Where can I legally declare a variable in C99?

问题:

When I was first introduced to C I was told to always declare my variables at the top of the function. Now that I have a strong grasp of the language I am focusing my efforts on coding style, particularly limiting the scope of my variables. I have read about the benefits to limiting the scope and I came across an interesting example. Apparently, C99 allows you to do this...

for (int i = 0; i < 10; i++)
{
   puts("hello");
}

I had thought that a variables scope was limited by the inner-most surrounding curly braces { }, but in the above example int i appears to be limited in scope by the curly braces of the for-loop even though it is declared outside of them.

I tried to extend the above example with fgets() to do what I thought was something similar but both of these gave me a syntax error.

fgets(char fpath[80], 80, stdin); *See Note**

fgets(char* fpath = malloc(80), 80, stdin);

So, just where exactly is it legal to declare variables in C99? Was the for-loop example an exception to the rule? Does this apply to while and do while loops as well?

*Note**: I'm not even sure this would be syntactically correct even if I could declare the char array there since fgets() is looking for pointer to char not pointer to array 80 of char. This is why I tried the malloc() version.

回答1:

In C99, you can declare your variables where you need them, just like C++ allows you to do that.

void somefunc(char *arg)
{
    char *ptr = "xyz";
    if (strcmp(arg, ptr) == 0)
    {
        int abc = 0;    /* Always could declare variables at a block start */

        somefunc(arg, &ptr, &abc);

        int def = another_func(abc, arg);   /* New in C99 */
        ...other code using def, presumably...
    }
}

• You can declare a variable in the control part of a 'for' loop:

  for (int x = 0; x < 10; x++)    /* New in C99 */
  

• You cannot declare a variable in the control part of a 'while' loop or an 'if' statement.

• You cannot declare a variable in a function call.
• Obviously, you can (and always could) declare variables in the block after any loop or an 'if' statement.

The C99 standard says:

6.8.5.3 The for statement

The statement

for ( clause-1 ; expression-2 ; expression-3 ) statement

behaves as follows: The expression expression-2 is the controlling expression that is evaluated before each execution of the loop body. The expression expression-3 is evaluated as a void expression after each execution of the loop body. If clause-1 is a declaration, the scope of any variables it declares is the remainder of the declaration and the entire loop, including the other two expressions; it is reached in the order of execution before the first evaluation of the controlling expression. If clause-1 is an expression, it is evaluated as a void expression before the first evaluation of the controlling expression.

回答2:

The first thing I'd note is that you shouldn't confuse

for (int i = 0; i < 10; i++) {
    puts("hello");
}

and

fgets(char* fpath = malloc(80), 80, stdin);

The first is a control structure while the second is a function call. The control structure evaluates the text inside it's parens() in a very different way from how a function call does.

The second thing is... I don't understand what you're trying to say by:

the compiler will promptly give you an error if you try to use i inside the for-loop body.

The code you listed for the for loop is a very common structure in C and the variable "i" should, indeed, be available inside the for loop body. Ie, the following should work:

int n = 0;
for (int i = 0; i < 10; i++) {
    n += i;
}

Am I misreading what you're saying?

回答3:

The bottom line with respect to your for/fgets confusion is that while "enclosing braces control scope" is the correct rule in C most of the time, there is another rule regarding scope in C99 (borrowed from C++) that says that a variable declared in the prologue of a control structure (i.e. for, while, if) is in scope in the body of the structure (and is not in scope outside the body).