破解C语言声明的“顺时针/螺旋规则”

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 encountering the following elements replace them with the corresponding english statements:

    1. [X] or [] => Array X size of… or Array undefined size of…
    2. (type1, type2) => function passing type1 and type2 returning…
    3. * => 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)

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.