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# tee() - Unix, Linux System Call

# NAME

tee - duplicating pipe content

# SYNOPSIS

    #define _GNU_SOURCE
    #include <fcntl.h>

    long tee(int fd_in, int fd_out, size_t len
    ", unsigned int " flags );

# DESCRIPTION

tee() duplicates up to len bytes of data from the pipe referred to by the file descriptor fd_in to 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).

flags is a series of modifier flags, which share the name space with splice(2) and vmsplice(2):

Tag | Description
--- | ---
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 errno is set to indicate the error.

# ERRORS

Tag | Description
--- | ---
EINVAL | fd_in or fd_out does not refer to a pipe; or fd_in and fd_out refer to the same pipe.
ENOMEM | Out of memory.

# 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 in the output by merely grabbing a reference to the input.

# EXAMPLE

The following example implements a basic tee(1) program using the tee(2) system call.

    #define _GNU_SOURCE
    #include <fcntl.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <assert.h>
    #include <errno.h>
    #include <limits.h>

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

        assert(argc == 2);

        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);
    }

# HISTORY

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

# CONFORMING TO

This system call is Linux specific.

# SEE ALSO

- splice (2)

- vmsplice (2)

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