nbuf.4

来自「<B>Digital的Unix操作系统VAX 4.2源码</B>」· 4 代码 · 共 141 行

.\" SCCSID: @(#)nbuf.4	8.1	9/11/90
.TH nbuf 4
.SH Name
nbuf \- select multiple-buffer operation to a raw device
.SH Syntax
.nf
.ft B
#include <sys/ioctl.h>
.PP
.nf
.ft B
ioctl(d, FIONBUF, count)
int d;
int *count;
.fi
.PP
.nf
.ft B
status=ioctl(d, FIONBDONE, buffer)
int d, status;
char **buffer;
.fi
.ft R
.SH Description
.NXR "nbuf keyword"
.NXR "I/O operation" "multiple buffers and"
The I/O operations to raw devices are usually performed through
a single buffer. This means that the issuing process
must wait for a buffer to complete before the process can
do anything else. An
.I N-buffered 
I/O operation allows a process
to begin an I/O operation and continue doing something else
until the operation has finished. Once 
.I N-buffered
operation
is enabled, 
.MS read 2
and 
.MS write 2
acts as before except that
buffer completion is not guaranteed when the call returns.
If the operation starts without errors,
.MS read 2 
and 
.MS write 2 
return
as if the operation were successful.
That is, the number of requested bytes
have transferred and file pointers are updated. 
On read operations,
the process must not use the contents of the started buffer
until the buffer actually completes. 
On write operations,
the process
must not reuse the buffer until the operation actually completes.
A second 
.PN ioctl 
is used to check the status of previously 
issued 
.I N-buffered 
read/write requests to determine when the 
operation has really completed.
.PP
.I N-buffered 
I/O is used through a set of 
.PN ioctl 
calls.
Setting the
.I request
argument in an
.PN ioctl
call to FIONBUF enables 
.I count 
buffers to be used with the raw device
associated with the file descriptor 
.IR d . 
If 
.I count
is zero,
the N-buffered operation is terminated and any pending buffers
are completed. A 
.I count
less than zero is invalid.
Any started I/O buffer's status is checked by the 
.PN ioctl
call with the
.I request
argument set to FIONBDONE,
with the address of the buffer used as an argument. 
The status field
returns the actual byte count transferred or any error encountered
on the I/O operation. The FIONBDONE ioctl must be called before 
re-using a buffer. FIONBDONE blocks the process until
the given buffer completes (unless FNDELAY has been specified
with
.MS fcntl 2 ,
at which point EWOULDBLOCK is returned).
In addition, a signal can be generated whenever a buffer completes, if 
FIOASYNC has been specified with
.MS fcntl 2 .
.PP
The
.MS select 2
call is also useful in checking on the status of pending 
buffers. 
The
.MS select 2
call returns immediately if less than 
.I count
operations have been started on an N-buffered channel. Otherwise,
.PN select 
blocks the specified amount of time for a buffer to 
become done. At this point, FIONBDONE must be used to return
actual status of the pending buffer.
.SH Diagnostics
The
.PN ioctl
call fails if one or more of the following are true:
.TP 15
[EBADF]
The \fId\fP argument is not a valid descriptor.
.TP 15
[ENOTTY]
The \fId\fP argument is not associated with a character
special device.
.TP 15
[ENOTTY]
The specified request does not apply to the kind
of object which the descriptor \fId\fP references.
.TP 15
[EINVAL]
The \fIrequest\fP or \fIargp\fP 
argument is not valid. Returned for FIONBDONE, if
requested buffer was never started. Also returned
for FIONBUF, if this device does not support 
.I N-buffered 
I/O.
.SH See Also
fcntl(2), ioctl(2), select(2)