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@(#)dup.2 8.1 (Berkeley) 6/4/93
$FreeBSD: releng/10.2/lib/libc/sys/dup.2 254410 2013-08-16 13:16:27Z jilles $
.Dd June 1, 2013 .Dt DUP 2 .Os .Sh NAME .Nm dup , .Nm dup2 .Nd duplicate an existing file descriptor .Sh LIBRARY .Lb libc .Sh SYNOPSIS n unistd.h .Ft int .Fn dup "int oldd" .Ft int .Fn dup2 "int oldd" "int newd" .Sh DESCRIPTION The .Fn dup system call duplicates an existing object descriptor and returns its value to the calling process .Fa ( newd = .Fn dup oldd ) . The argument .Fa 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.
p The object referenced by the descriptor does not distinguish between .Fa oldd and .Fa newd in any way. Thus if .Fa newd and .Fa oldd are duplicate references to an open file, .Xr read 2 , .Xr write 2 and .Xr 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 .Xr open 2 system call. The close-on-exec flag on the new file descriptor is unset.
p In .Fn dup2 , the value of the new descriptor .Fa newd is specified. If this descriptor is already in use and .Fa oldd \*(Ne .Fa newd , the descriptor is first deallocated as if the .Xr close 2 system call had been used. If .Fa oldd is not a valid descriptor, then .Fa newd is not closed. If .Fa oldd == .Fa newd and .Fa oldd is a valid descriptor, then .Fn dup2 is successful, and does nothing. .Sh RETURN VALUES These calls return the new file descriptor if successful; otherwise the value -1 is returned and the external variable .Va errno is set to indicate the cause of the error. .Sh ERRORS The .Fn dup system call fails if: l -tag -width Er t Bq Er EBADF The .Fa oldd argument is not a valid active descriptor t Bq Er EMFILE Too many descriptors are active. .El
p The .Fn dup2 system call fails if: l -tag -width Er t Bq Er EBADF The .Fa oldd argument is not a valid active descriptor or the .Fa newd argument is negative or exceeds the maximum allowable descriptor number .El .Sh SEE ALSO .Xr accept 2 , .Xr close 2 , .Xr fcntl 2 , .Xr getdtablesize 2 , .Xr open 2 , .Xr pipe 2 , .Xr socket 2 , .Xr socketpair 2 , .Xr dup3 3 .Sh STANDARDS The .Fn dup and .Fn dup2 system calls are expected to conform to .St -p1003.1-90 . .Sh HISTORY The .Fn dup and .Fn dup2 functions appeared in .At v7 .