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DUP(2)                     Linux Programmer's ManualDUP(2)NAME       dup, dup2, dup3 - duplicate a file descriptorSYNOPSIS       #include <unistd.h>       int dup(int oldfd);       int dup2(int oldfd, int newfd);       #define _GNU_SOURCE             /* Seefeature_test_macros(7) */       #include <fcntl.h>              /* Obtain O_* constant definitions */       #include <unistd.h>       int dup3(int oldfd, int newfd, int flags);DESCRIPTION       The  dup() system call creates a copy of the file descriptor oldfd, us-       ing the lowest-numbered unused file 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 (seeopen(2)) and thus share file offset and file status flags; for example,       if the file offset is modified by usinglseek(2) on one of the file de-       scriptors, the offset is also changed for the other.       The two file descriptors do not share file descriptor flags (the close-       on-exec  flag).   The close-on-exec flag (FD_CLOEXEC; seefcntl(2)) for       the duplicate descriptor is off.   dup2()       The dup2() system call performs the same task as dup(), but instead  of       using  the lowest-numbered unused file descriptor, it uses the file de-       scriptor number specified in newfd.  If the file descriptor  newfd  was       previously open, it is silently closed before being reused.       The  steps  of  closing  and reusing the file descriptor newfd are per-       formed atomically.  This is  important,  because  trying  to  implement       equivalent  functionality  usingclose(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()       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  descrip-          tion of the same flag inopen(2) for reasons why this may be useful.       *  If oldfd equals newfd, then dup3() fails with the error EINVAL.RETURN VALUE       On  success, these system calls return the new file descriptor.  On er-       ror, -1 is returned, and errno is set appropriately.ERRORS       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 ingetrlimit(2)).       EBUSY  (Linux  only)  This may be returned by dup2() or dup3() during a              race condition withopen(2) and dup().       EINTR  The dup2() or dup3() call was interrupted by a signal; see  sig-nal(7).       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  getr-limit(2)).VERSIONS       dup3() was added to Linux in version 2.6.27; glibc support is available       starting with version 2.9.CONFORMING TO       dup(), dup2(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.       dup3() is Linux-specific.NOTES       The error returned by dup2() is different from  that  returned  by  fc-       ntl(...,  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 atclose(2)       time are lost.  If this is of concern, then--unless the program is sin-       gle-threaded and does not allocate  file  descriptors  in  signal  han-       dlers--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 */               }           }SEE ALSOclose(2),fcntl(2),open(2)COLOPHON       This  page  is  part of release 5.05 of the Linux man-pages project.  A       description of the project, information about reporting bugs,  and  the       latest     version     of     this    page,    can    be    found    at       https://www.kernel.org/doc/man-pages/.Linux                             2017-09-15DUP(2)