dup (2)

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(The comments found at the beginning of the groff file "man2/dup.2freebsd".)

NAME

dup dup2 - duplicate an existing file descriptor

LIBRARY

Lb libc

SYNOPSIS

In unistd.h Ft int Fn dup int oldd Ft int Fn dup2 int oldd int newd

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.

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, 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 Fn dup2 , the value of the new descriptor Fa newd is specified. If this descriptor is already in use and Fa oldd Fa newd , the descriptor is first deallocated as if the 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.

RETURN VALUES

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.

ERRORS

The Fn dup system call fails if:

Bq Er EBADF

The Fa oldd argument is not a valid active descriptor

Bq Er EMFILE

Too many descriptors are active.

The Fn dup2 system call fails if:

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

SEE ALSO

accept(2), close(2), fcntl(2), getdtablesize(2), open(2), pipe(2), socket(2), socketpair(2), dup3(3)

STANDARDS

The Fn dup and Fn dup2 system calls are expected to conform to St -p1003.1-90 .

HISTORY

The Fn dup and Fn dup2 functions appeared in AT&T System v7 .