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NAME

       tee - duplicating pipe content

SYNOPSIS

       #define _GNU_SOURCE
       #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  series of modifier flags, which share the name space with
       splice(2) and vmsplice(2):

       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

       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.

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

EXAMPLE

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

       #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), feature_test_macros(7)

COLOPHON

       This  page  is  part of release 3.24 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.