1TEE(2) Linux Programmer's Manual TEE(2)
2
6 tee - duplicating pipe content
7
9 #define _GNU_SOURCE /* See feature_test_macros(7) */
10 #include <fcntl.h>
11 ssize_t tee(int fd_in, int fd_out, size_t len, unsigned int flags);
13
15 tee() duplicates up to len bytes of data from the pipe referred to by
16 the file descriptor fd_in to the pipe referred to by the file descrip‐
17 tor fd_out. It does not consume the data that is duplicated from
18 fd_in; therefore, that data can be copied by a subsequent splice(2).
19 flags is a bit mask that is composed by ORing together zero or more of
21 the following values:
22 SPLICE_F_MOVE Currently has no effect for tee(); see splice(2).
24 SPLICE_F_NONBLOCK Do not block on I/O; see splice(2) for further
26 details.
27 SPLICE_F_MORE Currently has no effect for tee(), but may be imple‐
29 mented in the future; see splice(2).
30 SPLICE_F_GIFT Unused for tee(); see vmsplice(2).
32
34 Upon successful completion, tee() returns the number of bytes that were
35 duplicated between the input and output. A return value of 0 means
36 that there was no data to transfer, and it would not make sense to
37 block, because there are no writers connected to the write end of the
38 pipe referred to by fd_in.
39 On error, tee() returns -1 and errno is set to indicate the error.
41
43 EAGAIN SPLICE_F_NONBLOCK was specified in flags or one of the file
44 descriptors had been marked as nonblocking (O_NONBLOCK), and the
45 operation would block.
46 EINVAL fd_in or fd_out does not refer to a pipe; or fd_in and fd_out
48 refer to the same pipe.
49 ENOMEM Out of memory.
51
53 The tee() system call first appeared in Linux 2.6.17; library support
54 was added to glibc in version 2.5.
55
57 This system call is Linux-specific.
58
60 Conceptually, tee() copies the data between the two pipes. In reality
61 no real data copying takes place though: under the covers, tee()
62 assigns data to the output by merely grabbing a reference to the input.
63
65 The example below implements a basic tee(1) program using the tee()
66 system call. Here is an example of its use:
67 $ date |./a.out out.log | cat
69 Tue Oct 28 10:06:00 CET 2014
70 $ cat out.log
71 Tue Oct 28 10:06:00 CET 2014
72 Program source
74 #define _GNU_SOURCE
76 #include <fcntl.h>
77 #include <stdio.h>
78 #include <stdlib.h>
79 #include <unistd.h>
80 #include <errno.h>
81 #include <limits.h>
82 int
84 main(int argc, char *argv[])
85 {
86 int fd;
87 int len, slen;
88 if (argc != 2) {
90 fprintf(stderr, "Usage: %s <file>\n", argv[0]);
91 exit(EXIT_FAILURE);
92 }
93 fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, 0644);
95 if (fd == -1) {
96 perror("open");
97 exit(EXIT_FAILURE);
98 }
99 do {
101 /*
102 * tee stdin to stdout.
103 */
104 len = tee(STDIN_FILENO, STDOUT_FILENO,
105 INT_MAX, SPLICE_F_NONBLOCK);
106 if (len < 0) {
108 if (errno == EAGAIN)
109 continue;
110 perror("tee");
111 exit(EXIT_FAILURE);
112 } else
113 if (len == 0)
114 break;
115 /*
117 * Consume stdin by splicing it to a file.
118 */
119 while (len > 0) {
120 slen = splice(STDIN_FILENO, NULL, fd, NULL,
121 len, SPLICE_F_MOVE);
122 if (slen < 0) {
123 perror("splice");
124 break;
125 }
126 len -= slen;
127 }
128 } while (1);
129 close(fd);
131 exit(EXIT_SUCCESS);
132 }
133
135 splice(2), vmsplice(2), pipe(7)
136
138 This page is part of release 5.07 of the Linux man-pages project. A
139 description of the project, information about reporting bugs, and the
140 latest version of this page, can be found at
141 https://www.kernel.org/doc/man-pages/.
142Linux 2020-06-09 TEE(2)