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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 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; 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 copy-
       ing takes place though: under the covers, tee() assigns data in the output by merely grab-
       bing 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
       This  page  is  part of release 3.53 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/.

Linux					    2012-05-04					   TEE(2)
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