SVR5

geteblk(D3)

geteblk -- get an empty buffer

Synopsis (Not in current DDI version)

   #include <sys/types.h>
   #include <sys/buf.h>
   #include <sys/ddi.h>
   
   buf_t *geteblk(void);

Description

geteblk retrieves a buffer [see buf(D4)] from the buffer cache and returns a pointer to the buffer header. If a buffer is not available, geteblk sleeps until one is available.

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 using the buffer for an I/O request.

The following list describes the state of these members when the buffer header is received from geteblk:

b_flags: is set for a synchronous write transfer from main memory to the device. The driver must turn on B_READ flag to transfer from the device to main memor. The driver may also turn on the B_ASYNC flag and set the b_iodone member for a synchronous transfer. the user's buffer.
b_edev: is set to NODEV and must be initialized by the driver.
b_bcount: is set to 1024.
b_un.b_addr: is set to the buffer's virtual address.
b_blkno: is not initialized by geteblk, and must be initialized by the driver
b_blkoff: ??
b_addrtype: is set to BA_KVIRT
b_bufsize: is set to bsize
b_iodone: is set to NULL

Typically, drivers do not allocate buffers. A buffer is allocated by the kernel, and the associated buffer header is used as an argument to the driver's strategy(D2) 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 or ngeteblk(D3). If the driver chooses to use its own memory for the buffer, it can allocate just a buffer header by using the getrbuf(D3) function.

Buffers allocated via geteblk must be freed using either brelse(D3) or biodone(D3).

Context and synchronization

User or blockable context.

Hardware applicability

All

Version applicability

ddi: 1, 2, 3, 4, 5, 5mp, 6, 6mp, 7, 7mp, 7.1, 7.1mp

Differences between versions

In DDI 8, use the ngeteblk(D3) function instead of geteblk( ).

References

biodone(D3), biowait(D3), brelse(D3), buf(D4) ngeteblk(D3)