tee(2) - f7

1tee(2)                     Linux Programmer's Manual                    tee(2)
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NAME

6       tee - duplicating pipe content
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SYNOPSIS

9       #define _GNU_SOURCE
10       #include <fcntl.h>
11
12       long tee(int fd_in, int fd_out, size_t len, unsigned int flags);
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DESCRIPTION

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).
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20       flags is a series of modifier flags, which share the  name  space  with
21       splice(2) and vmsplice(2):
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23       SPLICE_F_MOVE      Currently has no effect for tee(); see splice(2).
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25       SPLICE_F_NONBLOCK  Do  not  block  on  I/O;  see  splice(2) for further
26                          details.
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28       SPLICE_F_MORE      Currently has no effect for tee(), but may be imple‐
29                          mented in the future; see splice(2).
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31       SPLICE_F_GIFT      Unused for tee(); see vmsplice(2).
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RETURN VALUE

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.
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40       On error, tee() returns -1 and errno is set to indicate the error.
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ERRORS

43       EINVAL fd_in  or  fd_out  does not refer to a pipe; or fd_in and fd_out
44              refer to the same pipe.
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46       ENOMEM Out of memory.
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NOTES

49       Conceptually, tee() copies the data between the two pipes.  In  reality
50       no  real  data  copying  takes  place  though:  under the covers, tee()
51       assigns data in the output by merely grabbing a reference to the input.
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EXAMPLE

54       The following example implements  a  basic  tee(1)  program  using  the
55       tee(2) system call.
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57       #define _GNU_SOURCE
58       #include <fcntl.h>
59       #include <stdio.h>
60       #include <stdlib.h>
61       #include <unistd.h>
62       #include <assert.h>
63       #include <errno.h>
64       #include <limits.h>
65
66       int
67       main(int argc, char *argv[])
68       {
69           int fd;
70           int len, slen;
71
72           assert(argc == 2);
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74           fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, 0644);
75           if (fd == -1) {
76               perror("open");
77               exit(EXIT_FAILURE);
78           }
79
80           do {
81               /*
82                * tee stdin to stdout.
83                */
84               len = tee(STDIN_FILENO, STDOUT_FILENO,
85                         INT_MAX, SPLICE_F_NONBLOCK);
86
87               if (len < 0) {
88                   if (errno == EAGAIN)
89                       continue;
90                   perror("tee");
91                   exit(EXIT_FAILURE);
92               } else
93                   if (len == 0)
94                       break;
95
96               /*
97                * Consume stdin by splicing it to a file.
98                */
99               while (len > 0) {
100                   slen = splice(STDIN_FILENO, NULL, fd, NULL,
101                                 len, SPLICE_F_MOVE);
102                   if (slen < 0) {
103                       perror("splice");
104                       break;
105                   }
106                   len -= slen;
107               }
108           } while (1);
109
110           close(fd);
111           exit(EXIT_SUCCESS);
112       }
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HISTORY

115       The tee(2) system call first appeared in Linux-2.6.17.
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CONFORMING TO

118       This system call is Linux specific.
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