MirBSD manpage: dup(2), dup2(2)

DUP(2)                     BSD Programmer's Manual                      DUP(2)

NAME

     dup, dup2 - duplicate an existing file descriptor

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(3). 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 op-
     tions are shared between the references. If a separate pointer into the
     file is desired, a different object reference to the file must be ob-
     tained 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, it is first deallocated as if a close(2)
     call had been done first. When newd equals oldd, dup2() just returns
     without affecting the close-on-exec flag.

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), open(2), pipe(2), socket(2),
     socketpair(2), getdtablesize(3)

STANDARDS

     dup() and dup2() are expected to conform to IEEE Std 1003.1-1988
     ("POSIX.1").

MirBSD #10-current               June 4, 1993                                1