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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  descrip-
     tor  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 imple-
                        mented 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); }