Clockwise/Spiral Rule
[This was posted to comp.lang.c by its author,<br>David Anderson, on 1994-05-06.]<br>--><br>The ``Clockwise/Spiral Rule''
By David Anderson
There is a technique known as the ``Clockwise/Spiral Rule'' which enables<br>any C programmer to parse in their head any C declaration!
There are three simple steps to follow:
Starting with the unknown element, move in a spiral/clockwise direction;<br>when ecountering the following elements replace them with the corresponding<br>english statements:
[X] or []
=> Array X size of... or Array undefined size of...
(type1, type2)
=> function passing type1 and type2 returning...
=> pointer(s) to...
Keep doing this in a spiral/clockwise direction until all tokens have been<br>covered.
Always resolve anything in parenthesis first!
Example #1: Simple declaration
+-------+<br>| +-+ |<br>| ^ | |<br>char *str[10];<br>^ ^ | |<br>| +---+ |<br>+-----------+
Question we ask ourselves: What is str?
``str is an...
We move in a spiral clockwise direction starting with `str' and the first<br>character we see is a `[' so, that means we have an array, so...
``str is an array 10 of...
Continue in a spiral clockwise direction, and the next thing we encounter is<br>the `*' so, that means we have pointers, so...
``str is an array 10 of pointers to...
Continue in a spiral direction and we see the end of the line (the `;'), so<br>keep going and we get to the type `char', so...
``str is an array 10 of pointers to char''
We have now ``visited'' every token; therefore we are done!
Example #2: Pointer to Function declaration
+--------------------+<br>| +---+ |<br>| |+-+| |<br>| |^ || |<br>char *(*fp)( int, float *);<br>^ ^ ^ || |<br>| | +--+| |<br>| +-----+ |<br>+------------------------+
Question we ask ourselves: What is fp?
``fp is a...
Moving in a spiral clockwise direction, the first thing we see is a `)';<br>therefore, fp is inside parenthesis, so we continue the spiral inside the<br>parenthesis and the next character seen is the `*', so...
``fp is a pointer to...
We are now out of the parenthesis and continuing in a spiral clockwise<br>direction, we see the `('; therefore, we have a function, so...
``fp is a pointer to a function passing an int and a pointer to float<br>returning...
Continuing in a spiral fashion, we then see the `*' character, so...
``fp is a pointer to a function passing an int and a pointer to float<br>returning a pointer to...
Continuing in a spiral fashion we see the `;', but we haven't visited all<br>tokens, so we continue and finally get to the type `char', so...
``fp is a pointer to a function passing an int and a pointer to float<br>returning a pointer to a char''
Example #3: The ``Ultimate''
+-----------------------------+<br>| +---+ |<br>| +---+ |+-+| |<br>| ^ | |^ || |<br>void (*signal(int, void (*fp)(int)))(int);<br>^ ^ | ^ ^ || |<br>| +------+ | +--+| |<br>| +--------+ |<br>+----------------------------------+
Question we ask ourselves: What is `signal'?
Notice that signal is inside parenthesis, so we must resolve this first!
Moving in a clockwise direction we see `(' so we have...
``signal is a function passing an int and a...
Hmmm, we can use this same rule on `fp', so... What is fp? fp is also inside<br>parenthesis so continuing we see an `*', so...
fp is a pointer to...
Continue in a spiral clockwise direction and we get to `(', so...
``fp is a pointer to a function passing int returning...''
Now we continue out of the function parenthesis and we see void, so...
``fp is a pointer to a function passing int returning nothing (void)''
We have finished with fp so let's catch up with `signal', we now have...
``signal is a function passing an int and a pointer to a function passing an<br>int returning nothing (void) returning...
We are still inside parenthesis so the next character seen is a `*', so...
``signal is a function passing an int and a pointer to a function passing an<br>int returning nothing (void) returning a pointer to...
We have now resolved the items within parenthesis, so continuing clockwise,<br>we then see another `(', so...
``signal is a function passing an int and a pointer to a function passing an<br>int returning nothing (void) returning a pointer to a function passing an<br>int returning...
Finally we continue and the only thing left is the word `void', so the<br>final complete definition for signal is:
``signal is a function passing an int and a pointer to a function passing an<br>int returning nothing (void) returning a pointer to a function passing an<br>int returning nothing (void)''
The same rule is applied for const and volatile. For Example:
const char *chptr;
Now, what is chptr??
``chptr is a pointer to a char constant''
How about this one:
char * const chptr;
Now, what is chptr??
``chptr is a constant pointer to char''
Finally:
volatile char * const chptr;
Now, what is chptr??
``chptr is a constant pointer to a char volatile.''
Practice this rule with the examples found in K&R II on page 122.
Copyright © 1993,1994 David Anderson
This article may be freely distributed as long as...