Where can I legally declare a variable in C99?
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 vari开发者_Python百科ables. 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.
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.
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?
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).
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