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

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 len bytes of data from the pipe referred to by
       the file descriptor fd_in to the pipe referred to by the file  descrip-
       tor  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 de-
                          tails.

       SPLICE_F_MORE      Currently has no effect for tee(), but may be imple-
                          mented 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
       EAGAIN SPLICE_F_NONBLOCK  was specified in flags or one of the file de-
              scriptors had been marked as nonblocking (O_NONBLOCK),  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
       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()  as-
       signs 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 | 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
       splice(2), vmsplice(2), pipe(7)

COLOPHON
       This page is part of release 5.05 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                             2019-03-06                            TEE(2)
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON