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><H1
><A
NAME="USERLAND"
></A
>Communicating with userland</H1
><P
> Instead of reading (or writing) information directly from
kernel memory, procfs works with <I
CLASS="EMPHASIS"
>call back
functions</I
> for files: functions that are called when
a specific file is being read or written. Such functions have
to be initialised after the procfs file is created by setting
the <TT
CLASS="STRUCTFIELD"
><I
>read_proc</I
></TT
> and/or
<TT
CLASS="STRUCTFIELD"
><I
>write_proc</I
></TT
> fields in the
<SPAN
CLASS="STRUCTNAME"
>struct proc_dir_entry*</SPAN
> that the
function <TT
CLASS="FUNCTION"
>create_proc_entry</TT
> returned:
</P
><TABLE
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><TD
><PRE
CLASS="PROGRAMLISTING"
>struct proc_dir_entry* entry;
entry->read_proc = read_proc_foo;
entry->write_proc = write_proc_foo;
</PRE
></TD
></TR
></TABLE
><P
> If you only want to use a the
<TT
CLASS="STRUCTFIELD"
><I
>read_proc</I
></TT
>, the function
<TT
CLASS="FUNCTION"
>create_proc_read_entry</TT
> described in <A
HREF="c275.html#CONVENIENCE"
>the Section called <I
>Convenience functions</I
> in the chapter called <I
>Tips and tricks</I
></A
> may be used to create and initialise the
procfs entry in one single call.
</P
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN190"
></A
>Reading data</H1
><P
> The read function is a call back function that allows userland
processes to read data from the kernel. The read function
should have the following format:
</P
><DIV
CLASS="FUNCSYNOPSIS"
><A
NAME="AEN193"
></A
><P
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>int <TT
CLASS="FUNCTION"
>read_func</TT
></CODE
>(char* <TT
CLASS="PARAMETER"
><I
>page</I
></TT
>, char** <TT
CLASS="PARAMETER"
><I
>start</I
></TT
>, off_t <TT
CLASS="PARAMETER"
><I
>off</I
></TT
>, int <TT
CLASS="PARAMETER"
><I
>count</I
></TT
>, int* <TT
CLASS="PARAMETER"
><I
>eof</I
></TT
>, void* <TT
CLASS="PARAMETER"
><I
>data</I
></TT
>);</CODE
></P
><P
></P
></DIV
><P
> The read function should write its information into the
<TT
CLASS="PARAMETER"
><I
>page</I
></TT
>. For proper use, the function
should start writing at an offset of
<TT
CLASS="PARAMETER"
><I
>off</I
></TT
> in <TT
CLASS="PARAMETER"
><I
>page</I
></TT
> and
write at most <TT
CLASS="PARAMETER"
><I
>count</I
></TT
> bytes, but because
most read functions are quite simple and only return a small
amount of information, these two parameters are usually
ignored (it breaks pagers like <TT
CLASS="LITERAL"
>more</TT
> and
<TT
CLASS="LITERAL"
>less</TT
>, but <TT
CLASS="LITERAL"
>cat</TT
> still
works).
</P
><P
> If the <TT
CLASS="PARAMETER"
><I
>off</I
></TT
> and
<TT
CLASS="PARAMETER"
><I
>count</I
></TT
> parameters are properly used,
<TT
CLASS="PARAMETER"
><I
>eof</I
></TT
> should be used to signal that the
end of the file has been reached by writing
<TT
CLASS="LITERAL"
>1</TT
> to the memory location
<TT
CLASS="PARAMETER"
><I
>eof</I
></TT
> points to.
</P
><P
> The parameter <TT
CLASS="PARAMETER"
><I
>start</I
></TT
> doesn't seem to be
used anywhere in the kernel. The <TT
CLASS="PARAMETER"
><I
>data</I
></TT
>
parameter can be used to create a single call back function for
several files, see <A
HREF="x257.html"
>the Section called <I
>A single call back for many files</I
></A
>.
</P
><P
> The <TT
CLASS="FUNCTION"
>read_func</TT
> function must return the
number of bytes written into the <TT
CLASS="PARAMETER"
><I
>page</I
></TT
>.
</P
><P
> <A
HREF="c310.html"
>the chapter called <I
>Example</I
></A
> shows how to use a read call back
function.
</P
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