The C ``Clockwise/Spiral Rule''

# Clockwise/Spiral Rule Source: [https://c-faq.com/decl/spiral.anderson.html](https://c-faq.com/decl/spiral.anderson.html) \[This was posted to comp\.lang\.c by its author, David Anderson, on 1994\-05\-06\.\]## 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: 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\.\.\.> \`\`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 AndersonThis article may be freely distributed as long as the author's name and this notice are retained\. ---