Functions - C Interview Questions I

Q. Why should I prototype a function?

A function prototype tells the compiler what kind of arguments a function is looking to receive and what kind of return value a function is going to give back. This approach helps the compiler ensure that calls to a function are made correctly and that no erroneous type conversions are taking place. For instance, consider the following prototype:

int some_func(int, char*, long);

Looking at this prototype, the compiler can check all references (including the definition of some_func()) to ensure that three parameters are used (an integer, a character pointer, and then a long integer) and that a return value of type integer is received. If the compiler finds differences between the prototype and calls to the function or the definition of the function, an error or a warning can be generated to avoid errors in your source code. For instance, the following examples would be flagged as incorrect, given the preceding

prototype of some_func():

x = some_func(1);                    /* not enough arguments passed */

x = some_func("HELLO!", 1, "DUDE!"); /* wrong type of arguments used */

x = some_func(1, str, 2879, "T");    /* too many arguments passed */

/* In the following example, the return value expected

   from some_func() is not an integer: */

long* lValue;

lValue = some_func(1, str, 2879);    /* some_func() returns an int,

                                        not a long* */

Using prototypes, the compiler can also ensure that the function definition, or body, is correct and correlates with the prototype. For instance, the following definition of some_func() is not the same as its prototype, and it therefore would be flagged by the compiler:

int some_func(char* string, long lValue, int iValue)  /* wrong order of

                                                         parameters */

{

    ...

}

The bottom line on prototypes is that you should always include them in your source code because they provide a good error-checking mechanism to ensure that your functions are being used correctly. Besides, many of today's popular compilers give you warnings when compiling if they can't find a prototype for a function that is being referenced.