CareerCup

Question needs some clarification.

The input string cannot be modified as the replacement word can be a different size than the word it is replacing. So, implicitly, it needs new string memory.

As you specify, we cannot allocate a local variable and return. So, why not use malloc i.e. dynamic memory allocation and return the pointer?

It is possible to return an array in c. Below is the function prototype .

Char [] replace_string( char const *const str )

Inside the function you can declare the array as static and return the array or can dynamically allocate memory for array.

another way you could return the array as a parameter in the function

Static void replace_str(char const *const str, char dst[length] )

The prototype he gave is not pure C/C++ style.

use below instead
char * replace_string( char * str )

You can return an array by putting it in a structure. There is no way of returning an array otherwise in C because its always converted to a pointer.

struct ArrayHolder {
char a[100];
}

Now you can just return the instance of the structure you create.