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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             /* See feature_test_macros(7) */
       #include <fcntl.h>              /* Obtain O_* constant definitions */
       #include <unistd.h>

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


       The  dup()  system  call  creates  a copy of the file descriptor oldfd,
       using the lowest-numbered unused descriptor for the new descriptor.

       After a successful return, 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.

       The dup2() system call performs the same task as dup(), but instead  of
       using   the   lowest-numbered  unused  file  descriptor,  it  uses  the
       descriptor number specified in newfd.   If  the  descriptor  newfd  was
       previously open, it is silently closed before being reused.

       The  steps  of  closing  and  reusing  the  file  descriptor  newfd are
       performed atomically.  This is important, because trying  to  implement
       equivalent  functionality  using close(2) and dup() would be subject to
       race conditions, whereby newfd might be reused between the  two  steps.
       Such  reuse  could  happen because the main program is interrupted by a
       signal handler that allocates a file descriptor, or because a  parallel
       thread allocates a file descriptor.

       Note the following points:

       *  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.

       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.

       EBADF  newfd  is out of the allowed range for file descriptors (see the
              discussion of RLIMIT_NOFILE in getrlimit(2)).

       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.

       EINVAL (dup3()) oldfd was equal to newfd.

       EMFILE The per-process limit on the number of open file descriptors has
              been   reached   (see   the   discussion   of  RLIMIT_NOFILE  in


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


       dup(), dup2(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

       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.  If this is  of  concern,  then—unless  the  program  is
       single-threaded  and  does  not  allocate  file  descriptors  in signal
       handlers—the correct approach is not  to  close  newfd  before  calling
       dup2(),  because  of the race condition described above.  Instead, code
       something like the following could be used:

           /* Obtain a duplicate of 'newfd' that can subsequently
              be used to check for close() errors; an EBADF error
              means that 'newfd' was not open. */

           tmpfd = dup(newfd);
           if (tmpfd == -1 && errno != EBADF) {
               /* Handle unexpected dup() error */

           /* Atomically duplicate 'oldfd' on 'newfd' */

           if (dup2(oldfd, newfd) == -1) {
               /* Handle dup2() error */

           /* Now check for close() errors on the file originally
              referred to by 'newfd' */

           if (tmpfd != -1) {
               if (close(tmpfd) == -1) {
                   /* Handle errors from close */


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


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