DUP(2) BSD System Calls Manual DUP(2)
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 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.
These calls return the new file descriptor if successful; otherwise the value -1 is returned
and the external variable errno is set to indicate the cause of the error.
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
accept(2), close(2), fcntl(2), getdtablesize(2), open(2), pipe(2), socket(2), socketpair(2),
The dup() and dup2() system calls are expected to conform to ISO/IEC 9945-1:1990
The dup() and dup2() functions appeared in Version 7 AT&T UNIX.
BSD June 1, 2013 BSD