Manpage of TEE

TEE

Section: Linux Programmer's Manual (2)
Updated: 2014-12-31
Index
 

NAME

tee - duplicating pipe content  

SYNOPSIS

#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <fcntl.h>ssize_t tee(int fd_in, int fd_out, size_t len, unsigned int flags);
 

DESCRIPTION

tee() duplicates up to lenbytes of data from the pipe referred to by the file descriptor fd_into the pipe referred to by the file descriptor fd_out. It does not consume the data that is duplicated from fd_in; therefore, that data can be copied by a subsequent splice(2).

flagsis a bit mask that is composed by ORing together zero or more of the following values:

SPLICE_F_MOVE
Currently has no effect for tee(); see splice(2).
SPLICE_F_NONBLOCK
Do not block on I/O; see splice(2) for further details.
SPLICE_F_MORE
Currently has no effect for tee(), but may be implemented in the future; see splice(2).
SPLICE_F_GIFT
Unused for tee(); see vmsplice(2).
 

RETURN VALUE

Upon successful completion, 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 fd_in.

On error, tee() returns -1 and errnois set to indicate the error.  

ERRORS

EAGAIN
SPLICE_F_NONBLOCKwas specified in flags, and the operation would block.
EINVAL
fd_inor fd_outdoes not refer to a pipe; or fd_inand fd_outrefer to the same pipe.
ENOMEM
Out of memory.
 

VERSIONS

The tee() system call first appeared in Linux 2.6.17; library support was added to glibc in version 2.5.  

CONFORMING TO

This system call is Linux-specific.  

NOTES

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

EXAMPLE

The example below implements a basic tee(1) program using the tee() system call. Here is an example of its use:

$ date |./a.out out.log | catTue Oct 28 10:06:00 CET 2014
$ cat out.logTue Oct 28 10:06:00 CET 2014
 

Program source

#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>\n", 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);
}
 

SEE ALSO

splice(2), vmsplice(2)


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
VERSIONS
CONFORMING TO
NOTES
EXAMPLE
Program source
SEE ALSO

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