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

     The dup() system call 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 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) system 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 the close(2) system 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.

     The related cap_new(2) system call allows file descriptors to be duplicated with
     restrictions on their use.

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

     The dup() system call fails if:

     [EBADF]            The oldd argument is not a valid active descriptor

     [EMFILE]           Too many descriptors are active.
     The dup2() system call fails if:

     [EBADF]            The oldd argument is not a valid active descriptor or the newd argument
                        is negative or exceeds the maximum allowable descriptor number

SEE ALSO

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

STANDARDS

     The dup() and dup2() system calls are expected to conform to ISO/IEC 9945-1:1990
     (“POSIX.1”).

HISTORY

     The dup() and dup2() functions appeared in Version 7 AT&T UNIX.