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Annotate NULL

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Jim Huang 2021-08-09 23:57:30 +08:00
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lkmpg.tex
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@ -389,7 +389,7 @@ module_param(myint, int, 0);
Arrays are supported too, but things are a bit different now than they were in the olden days.
To keep track of the number of parameters you need to pass a pointer to a count variable as third parameter.
At your option, you could also ignore the count and pass NULL instead. We show both possibilities here:
At your option, you could also ignore the count and pass \cpp|NULL| instead. We show both possibilities here:
\begin{code}
int myintarray[2];
@ -799,7 +799,7 @@ struct file_operations {
Some operations are not implemented by a driver.
For example, a driver that handles a video card will not need to read from a directory structure.
The corresponding entries in the \cpp|file_operations| structure should be set to NULL.
The corresponding entries in the \cpp|file_operations| structure should be set to \cpp|NULL|.
There is a gcc extension that makes assigning to this structure more convenient.
You will see it in modern drivers, and may catch you by surprise.
@ -827,9 +827,9 @@ struct file_operations fops = {
};
\end{code}
The meaning is clear, and you should be aware that any member of the structure which you do not explicitly assign will be initialized to NULL by gcc.
The meaning is clear, and you should be aware that any member of the structure which you do not explicitly assign will be initialized to \cpp|NULL| by gcc.
An instance of \cpp|struct file_operations| containing pointers to functions that are used to implement read, write, open, \ldots{} syscalls is commonly named fops.
An instance of \cpp|struct file_operations| containing pointers to functions that are used to implement \cpp|read|, \cpp|write|, \cpp|open|, \ldots{} system calls is commonly named \cpp|fops|.
Sin Linux v5.6, the \cpp|proc_ops| structure was introduced to replace the use of the \cpp|file_operations| structure when registering proc handlers.
@ -1032,16 +1032,16 @@ It is based on sequence, which is composed of 3 functions: \cpp|start()|, \cpp|n
The \cpp|seq_file| API starts a sequence when a user read the /proc file.
A sequence begins with the call of the function \cpp|start()|.
If the return is a non NULL value, the function \cpp|next()| is called.
If the return is a non \cpp|NULL| value, the function \cpp|next()| is called.
This function is an iterator, the goal is to go through all the data.
Each time \cpp|next()| is called, the function \cpp|show()| is also called.
It writes data values in the buffer read by the user.
The function \cpp|next()| is called until it returns NULL.
The sequence ends when \cpp|next()| returns NULL, then the function \cpp|stop()| is called.
The function \cpp|next()| is called until it returns \cpp|NULL|.
The sequence ends when \cpp|next()| returns \cpp|NULL|, then the function \cpp|stop()| is called.
BE CAREFUL: when a sequence is finished, another one starts.
That means that at the end of function \cpp|stop()|, the function \cpp|start()| is called again.
This loop finishes when the function \cpp|start()| returns NULL.
This loop finishes when the function \cpp|start()| returns \cpp|NULL|.
You can see a scheme of this in the Figure~\ref{img:seqfile}.
\begin{figure}