NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

TEE(2)                    Linux Programmer's Manual                   TEE(2)

NAME         top

       tee - duplicating pipe content

SYNOPSIS         top

       #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         top

       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 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         top

       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         top

       EAGAIN SPLICE_F_NONBLOCK was specified in flags, and the operation
              would block.

       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.

VERSIONS         top

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

CONFORMING TO         top

       This system call is Linux-specific.

NOTES         top

       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         top

       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 | cat
           Tue Oct 28 10:06:00 CET 2014
           $ cat out.log
           Tue 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         top

       splice(2), vmsplice(2)

COLOPHON         top

       This page is part of release 4.07 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                            2014-12-31                           TEE(2)