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Tuesday, October 25, 2011

Demystifying complex C declarations.

I have seen many times people complaining of interviewer giving them shit looking declaration and ask us to tell them what it is..
Ex. :
int * p :
pointer to an integer :
[:D]..
okay.. how about this..

 void (*signal(int, void (*fp)(int)))(int); 


To some even this may look easy but to me I put down my pencil and got back to facebook to relieve from tension.
Following is the illustration of one simple method I stumbled across and felt it worked for any kind of declaration.

If there is/are any exception(s) to the following, please feel free to discuss.


[This was posted to comp.lang.c by its author, David Anderson, on 1994-05-06.].. and I read on c-facts ;)

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...
    ``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.


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