man2/tee.2

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  26 .TH TEE 2 2009-09-15 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 tee \- duplicating pipe content
  29 .SH SYNOPSIS
  30 .nf
  31 .B #define _GNU_SOURCE
  32 .B #include <fcntl.h>
  33
  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
  57 .I flags
  58 is a series of modifier flags, which share the name space with
  59 .BR splice (2)
  60 and
  61 .BR vmsplice (2):
  62 .TP 1.9i
  63 .B SPLICE_F_MOVE
  64 Currently has no effect for
  65 .BR tee ();
  66 see
  67 .BR splice (2).
  68 .TP
  69 .B SPLICE_F_NONBLOCK
  70 Do not block on I/O; see
  71 .BR splice (2)
  72 for further details.
  73 .TP
  74 .B SPLICE_F_MORE
  75 Currently has no effect for
  76 .BR tee (),
  77 but may be implemented in the future; see
  78 .BR splice (2).
  79 .TP
  80 .B SPLICE_F_GIFT
  81 Unused for
  82 .BR tee ();
  83 see
  84 .BR vmsplice (2).
  85 .SH RETURN VALUE
  86 Upon successful completion,
  87 .BR tee ()
  88 returns the number of bytes that were duplicated between the input
  89 and output.
  90 A return value of 0 means that there was no data to transfer,
  91 and it would not make sense to block, because there are no
  92 writers connected to the write end of the pipe referred to by
  93 .IR fd_in .
  94
  95 On error,
  96 .BR tee ()
  97 returns \-1 and
  98 .I errno
  99 is set to indicate the error.
 100 .SH ERRORS
 101 .TP
 102 .B EINVAL
 103 .I fd_in
 104 or
 105 .I fd_out
 106 does not refer to a pipe; or
 107 .I fd_in
 108 and
 109 .I fd_out
 110 refer to the same pipe.
 111 .TP
 112 .B ENOMEM
 113 Out of memory.
 114 .SH VERSIONS
 115 The
 116 .BR tee ()
 117 system call first appeared in Linux 2.6.17.
 118 .SH "CONFORMING TO"
 119 This system call is Linux-specific.
 120 .SH NOTES
 121 Conceptually,
 122 .BR tee ()
 123 copies the data between the two pipes.
 124 In reality no real data copying takes place though:
 125 under the covers,
 126 .BR tee ()
 127 assigns data in the output by merely grabbing
 128 a reference to the input.
 129 .SH EXAMPLE
 130 The following example implements a basic
 131 .BR tee (1)
 132 program using the
 133 .BR tee ()
 134 system call.
 135 .nf
 136
 137 #define _GNU_SOURCE
 138 #include <fcntl.h>
 139 #include <stdio.h>
 140 #include <stdlib.h>
 141 #include <unistd.h>
 142 #include <assert.h>
 143 #include <errno.h>
 144 #include <limits.h>
 145
 146 int
 147 main(int argc, char *argv[])
 148 {
 149     int fd;
 150     int len, slen;
 151
 152     assert(argc == 2);
 153
 154     fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, 0644);
 155     if (fd == \-1) {
 156         perror("open");
 157         exit(EXIT_FAILURE);
 158     }
 159
 160     do {
 161         /*
 162          * tee stdin to stdout.
 163          */
 164         len = tee(STDIN_FILENO, STDOUT_FILENO,
 165                   INT_MAX, SPLICE_F_NONBLOCK);
 166
 167         if (len < 0) {
 168             if (errno == EAGAIN)
 169                 continue;
 170             perror("tee");
 171             exit(EXIT_FAILURE);
 172         } else
 173             if (len == 0)
 174                 break;
 175
 176         /*
 177          * Consume stdin by splicing it to a file.
 178          */
 179         while (len > 0) {
 180             slen = splice(STDIN_FILENO, NULL, fd, NULL,
 181                           len, SPLICE_F_MOVE);
 182             if (slen < 0) {
 183                 perror("splice");
 184                 break;
 185             }
 186             len \-= slen;
 187         }
 188     } while (1);
 189
 190     close(fd);
 191     exit(EXIT_SUCCESS);
 192 }
 193 .fi
 194 .SH SEE ALSO
 195 .BR splice (2),
 196 .BR vmsplice (2),
 197 .BR feature_test_macros (7)