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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 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)

COLOPHON

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       project,  and information about reporting bugs, can be found at http://man7.org/linux/man-
       pages/.