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       dup, dup2, dup3 - duplicate a file descriptor


       #include <unistd.h>

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

       #define _GNU_SOURCE
       #include <unistd.h>

       int dup3(int oldfd, int newfd, int flags);


       These system calls create a copy of the file descriptor oldfd.

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

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

       *  If  oldfd  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 one of these system calls, 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.

       dup3() is the same as dup2(), except that:

       *  The caller can force the close-on-exec flag to be set  for  the  new
          file   descriptor   by  specifying  O_CLOEXEC  in  flags.   See  the
          description of the same flag in open(2) for reasons why this may  be

       *  If oldfd equals newfd, then dup3() fails with the error EINVAL.


       On success, these system calls return the new descriptor.  On error, -1
       is returned, and 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() or dup3() during a
              race condition with open(2) and dup().

       EINTR  The dup2() or dup3() call  was  interrupted  by  a  signal;  see

       EINVAL (dup3()) flags contain an invalid value.  Or, oldfd was equal to

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


       dup3() was added to Linux in version 2.6.27; glibc support is available
       starting with version 2.9.


       dup(), dup2(): SVr4, 4.3BSD, POSIX.1-2001.

       dup3() is Linux-specific.


       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() or dup3()
       without closing newfd first.


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


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