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DUP(2)                   DragonFly System Calls Manual                  DUP(2)


NAME

     dup, dup2 - duplicate an existing file descriptor


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <unistd.h>

     int
     dup(int oldd);

     int
     dup2(int oldd, int newd);


DESCRIPTION

     Dup() duplicates an existing object descriptor and returns its value to
     the calling process (newd = dup(oldd)).  The argument oldd is a small
     non-negative integer index in the per-process descriptor table.  The
     value must be less than the size of the table, which is returned by
     getdtablesize(2).  The new descriptor returned by the call is the lowest
     numbered descriptor currently not in use by the process.

     The object referenced by the descriptor does not distinguish between oldd
     and newd in any way.  Thus if newd and oldd are duplicate references to
     an open file, read(2), write(2) and lseek(2) calls all move a single
     pointer into the file, and append mode, non-blocking I/O and asynchronous
     I/O options are shared between the references.  If a separate pointer
     into the file is desired, a different object reference to the file must
     be obtained by issuing an additional open(2) call.  The close-on-exec
     flag on the new file descriptor is unset.

     In dup2(), the value of the new descriptor newd is specified.  If this
     descriptor is already in use and oldd != newd, the descriptor is first
     deallocated as if a close(2) call had been used.  If oldd is not a valid
     descriptor, then newd is not closed.  If oldd == newd and oldd is a valid
     descriptor, then dup2() is successful, and does nothing.


RETURN VALUES

     The value -1 is returned if an error occurs in either call.  The external
     variable errno indicates the cause of the error.


ERRORS

     Dup() and dup2() fail if:

     [EBADF]            Oldd or newd is not a valid active descriptor

     [EMFILE]           Too many descriptors are active.


SEE ALSO

     accept(2), close(2), fcntl(2), getdtablesize(2), open(2), pipe(2),
     socket(2), socketpair(2), dup3(3)


STANDARDS

     The dup() and dup2() function calls are expected to conform to IEEE Std
     1003.1-1990 ("POSIX.1").


HISTORY

     A dup() and a dup2() function call appeared in Version 7 AT&T UNIX.

DragonFly 5.5-DEVELOPMENT         May 6, 2019        DragonFly 5.5-DEVELOPMENT

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