[This was posted to comp.lang.c by its author, David Anderson, on 1994-05-06.]

The ``Clockwise/Spiral Rule''

By David Anderson

There is a technique known as the ``Clockwise/Spiral Rule'' which enables any C programmer to parse in their head any C declaration!

There are three simple steps to follow:

1. Starting with the unknown element, move in a spiral/clockwise direction; when ecountering the following elements replace them with the corresponding english statements:

   [X] or []

   => Array X size of... or Array undefined size of...

   (type1, type2)

   => function passing type1 and type2 returning...

   *

   => pointer(s) to...

2. Keep doing this in a spiral/clockwise direction until all tokens have been covered.
3. Always resolve anything in parenthesis first!

Example #1: Simple declaration

                     +-------+
                     | +-+   |
                     | ^ |   |
                char *str[10];
                 ^   ^   |   |
                 |   +---+   |
                 +-----------+

Question we ask ourselves: What is str?

``str is an...

• We move in a spiral clockwise direction starting with `str' and the first 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 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 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

                     +--------------------+
                     | +---+              |
                     | |+-+|              |
                     | |^ ||              |
                char *(*fp)( int, float *);
                 ^   ^ ^  ||              |
                 |   | +--+|              |
                 |   +-----+              |
                 +------------------------+

Question we ask ourselves: What is fp?

``fp is a...

• Moving in a spiral clockwise direction, the first thing we see is a `)'; therefore, fp is inside parenthesis, so we continue the spiral inside the 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 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 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 returning a pointer to...

• Continuing in a spiral fashion we see the `;', but we haven't visited all 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 returning a pointer to a char''

Example #3: The ``Ultimate''

                      +-----------------------------+
                      |                  +---+      |
                      |  +---+           |+-+|      |
                      |  ^   |           |^ ||      |
                void (*signal(int, void (*fp)(int)))(int);
                 ^    ^      |      ^    ^  ||      |
                 |    +------+      |    +--+|      |
                 |                  +--------+      |
                 +----------------------------------+

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 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 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 int returning nothing (void) returning a pointer to...

• We have now resolved the items within parenthesis, so continuing clockwise, we then see another `(', so...

  ``signal is a function passing an int and a pointer to a function passing an int returning nothing (void) returning a pointer to a function passing an int returning...

• Finally we continue and the only thing left is the word `void', so the final complete definition for signal is:

  ``signal is a function passing an int and a pointer to a function passing an int returning nothing (void) returning a pointer to a function passing an 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 the author's name and this notice are retained.