C

Table des matières

1. .
2. Operator precedence
3. Reading declarations

1 .

I used C for a lot of years and in a flurry of projects. I selected it mainly for portability or speed considerations, that is, for practical matters. As for the language itself, I found its syntax repulsive at first (I had a strong Pascal background at the time, and much enjoyed Pascal design and syntax). But in the long run, I tamed myself to C in some deep way, and many surrounding aspects (like Flex, Bison, Autoconf, Automake, Gettext, and of course, coding standards).

2 Operator precedence

A quick reminder chart. Each line gives operators sharing a precedence level. The first line gives the most binding operators, the last line gives the least binding operators. The initial column indicates if operators associate from left to right (→) or from right to left (←):

Assoc       Operators

   →        ()  []  ->  .
 ←          !   ~   ++  --  +   -   *   &   (type) sizeof
   →        *   /   %
   →        +   -
   →        <<  >>
   →        <   <=  >   >=
   →        ==  !=
   →        &
   →        ^
   →        |
   →        &&
   →        ||
   →        ?:
 ←          =   +=  -=  *=  /=  %=  &=  ^=  !=  <<=  >>=
   →        ,

3 Reading declarations

I found the following description somewhere on the Net, a while ago, and saved it without reference (in a word: I did not write it).

The right-left rule is a completely regular rule for deciphering C declarations. It can also be useful in creating them.

3.1 Reading rules

First, symbols. Read

*	as pointer to
[]	as array of
()	as function returning

as you encounter them in the declaration.

Step 1 — Find the identifier. This is your starting point. Then say to yourself, identifier is. You've started your declaration.
Step 2 — Look at the symbols on the right of the identifier. If, say, you find () there, then you know that this is the declaration for a function. So you would then have identifier is function returning. Or if you found a [] there, you would say identifier is array of. Continue right until you run out of symbols or hit a left parenthesis. (If you hit a right parenthesis, that's the beginning of a () symbol, even if there is stuff in between the parentheses. More on that below.)
Step 3 — Look at the symbols to the left of the identifier. If it is not one of our symbols above (say, something like int), just say it. Otherwise, translate it into English using that table above. Keep going left until you run out of symbols or hit a right parenthesis.
Now repeat steps 2 and 3 until you've formed your declaration.

3.2 A first example

int *p[];

Find identifier:
```
int *p[];
     ^
```
p is
Move right until out of symbols or left parenthesis hit:
```
int *p[];
      ^^
```
p is array of
Can't move right anymore (out of symbols), so move left and find:
```
int *p[];
        ^
```
p is array of pointer to
Keep going left and find:
```
int *p[];
^^^
```
p is array of pointer to int.

3.3 Another example

int *(*func())();

Find the identifier:
```
int *(*func())();
       ^^^^
```
func is
Move right:
```
int *(*func())();
           ^^
```
func is function returning
Can't move right anymore because of the left parenthesis, so move left:
```
int *(*func())();
      ^
```
func is function returning pointer to
Can't move left anymore because of the right parenthesis, so keep going right:
```
int *(*func())();
              ^^
```
func is function returning pointer to function returning
Can't move right anymore because we're out of symbols, so go left:
```
int *(*func())();
    ^
```
func is function returning pointer to function returning pointer to
And finally, keep going left, because there's nothing left on the right:
```
int *(*func())();
^^^
```
func is function returning pointer to function returning pointer to int.

As you can see, this rule can be quite useful. You can also use it to sanity check yourself while you are creating declarations, and to give you a hint about where the put the next symbol and whether parentheses are required.

3.4 More complex cases

Some declarations look much more complicated than they are due to array sizes and argument lists in prototype form. If you see [3], that's read as array (size 3) of …. If you see (char *, int) that's read as function expecting (char *, int) and returning …. Here's a fun one:

int (*(*fun_one)(char *, double))[9][20];

I won't go through each of the steps to decipher this one. It's:

fun_one is pointer to function expecting (char *, double) and returning pointer to array (size 9) of array (size 20) of int.

As you can see, it's not as complicated if you get rid of the array sizes and argument lists:

int (*(*fun_one)())[][];

You can decipher it that way, and then put in the array sizes and argument lists later.

3.5 Some final words

It is quite possible to make illegal declarations using this rule, so some knowledge of what's legal in C is necessary. For instance, if the above had been:

int *((*fun_one)())[][];

it would have been fun_one is pointer to function returning array of array of pointer to int. Since a function cannot return an array, but only a pointer to an array, that declaration is illegal.

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C