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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
                          details.
       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,  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()
       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 | 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 4.15 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                             2017-09-15                            TEE(2)