dup.2

来自「<B>Digital的Unix操作系统VAX 4.2源码</B>」· 2 代码 · 共 80 行

.\" SCCSID: @(#)dup.2	8.1	9/11/90
.TH dup 2
.SH Name
dup, dup2 \- duplicate an open file descriptor
.SH Syntax
.nf
newd = dup(\fIoldd\fP)
int newd, \fIoldd\fP;
.PP
dup2(\fIoldd, newd\fP)
int \fIoldd, newd\fP;
.fi
.SH Description
.NXR "dup system call"
.NXR "dup2 system call"
.NXR "file descriptor" "duplicating"
.NXR "file descriptor" "process reference table"
The 
.PN dup
system call duplicates an existing object descriptor.
The argument \fIoldd\fP 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 
.PN getdtablesize .
The new descriptor,
.I newd,
returned by the call is the lowest numbered descriptor that is
not currently in use by the process.
.PP
The object referenced by the descriptor does not distinguish
between references using \fIoldd\fP and \fInewd\fP in any way.
Thus, if \fInewd\fP and \fIoldd\fP are duplicate references to an open
file, 
.PN read ,
.PN write ,
and
.PN lseek
calls all move a single pointer into the file.
If a separate pointer into the file is desired, a different
object reference to the file must be obtained by issuing an
additional 
.PN open
call.
.PP
In the second form of the call, specify the value of
.IR newd
needed. If this descriptor is already
in use, the descriptor is first deallocated as if a
.PN close
call had been done.
.SH Return Values
The value \-1 is returned if an error occurs in either call.
The external variable
.I errno
indicates the cause of the error.
.SH Diagnostics
The
.PN dup
and
.PN dup2
system calls fail under the following conditions:
.TP 15
[EBADF]
The
\fIoldd\fP or
\fInewd\fP is not a valid active descriptor.
.TP 15
[EMFILE]
Too many descriptors are active.
.TP 15
[EINTR]
The
.PN dup()
or
.PN dup2()
function was terminated prematurely by a signal.
.SH See Also
accept(2), close(2), getdtablesize(2), lseek(2), open(2), pipe(2), read(2), socket(2),
socketpair(2), write(2)