man2/tee.2

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  26 .TH TEE 2 2019-03-06 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 tee \- duplicating pipe content
  29 .SH SYNOPSIS
  30 .nf
  31 .BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
  32 .B #include <fcntl.h>
  33 .PP
  34 .BI "ssize_t tee(int " fd_in ", int " fd_out ", size_t " len \
  35 ", unsigned int " flags );
  36 .fi
  37 .\" Return type was long before glibc 2.7
  38 .SH DESCRIPTION
  39 .\" Example programs http://brick.kernel.dk/snaps
  40 .\"
  41 .\"
  42 .\" add a "tee(in, out1, out2)" system call that duplicates the pages
  43 .\" (again, incrementing their reference count, not copying the data) from
  44 .\" one pipe to two other pipes.
  45 .BR tee ()
  46 duplicates up to
  47 .I len
  48 bytes of data from the pipe referred to by the file descriptor
  49 .I fd_in
  50 to the pipe referred to by the file descriptor
  51 .IR fd_out .
  52 It does not consume the data that is duplicated from
  53 .IR fd_in ;
  54 therefore, that data can be copied by a subsequent
  55 .BR splice (2).
  56 .PP
  57 .I flags
  58 is a bit mask that is composed by ORing together
  59 zero or more of the following values:
  60 .TP 1.9i
  61 .B SPLICE_F_MOVE
  62 Currently has no effect for
  63 .BR tee ();
  64 see
  65 .BR splice (2).
  66 .TP
  67 .B SPLICE_F_NONBLOCK
  68 Do not block on I/O; see
  69 .BR splice (2)
  70 for further details.
  71 .TP
  72 .B SPLICE_F_MORE
  73 Currently has no effect for
  74 .BR tee (),
  75 but may be implemented in the future; see
  76 .BR splice (2).
  77 .TP
  78 .B SPLICE_F_GIFT
  79 Unused for
  80 .BR tee ();
  81 see
  82 .BR vmsplice (2).
  83 .SH RETURN VALUE
  84 Upon successful completion,
  85 .BR tee ()
  86 returns the number of bytes that were duplicated between the input
  87 and output.
  88 A return value of 0 means that there was no data to transfer,
  89 and it would not make sense to block, because there are no
  90 writers connected to the write end of the pipe referred to by
  91 .IR fd_in .
  92 .PP
  93 On error,
  94 .BR tee ()
  95 returns \-1 and
  96 .I errno
  97 is set to indicate the error.
  98 .SH ERRORS
  99 .TP
 100 .B EAGAIN
 101 .B SPLICE_F_NONBLOCK
 102 was specified in
 103 .IR flags
 104 or one of the file descriptors had been marked as nonblocking
 105 .RB ( O_NONBLOCK ) ,
 106 and the operation would block.
 107 .TP
 108 .B EINVAL
 109 .I fd_in
 110 or
 111 .I fd_out
 112 does not refer to a pipe; or
 113 .I fd_in
 114 and
 115 .I fd_out
 116 refer to the same pipe.
 117 .TP
 118 .B ENOMEM
 119 Out of memory.
 120 .SH VERSIONS
 121 The
 122 .BR tee ()
 123 system call first appeared in Linux 2.6.17;
 124 library support was added to glibc in version 2.5.
 125 .SH CONFORMING TO
 126 This system call is Linux-specific.
 127 .SH NOTES
 128 Conceptually,
 129 .BR tee ()
 130 copies the data between the two pipes.
 131 In reality no real data copying takes place though:
 132 under the covers,
 133 .BR tee ()
 134 assigns data to the output by merely grabbing
 135 a reference to the input.
 136 .SH EXAMPLE
 137 The example below implements a basic
 138 .BR tee (1)
 139 program using the
 140 .BR tee ()
 141 system call.
 142 Here is an example of its use:
 143 .PP
 144 .in +4n
 145 .EX
 146 $ \fBdate |./a.out out.log | cat\fP
 147 Tue Oct 28 10:06:00 CET 2014
 148 $ \fBcat out.log\fP
 149 Tue Oct 28 10:06:00 CET 2014
 150 .EE
 151 .in
 152 .SS Program source
 153 \&
 154 .EX
 155 #define _GNU_SOURCE
 156 #include <fcntl.h>
 157 #include <stdio.h>
 158 #include <stdlib.h>
 159 #include <unistd.h>
 160 #include <errno.h>
 161 #include <limits.h>
 162
 163 int
 164 main(int argc, char *argv[])
 165 {
 166     int fd;
 167     int len, slen;
 168
 169     if (argc != 2) {
 170         fprintf(stderr, "Usage: %s <file>\en", argv[0]);
 171         exit(EXIT_FAILURE);
 172     }
 173
 174     fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, 0644);
 175     if (fd == \-1) {
 176         perror("open");
 177         exit(EXIT_FAILURE);
 178     }
 179
 180     do {
 181         /*
 182          * tee stdin to stdout.
 183          */
 184         len = tee(STDIN_FILENO, STDOUT_FILENO,
 185                   INT_MAX, SPLICE_F_NONBLOCK);
 186
 187         if (len < 0) {
 188             if (errno == EAGAIN)
 189                 continue;
 190             perror("tee");
 191             exit(EXIT_FAILURE);
 192         } else
 193             if (len == 0)
 194                 break;
 195
 196         /*
 197          * Consume stdin by splicing it to a file.
 198          */
 199         while (len > 0) {
 200             slen = splice(STDIN_FILENO, NULL, fd, NULL,
 201                           len, SPLICE_F_MOVE);
 202             if (slen < 0) {
 203                 perror("splice");
 204                 break;
 205             }
 206             len \-= slen;
 207         }
 208     } while (1);
 209
 210     close(fd);
 211     exit(EXIT_SUCCESS);
 212 }
 213 .EE
 214 .SH SEE ALSO
 215 .BR splice (2),
 216 .BR vmsplice (2),
 217 .BR pipe (7)