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       dup, dup2 - duplicate a file descriptor


       #include <unistd.h>

       int dup(int oldfd);
       int dup2(int oldfd, int newfd);


       dup() and dup2() create a copy of the file descriptor oldfd.

       After  a  successful  return from dup() or dup2(), the old and new file
       descriptors may be used interchangeably.  They refer to the  same  open
       file  description  (see  open(2))  and  thus share file offset and file
       status flags; for example, if the file  offset  is  modified  by  using
       lseek(2)  on one of the descriptors, the offset is also changed for the

       The two descriptors do not share file descriptor flags  (the  close-on-
       exec  flag).  The close-on-exec flag (FD_CLOEXEC; see fcntl(2)) for the
       duplicate descriptor is off.

       dup()  uses  the  lowest-numbered  unused  descriptor   for   the   new

       dup2()  makes  newfd  be  the  copy  of  oldfd,  closing newfd first if


       dup() and dup2() return the new descriptor, or -1 if an error  occurred
       (in which case, errno is set appropriately).


       EBADF  oldfd  isn’t  an  open  file  descriptor, or newfd is out of the
              allowed range for file descriptors.

       EBUSY  (Linux only) This may  be  returned  by  dup2()  during  a  race
              condition with open() and dup().

       EINTR  The dup2() call was interrupted by a signal.

       EMFILE The  process  already has the maximum number of file descriptors
              open and tried to open a new one.


       The error returned  by  dup2()  is  different  from  that  returned  by
       fcntl(...,  F_DUPFD,  ...)  when newfd is out of range. On some systems
       dup2() also sometimes returns EINVAL like F_DUPFD.

       If newfd was open, any errors that would have been reported at  close()
       time,  are  lost.  A  careful  programmer  will  not use dup2() without
       closing newfd first.


       SVr4, SVID, POSIX, X/OPEN, 4.3BSD. SVr4 documents additional EINTR  and
       ENOLINK  error  conditions.   POSIX.1  adds EINTR.  The EBUSY return is


       close(2), fcntl(2), open(2)