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Function declarations and their scope

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• Function declaration: providing the C compiler with the function prototype information
  • Syntax to declare a function in C:

    [extern] return-type FuncName ( param1_type [var1], param2_type [var2], .... ) ;

    Note:

    The keyword extern is optional The parameter variable names are also optional

    Example of function declarations:

    extern float f( float x ) ; float f( float x ) ; // extern can be omitted float f( float ) ; // the parameter name can be omitted

    (Personally, I prefer to write float f(float x), it looks like the function definition (so I can cut and paste to create the function declaration easily)

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  • Note:

    The function body is (must be) omitted in the function declaration

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• How is the function declaration used by the C compiler ?
  • The function declaration provides the data type information to the C compiler

  • How is the function declaration (information) used:

    Suppose the C compiler processes this function call: z = f( x , y) ; ^ ^ ^ | | | receiving function variable input The C compiler will convert the input parameters to the correct data type of each parameters: The C compiler knows the data type of each parameter of the function from the function declaration The C compiler knows the data type of each function input of the function from the function call If the data types are different, the C compiler will perform type conversion before the input value is passed to the function !!! The C compiler will convert the return value to the correct data type of the receiving variable: The C compiler knows the data type of the return value of the function from the function declaration The C compiler knows the data type of the receiving variable If the data types are different, the C compiler will perform type conversion before the return value is stored in the receiving variable !!!

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• Scope of a function declaration
  • Scope:

    Scope = the range (= area in a program) where a given object (e.g., variable) is accessible (or valid)

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  • The scope of a function declaration:

    A function declaration can be placed anywhere in a C program When a function declaration is placed outside a function, then the scope of this function declaration is: The scope starts at the location of the function declaration in the program file And remains valid till the end of the program file Graphically: When a function declaration is placed inside a function, then the scope of this function declaration is: The scope starts at the location of the function declaration in the program file And remains valid till the end of the function Graphically:

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• Common practice
  • Common practice of C programmers:

    We always place all function declarations at the top of all our C program files This will maximize the scope (area) of all of the function declarations !!!

    Grpahically:

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• Example of using function declaration
  • Recall in the last example the C program did not compiled due to conflicting information in the default parameters assumption and the actual function definition:

    #include <stdio.h> int main(int argc, char *argv[]) { short x = 2; // *** short !!! int y = 0; y = f(x); // Assumes: int f(int x) printf("x = %d, y = %d\n", x, y); } /* --------------------------------------------------- Function f( ) is defined AFTER main's use of f( ) --------------------------------------------------- */ int f(short x) { return(x*x); }

    We can declare the function f( ) at the start of the program to solve this problem:

    #include <stdio.h> int f(short x); // function declaration !!! int main(int argc, char *argv[]) { short x = 2; // *** short !!! int y = 0; y = f(x); // Assumes: int f(int x) printf("x = %d, y = %d\n", x, y); } /* --------------------------------------------------- Function f( ) is defined AFTER main's use of f( ) --------------------------------------------------- */ int f(short x) { return(x*x); }

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set1/impl-declare4.c

    How to run the program:

    To compile:   gcc impl-declare4.c          To run: ./a.out

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  • Comment:

    You can declare a function multiple times (as long as the declarations are non-conflicting !!!) (You can define a function only once !!!)