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, and the operation
44 would block.
45 EINVAL fd_in or fd_out does not refer to a pipe; or fd_in and fd_out
47 refer to the same pipe.
48 ENOMEM Out of memory.
50
52 The tee() system call first appeared in Linux 2.6.17; library support
53 was added to glibc in version 2.5.
54
56 This system call is Linux-specific.
57
59 Conceptually, tee() copies the data between the two pipes. In reality
60 no real data copying takes place though: under the covers, tee()
61 assigns data to the output by merely grabbing a reference to the input.
62
64 The example below implements a basic tee(1) program using the tee()
65 system call. Here is an example of its use:
66 $ date |./a.out out.log | cat
68 Tue Oct 28 10:06:00 CET 2014
69 $ cat out.log
70 Tue Oct 28 10:06:00 CET 2014
71 Program source
73 #define _GNU_SOURCE
75 #include <fcntl.h>
76 #include <stdio.h>
77 #include <stdlib.h>
78 #include <unistd.h>
79 #include <errno.h>
80 #include <limits.h>
81 int
83 main(int argc, char *argv[])
84 {
85 int fd;
86 int len, slen;
87 if (argc != 2) {
89 fprintf(stderr, "Usage: %s <file>\n", argv[0]);
90 exit(EXIT_FAILURE);
91 }
92 fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, 0644);
94 if (fd == -1) {
95 perror("open");
96 exit(EXIT_FAILURE);
97 }
98 do {
100 /*
101 * tee stdin to stdout.
102 */
103 len = tee(STDIN_FILENO, STDOUT_FILENO,
104 INT_MAX, SPLICE_F_NONBLOCK);
105 if (len < 0) {
107 if (errno == EAGAIN)
108 continue;
109 perror("tee");
110 exit(EXIT_FAILURE);
111 } else
112 if (len == 0)
113 break;
114 /*
116 * Consume stdin by splicing it to a file.
117 */
118 while (len > 0) {
119 slen = splice(STDIN_FILENO, NULL, fd, NULL,
120 len, SPLICE_F_MOVE);
121 if (slen < 0) {
122 perror("splice");
123 break;
124 }
125 len -= slen;
126 }
127 } while (1);
128 close(fd);
130 exit(EXIT_SUCCESS);
131 }
132
134 splice(2), vmsplice(2), pipe(7)
135
137 This page is part of release 4.15 of the Linux man-pages project. A
138 description of the project, information about reporting bugs, and the
139 latest version of this page, can be found at
140 https://www.kernel.org/doc/man-pages/.
141Linux 2017-09-15 TEE(2)