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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 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

       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 descriptors 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() 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

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       found at https://www.kernel.org/doc/man-pages/.