Where can I legally declare a variable in C99?

Question

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 effor

Answer 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.

Answer 2

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).

Answer 3

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?