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NAME

       dup, dup2 - duplicate a file descriptor

SYNOPSIS

       #include <unistd.h>

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

DESCRIPTION

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

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

       dup2() makes newfd be  the  copy  of  oldfd,  closing  newfd  first  if
       necessary, but note the following:

       *  If  olfd  is  not  a valid file descriptor, then the call fails, and
          newfd is not closed.

       *  If oldfd is a valid file descriptor, and newfd has the same value as
          oldfd, then dup2() does nothing, and returns newfd.

       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
       other.

       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.

RETURN VALUE

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

ERRORS

       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(2) 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.

CONFORMING TO

       SVr4, 4.3BSD, POSIX.1-2001.

NOTES

       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(2)
       time  are  lost.   A  careful  programmer  will  not use dup2() without
       closing newfd first.

SEE ALSO

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

COLOPHON

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