nbuf(4)nbuf(4)Namenbuf - select multiple-buffer operation to a raw device
Syntax
#include <sys/ioctl.h>
ioctl(d, FIONBUF, count)
int d;
int *count;
status=ioctl(d, FIONBDONE, buffer)
int d, status;
char **buffer;
Description
The I/O operations to raw devices are usually performed through a sin‐
gle buffer. This means that the issuing process must wait for a buffer
to complete before the process can do anything else. An 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 N-buffered opera‐
tion is enabled, and acts as before except that buffer completion is
not guaranteed when the call returns. If the operation starts without
errors, and 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 oper‐
ations, the process must not reuse the buffer until the operation actu‐
ally completes. A second is used to check the status of previously
issued N-buffered read/write requests to determine when the operation
has really completed.
N-buffered I/O is used through a set of calls. Setting the request
argument in an call to FIONBUF enables count buffers to be used with
the raw device associated with the file descriptor d.
The count returns the actual number of buffers allocated to the user
process. You can set the maximum number of buffers allowed for user
processes by setting the value of the global definition in the system
configuration file. The global definition defines the upper bound for
the number on N-buffered I/O buffer headers any user process can allo‐
cate.
If count is zero, the N-buffered operation is terminated and any pend‐
ing buffers are completed. A count less than zero is invalid. Any
started I/O buffer's status is checked by the call with the 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 at
which point EWOULDBLOCK is returned). In addition, a signal can be
generated whenever a buffer completes, if FIOASYNC has been specified
with
The call is also useful in checking on the status of pending buffers.
The call returns immediately if less than count operations have been
started on an N-buffered channel. Otherwise, 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.
Diagnostics
The call fails if one or more of the following are true:
[EBADF] The d argument is not a valid descriptor.
[ENOTTY] The d argument is not associated with a character spe‐
cial device.
[ENOTTY] The specified request does not apply to the kind of
object which the descriptor d references.
[EINVAL] The request or argp argument is not valid. Returned for
FIONBDONE, if requested buffer was never started. Also
returned for FIONBUF, if this device does not support N-
buffered I/O.
See Alsofcntl(2), ioctl(2), select(2)nbuf(4)
