[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!
* 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!
* 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''
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.
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This article may be freely distributed as long as the author's name and this
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