TEE(2)                                              Linux Programmer's Manual                                             TEE(2)



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 3.25 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                                                      2009-09-15                                                     TEE(2)