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>Writing data</H1
><P
> The write call back function allows a userland process to write
data to the kernel, so it has some kind of control over the
kernel. The write function should have the following format:
</P
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><CODE
><CODE
CLASS="FUNCDEF"
>int <TT
CLASS="FUNCTION"
>write_func</TT
></CODE
>(struct file* <TT
CLASS="PARAMETER"
><I
>file</I
></TT
>, const char* <TT
CLASS="PARAMETER"
><I
>buffer</I
></TT
>, unsigned long <TT
CLASS="PARAMETER"
><I
>count</I
></TT
>, void* <TT
CLASS="PARAMETER"
><I
>data</I
></TT
>);</CODE
></P
><P
></P
></DIV
><P
> The write function should read <TT
CLASS="PARAMETER"
><I
>count</I
></TT
>
bytes at maximum from the <TT
CLASS="PARAMETER"
><I
>buffer</I
></TT
>. Note
that the <TT
CLASS="PARAMETER"
><I
>buffer</I
></TT
> doesn't live in the
kernel's memory space, so it should first be copied to kernel
space with <TT
CLASS="FUNCTION"
>copy_from_user</TT
>. The
<TT
CLASS="PARAMETER"
><I
>file</I
></TT
> parameter is usually
ignored. <A
HREF="x257.html"
>the Section called <I
>A single call back for many files</I
></A
> shows how to use the
<TT
CLASS="PARAMETER"
><I
>data</I
></TT
> parameter.
</P
><P
> Again, <A
HREF="c310.html"
>the chapter called <I
>Example</I
></A
> shows how to use this call back
function.
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