdin ,
	98	.IR stdout ,
	99	and
	100	.I stderr
	101	are specified to be macros, assigning to them is nonportable.
	102	The standard streams can be made to refer to different files
	103	with help of the library function
	104	.BR freopen (3),
	105	specially introduced to make it possible to reassign
	106	.IR stdin ,
	107	.IR stdout ,
	108	and
	109	.IR stderr .
	110	The standard streams are closed by a call to
	111	.BR exit (3)
	112	and by normal program termination.
	113	.SH CONFORMING TO
	114	The
	115	.IR stdin ,
	116	.IR stdout ,
	117	and
	118	.I stderr
	119	macros conform to C89
	120	and this standard also stipulates that these three
	121	streams shall be open at program startup.
	122	.SH NOTES
	123	The stream
	124	.I stderr
	125	is unbuffered.
	126	The stream
	127	.I stdout
	128	is line-buffered when it points to a terminal.
	129	Partial lines will not
	130	appear until
	131	.BR fflush (3)
	132	or
	133	.BR exit (3)
	134	is called, or a newline is printed.
	135	This can produce unexpected
	136	results, especially with debugging output.
	137	The buffering mode of the standard streams (or any other stream)
	138	can be changed using the
	139	.BR setbuf (3)
	140	or
	141	.BR setvbuf (3)
	142	call.
	143	Note that in case
	144	.I stdin
	145	is associated with a terminal, there may also be input buffering
	146	in the terminal driver, entirely unrelated to stdio buffering.
	147	(Indeed, normally terminal input is line buffered in the kernel.)
	148	This kernel input handling can be modified using calls like
	149	.BR tcsetattr (3);
	150	see also
	151	.BR stty (1),
	152	and
	153	.BR termios (3).
	154	.SH SEE ALSO
	155	.BR csh (1),
	156	.BR sh (1),
	157	.BR open (2),
	158	.BR fopen (3),
	159	.BR stdio (3)