NAME

dup2 − duplicate a file descriptor

SYNOPSIS

dup2(oldd, newd)
int oldd, newd;

DESCRIPTION

Dup2 duplicates an existing file descriptor.  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 value of newd desired for the new descriptor is specified.  If this descriptor is already in use, the descriptor is first deallocated as if a close(2) call had been done.

The file referenced by the descriptor does not distinguish between references using 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 reference to the file must be obtained by issuing an additional open(2) call. The close-on-exec flag on the new file descriptor is unset.

RETURN VALUE

The value −1 is returned if an error occurs in either call.  The external variable errno indicates the cause of the error. 

ERRORS

Dup2 fails if:

[EBADF] Oldd or newd is not a valid active descriptor

[EMFILE] Too many descriptors are active. 

SEE ALSO

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

Version 3.6  —  December 20, 1987