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
   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,  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)

COLOPHON

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