DUP

Section: System Calls (2)
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BSD mandoc
BSD 4

NAME

dup dup2 - duplicate an existing file descriptor

SYNOPSIS

Fd #include <unistd.h> Ft int Fn dup int oldd Ft int Fn dup2 int oldd int newd

DESCRIPTION

Fn Dup 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 value must be less than the size of the table, which is returned by getdtablesize(2). 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) 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, the descriptor is first deallocated as if a close(2) call had been done first.

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

Fn Dup and Fn dup2 fail if:

Bq Er EBADF: Fa Oldd or Fa newd is not a valid active descriptor
Bq Er EMFILE: Too many descriptors are active.

STANDARDS

Fn Dup and Fn dup2 are expected to conform to St -p1003.1-88 .

This document was created by man2html, using the manual pages.
Time: 16:29:00 GMT, April 18, 2022

DUP