[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:
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… Keep doing this in a spiral/clockwise direction until all tokens have been covered. 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.