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 encountering 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 typechar
, 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 typechar
, 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)
Other Examples
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.