trusty (2) dup.2.gz

NAME

       dup, dup2, dup3 - duplicate a file descriptor

SYNOPSIS

       #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);

DESCRIPTION

       These system calls 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 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 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.

       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
          useful.

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

RETURN VALUE

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

       EINTR  The dup2() or dup3() call was interrupted by a signal; see signal(7).

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

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

VERSIONS

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

CONFORMING TO

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

       dup3() is Linux-specific.

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() or dup3() without closing newfd first.

SEE ALSO

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

COLOPHON

       This page is part of release 3.54 of the Linux man-pages project.  A  description  of  the  project,  and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.