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.TH TEE 2 2020-06-09 "Linux" "Linux Programmer's Manual"
.SH NAME
tee \- duplicating pipe content
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
.B #include <fcntl.h>
.PP
.BI "ssize_t tee(int " fd_in ", int " fd_out ", size_t " len \
", unsigned int " flags );
.fi
.\" Return type was long before glibc 2.7
.SH DESCRIPTION
.\" Example programs http://brick.kernel.dk/snaps
.\"
.\"
.\" add a "tee(in, out1, out2)" system call that duplicates the pages
.\" (again, incrementing their reference count, not copying the data) from
.\" one pipe to two other pipes.
.BR tee ()
duplicates up to
.I len
bytes of data from the pipe referred to by the file descriptor
.I fd_in
to the pipe referred to by the file descriptor
.IR fd_out .
It does not consume the data that is duplicated from
.IR fd_in ;
therefore, that data can be copied by a subsequent
.BR splice (2).
.PP
.I flags
is a bit mask that is composed by ORing together
zero or more of the following values:
.TP 1.9i
.B SPLICE_F_MOVE
Currently has no effect for
.BR tee ();
see
.BR splice (2).
.TP
.B SPLICE_F_NONBLOCK
Do not block on I/O; see
.BR splice (2)
for further details.
.TP
.B SPLICE_F_MORE
Currently has no effect for
.BR tee (),
but may be implemented in the future; see
.BR splice (2).
.TP
.B SPLICE_F_GIFT
Unused for
.BR tee ();
see
.BR vmsplice (2).
.SH RETURN VALUE
Upon successful completion,
.BR tee ()
returns the number of bytes that were duplicated between the input
and output.
A return value of 0 means that there was no data to transfer,
and it would not make sense to block, because there are no
writers connected to the write end of the pipe referred to by
.IR fd_in .
.PP
On error,
.BR tee ()
returns \-1 and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EAGAIN
.B SPLICE_F_NONBLOCK
was specified in
.IR flags
or one of the file descriptors had been marked as nonblocking
.RB ( O_NONBLOCK ) ,
and the operation would block.
.TP
.B EINVAL
.I fd_in
or
.I fd_out
does not refer to a pipe; or
.I fd_in
and
.I fd_out
refer to the same pipe.
.TP
.B ENOMEM
Out of memory.
.SH VERSIONS
The
.BR tee ()
system call first appeared in Linux 2.6.17;
library support was added to glibc in version 2.5.
.SH CONFORMING TO
This system call is Linux-specific.
.SH NOTES
Conceptually,
.BR tee ()
copies the data between the two pipes.
In reality no real data copying takes place though:
under the covers,
.BR tee ()
assigns data to the output by merely grabbing
a reference to the input.
.SH EXAMPLES
The example below implements a basic
.BR tee (1)
program using the
.BR tee ()
system call.
Here is an example of its use:
.PP
.in +4n
.EX
$ \fBdate |./a.out out.log | cat\fP
Tue Oct 28 10:06:00 CET 2014
$ \fBcat out.log\fP
Tue Oct 28 10:06:00 CET 2014
.EE
.in
.SS Program source
\&
.EX
#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <limits.h>

int
main(int argc, char *argv[])
{
    int fd;
    int len, slen;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s <file>\en", argv[0]);
        exit(EXIT_FAILURE);
    }

    fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, 0644);
    if (fd == \-1) {
        perror("open");
        exit(EXIT_FAILURE);
    }

    do {
        /*
         * tee stdin to stdout.
         */
        len = tee(STDIN_FILENO, STDOUT_FILENO,
                  INT_MAX, SPLICE_F_NONBLOCK);

        if (len < 0) {
            if (errno == EAGAIN)
                continue;
            perror("tee");
            exit(EXIT_FAILURE);
        } else
            if (len == 0)
                break;

        /*
         * Consume stdin by splicing it to a file.
         */
        while (len > 0) {
            slen = splice(STDIN_FILENO, NULL, fd, NULL,
                          len, SPLICE_F_MOVE);
            if (slen < 0) {
                perror("splice");
                break;
            }
            len \-= slen;
        }
    } while (1);

    close(fd);
    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR splice (2),
.BR vmsplice (2),
.BR pipe (7)