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