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.

       After  successful  return  of dup or dup2, the old and new
       descriptors may be used interchangeably. They share locks,
       file position pointers and flags; for example, if the file
       position is modified by using lseek on one of the descrip­
       tors, the position is also changed for the other.

       The  two  descriptors do not share the close-on-exec flag,
       however.

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

       dup2 makes newfd be the copy of oldfd, closing newfd first
       if necessary.


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.

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


WARNING

       The  error  returned by dup2 is different to that returned
       by fcntl(..., F_DUPFD, ...)  when newfd is out  of  range.
       On  some  systems  dup2 also sometimes returns EINVAL like
       F_DUPFD.


CONFORMING TO

       SVr4, SVID, POSIX, X/OPEN, BSD 4.3. SVr4  documents  addi­
       tional  EINTR  and ENOLINK error conditions.  POSIX.1 adds
       EINTR.


SEE ALSO

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