1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
|
.\" This manpage is Copyright (C) 2006 Jens Axboe
.\" and Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH tee 2 2023-05-03 "Linux man-pages 6.05.01"
.SH NAME
tee \- duplicating pipe content
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
.B #include <fcntl.h>
.PP
.BI "ssize_t tee(int " fd_in ", int " fd_out ", size_t " len \
", unsigned int " flags );
.fi
.\" Return type was long before glibc 2.7
.SH DESCRIPTION
.\" Example programs http://brick.kernel.dk/snaps
.\"
.\"
.\" add a "tee(in, out1, out2)" system call that duplicates the pages
.\" (again, incrementing their reference count, not copying the data) from
.\" one pipe to two other pipes.
.BR tee ()
duplicates up to
.I len
bytes of data from the pipe referred to by the file descriptor
.I fd_in
to the pipe referred to by the file descriptor
.IR fd_out .
It does not consume the data that is duplicated from
.IR fd_in ;
therefore, that data can be copied by a subsequent
.BR splice (2).
.PP
.I flags
is a bit mask that is composed by ORing together
zero or more of the following values:
.TP 1.9i
.B SPLICE_F_MOVE
Currently has no effect for
.BR tee ();
see
.BR splice (2).
.TP
.B SPLICE_F_NONBLOCK
Do not block on I/O; see
.BR splice (2)
for further details.
.TP
.B SPLICE_F_MORE
Currently has no effect for
.BR tee (),
but may be implemented in the future; see
.BR splice (2).
.TP
.B SPLICE_F_GIFT
Unused for
.BR tee ();
see
.BR vmsplice (2).
.SH RETURN VALUE
Upon successful completion,
.BR tee ()
returns the number of bytes that were duplicated between the input
and output.
A return value of 0 means that there was no data to transfer,
and it would not make sense to block, because there are no
writers connected to the write end of the pipe referred to by
.IR fd_in .
.PP
On error,
.BR tee ()
returns \-1 and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EAGAIN
.B SPLICE_F_NONBLOCK
was specified in
.I flags
or one of the file descriptors had been marked as nonblocking
.RB ( O_NONBLOCK ) ,
and the operation would block.
.TP
.B EINVAL
.I fd_in
or
.I fd_out
does not refer to a pipe; or
.I fd_in
and
.I fd_out
refer to the same pipe.
.TP
.B ENOMEM
Out of memory.
.SH STANDARDS
Linux.
.SH HISTORY
Linux 2.6.17,
glibc 2.5.
.SH NOTES
Conceptually,
.BR tee ()
copies the data between the two pipes.
In reality no real data copying takes place though:
under the covers,
.BR tee ()
assigns data to the output by merely grabbing
a reference to the input.
.SH EXAMPLES
The example below implements a basic
.BR tee (1)
program using the
.BR tee ()
system call.
Here is an example of its use:
.PP
.in +4n
.EX
$ \fBdate | ./a.out out.log | cat\fP
Tue Oct 28 10:06:00 CET 2014
$ \fBcat out.log\fP
Tue Oct 28 10:06:00 CET 2014
.EE
.in
.SS Program source
\&
.\" SRC BEGIN (tee.c)
.EX
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
\&
int
main(int argc, char *argv[])
{
int fd;
ssize_t len, slen;
\&
if (argc != 2) {
fprintf(stderr, "Usage: %s <file>\en", argv[0]);
exit(EXIT_FAILURE);
}
\&
fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, 0644);
if (fd == \-1) {
perror("open");
exit(EXIT_FAILURE);
}
\&
for (;;) {
/*
* tee stdin to stdout.
*/
len = tee(STDIN_FILENO, STDOUT_FILENO,
INT_MAX, SPLICE_F_NONBLOCK);
if (len < 0) {
if (errno == EAGAIN)
continue;
perror("tee");
exit(EXIT_FAILURE);
}
if (len == 0)
break;
\&
/*
* Consume stdin by splicing it to a file.
*/
while (len > 0) {
slen = splice(STDIN_FILENO, NULL, fd, NULL,
len, SPLICE_F_MOVE);
if (slen < 0) {
perror("splice");
exit(EXIT_FAILURE);
}
len \-= slen;
}
}
\&
close(fd);
exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR splice (2),
.BR vmsplice (2),
.BR pipe (7)
|
