ngeteblk(D3)ngeteblk(D3)NAMEngeteblk - get an empty buffer of the specified size
SYNOPSIS
#include <sys/types.h>
#include <sys/buf.h>
#include <sys/ddi.h>
buf_t *ngeteblk(size_t bsize);
Arguments
bsize Size of the buffer being requested.
DESCRIPTIONngeteblk retrieves a buffer [see buf(D4)] of size bsize from the buffer
cache and returns a pointer to the buffer header. If a buffer is not
available, ngeteblk dynamically allocates one. If memory is not
immediately available, ngeteblk will sleep until enough memory has been
freed to allocate the buffer.
Return Values
A pointer to the buffer header structure is returned.
USAGE
When the driver strategy(D2) routine receives a buffer header from the
kernel, all the necessary members are already initialized. However, when
a driver allocates buffers for its own use, it must set up some of the
members before calling its strategy routine.
The following list describes the state of these members when the buffer
header is received from ngeteblk:
b_flags is set to indicate the transfer is from the user's
buffer to the kernel. The driver must set the B_READ
flag if the transfer is from the kernel to the user's
buffer.
b_edev is set to NODEV and must be initialized by the driver.
b_bcount is set to bsize.
b_un.b_addr is set to the buffer's virtual address.
b_blkno is not initialized by ngeteblk, and must be
initialized by the driver
Typically, block drivers do not allocate buffers. The buffer is
allocated by the kernel, and the associated buffer header is used as an
argument to the driver strategy routine. However, to implement some
special features, such as ioctl(D2) commands that perform I/O, the driver
may need its own buffer space. The driver can get the buffer space from
the system by using geteblk(D3) or ngeteblk. Or the driver can choose to
use its own memory for the buffer and only allocate a buffer header with
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ngeteblk(D3)ngeteblk(D3)getrbuf(D3).
Note that buffers allocated via ngeteblk must be freed using either
brelse(D3) or biodone(D3).
Level
Base only.
Synchronization Constraints
Can sleep.
Driver-defined basic locks and read/write locks may not be held across
calls to this function.
Driver-defined sleep locks may be held across calls to this function.
REFERENCESbiodone(D3), brelse(D3), buf(D4), geteblk(D3)
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