trusty (2) dup2.2freebsd.gz

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.