The ``Clockwise/Spiral Rule''
\[This was posted to comp.lang.c by its author, David Anderson, on 1994-05-06.\]
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...
- 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...
- We have now \`\`visited'' every token; therefore we are done!
- We are now out of the parenthesis and continuing in a spiral clockwise direction, we see the \`('; therefore, we have a function, so...
- 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...
- Moving in a clockwise direction we see \`(' so we have...
- Continue in a spiral clockwise direction and we get to \`(', so...
- Now we continue out of the function parenthesis and we see void, so...
- We have finished with fp so let's catch up with \`signal', we now have...
- We have now resolved the items within parenthesis, so continuing clockwise, we then see another \`(', so...
- Finally we continue and the only thing left is the word \`void', so the final complete definition for signal is:
- Now, what is chptr??
- Now, what is chptr??
- Now, what is chptr??
\`\`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...
\`\`str is an array 10 of pointers to char''
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...
\`\`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...
\`\`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!
\`\`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...
\`\`fp is a pointer to a function passing int returning...''
\`\`fp is a pointer to a function passing int returning nothing (void)''
\`\`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...
\`\`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...
\`\`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;
\`\`chptr is a pointer to a char constant''
How about this one:
char * const chptr;
\`\`chptr is a constant pointer to char''
Finally:
volatile char * const 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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Copyright © 1993,1994 David Anderson
This article may be freely distributed as long as the author's name and this notice are retained.
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