dup, dup2 -- duplicate an existing file descriptor
Standard C Library (libc, -lc)
dup2(int oldd, int newd);
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 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
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-onexec
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 value -1 is returned if an error occurs in either call. The external
variable errno indicates the cause of the error.
The dup() and dup2() system calls fail if:
[EBADF] The oldd or newd argument is not a valid active
[EMFILE] Too many descriptors are active.
accept(2), close(2), fcntl(2), getdtablesize(2), open(2), pipe(2),
The dup() and dup2() system calls are expected to conform to ISO/IEC
The dup() and dup2() functions appeared in Version 7 AT&T UNIX.
FreeBSD 5.2.1 June 4, 1993 FreeBSD 5.2.1 [ Back ]