Implementing atoi() in C
Be consistent with stdlib.h declaration of atoi()

The standard declaration is int atoi(const char *str);. Your declaration should match (i.e., take a const char * instead of char *).

What happens if a NULL pointer is passed?

As written, you dereference ato without checking if it is NULL (0). That is a big no-no in C. At the very top of the function, you should check if it's 0 and return 0 if it is:

if (!ato) { return 0; }


Edit: Martin R points out that silently returning 0 when a NULL pointer is passed to your function, as I have suggested, is bad programming practice, because it hides the error. If there's a possibility of calling myatoi() with a NULL pointer, there's no way for the function to notify the caller of the error. Therefore, it's incumbent upon the caller to check before calling. 

No need for separate variable definition and initialization

Do:

int res = 0;
int sign = 0;


Don't:

int res;
int sign;
res = 0;
sign = 1;


Don't reinvent the wheel

Your first loop that skips over whitespace could use the standard library function isspace():

Edit: spot the bug in my original code... :/

Buggy (oops): while (isspace(*ato++));
Correct:      while (isspace(*ato)) ato++;


The condition check in the second loop can use the stdlib function isdigit():

while (isdigit(*ato)) {
    ...
    ato++;
}


If your instructor or assignment instructions prevent you from using isspace() and isdigit(), why not go ahead and define them yourself? You already have the logic!*

* Except you have a minor bug in your check for space characters. You also need to check for vertical tab (\v).

Edit: Chux points out that calling isspace(*ato) and isdigit(*ato) when *ato &lt; 0 invokes undefined behavior (UB). Specifically, the C11 standard, section 7.4, Character Handling, states,


  The header &lt;ctype.h&gt; declares several functions useful for classifying and mapping characters. In all cases the argument is an int, the value of which shall be representable as an unsigned char or shall equal the value of the macro EOF. If the argument has any other value, the behavior is undefined.


The correct usage, then, of the isspace() and isdigit() functions in this code would be:

while (isspace(*(unsigned char *)ato)) { atoi++; }
//...
while (isdigit(*(unsigned char *)ato)) {
    //...
    ato++;
}


Chux also pointed out that your original code as written did not have this problem; you weren't invoking undefined behavior. Moral of the story: be careful of the advice you receive from strangers on the Internet...? =)



Other than the issues mentioned, you have a decent implementation!