Buffer allocation using *geteblk() in sysV rel3

[KENG%NUSEEV.BITNET at cunyvm.cuny.edu]

Can anyone tell me more about allocation of buffer in
system V. The AT&T device driver manual says that after
obtaining a buffer using geteblk() I must fill in the
b_flags & b_bcount fields.

My questions are:

Since the b_bcount field specify the amount of bytes to
be transfered, can I specify any number of bytes as the
situation arise or the buffer size is fixed by the system.
(buf.h or anywhere else didn't mention the system buffer size).

The AT&T device driver reference manual for system V says
that some of the fields in the buf structure is not changable
by the driver when the buffer is allocated by the kernel unless
the buffer is allocated by the driver. I don't see the different
between a buffer allocated by the kernel from one allocated by
the driver since the call *geteblk() obtain buffer from the
system free buffer list.

The machine I am developing the device driver on is Motorola
sysV/68.

Many thanks in advance.

keng at nuseev.bitnet