---

Declaring functions and organizing the function declarations

---

• The official way to declare a function in C
  • Recall: declaring a function:

    Declaring a function = inform the C compiler on the type (encoding) of the input parameters and output value of the function.

    What I have shown you is what I normally used.

    Here is the official syntax....

    ---

  • Syntax of a function declaration in C:

    extern returnType functionName ( param1type, param2type, .... ) Example: extern double square( double ) ;

    Meaning:

    The keyword extern tells the C compiler that the function functionName is defined somewhere else Note: the keyword extern is optional in a function declaration The function definition can appear: inside another C program file (or later inside the same C program file)

    ---

    ---

    Example:

    File func3a.c containing the main function	File func3b.c containing the other functions
    extern double square ( double ); int main( int argc, char *argv[] ) { double a, b; a = 4.0; b = square( a ); // Call square printf("Square of %d = %d\n", a, b); }	double square ( double x ) { double r; // Local variable r = x * x; // Statement return ( r ); // Return }

    ---

  • Example Program: (Demo above code)                                                
    • Prog file func3a.c: click here
    • Prog file func3b.c: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   gcc func3a.c func3b.c To compile:   a.out You will now get the correct result !!!

    Result:

    Number = 4.000000 Number = 16.000000 (correct !)

  ---

  ---

  ---

  ---

  ---

  ---

• A more commonly used function declaration format
  • Fact:

    The C compiler will treat everything after the parameter type in a function declaration as comment

    ---

  • Exploiting this feature:

    It is a common practice to include the parameter name in the function declaration (although the parameter name is not necessary and the C compiler will treat it as comment): extern double square ( double x ); ^ | C compiler treats x as a comment !!!

    Example:

    File func4a.c containing the main function	File func4b.c containing the other functions
    extern double square ( double x ); int main( int argc, char *argv[] ) { double a, b; a = 4.0; b = square( a ); // Call square }	double square ( double x ) { double r; // Local variable r = x * x; // Statement return ( r ); // Return }

    ---

  • Example Program: (Demo above code)                                                
    • Prog file func4a.c: click here
    • Prog file func4b.c: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   gcc func4a.c func4b.c To compile:   a.out You will now get the correct result !!!

    Result:

    Number = 4.000000 Number = 16.000000 (correct !)

  ---

  ---

  ---

  ---

• Scoping rules for declarations
  • Recall from CS170:

    Variables are only defined inside the scope of its definition: int f( ... ) { int x; ... } int g( ... ) { x is not defined outside its defining scope So: x is not accessible here !!! }

    ---

    ---

  • Fact:

    A declaration uses the same scoping rule as a definiation that is

    ---

    ---

    ---

  • Consequently:

    A declaration is only valid (= recognized) within the declaring scope of the declaration

    Example:

    int f( ... ) { extern double square( double x); // declare square() function ... } int g( ... ) { square() is not declared here !!! }

  ---

  ---

  ---

  ---

• The program (external) scope
  • Program scope:

    When a definition/declaration is outside a function, the definition/declaration is visible in all functions that follows the definition/declaration

    ---

  • Example:

    C program file: void func1( ... ) { ... } extern square( double x ); void func2( ... ) { ... } void func3( ... ) { ... }

    Then:

    The declaration of square() is valid inside func2() and func3() The declaration of square() is not valid inside func1()

  ---

  ---

  ---

  ---

  ---

  ---

• Organizing the function declarations
  • Common practice in C: write a header file for you (library) functions !!!

    When you write a collection of useful functions, you will typically organize the functions in the manner: The function definitions (i.e., the actual function that contains the statements) are stored in one or more C program files You will also create a header file that contains the function declarations The header file must be be included by a C program that needs to use one or more of the functions

    ---

    ---

  • Example:

    File(s) containing the function definitions	Header file containing function declarations
    void print ( double x ) { printf("Number = %lf\n", x ); } double square ( double x ) { double r; // Local variable r = x * x; // Statement return ( r ); // Return }	extern void print( double x ); extern double square ( double x );

    ---

    ---

  • A C program that makes use of the functions will inlcude the header file:

    #include "header.h" // Include the function declarations int main( int argc, char *argv[] ) { double a, b; a = 4.0; b = square( a ); // Call square print( a ); // Call print print( b ); // Call print }

    This will provide all function declarations to the program !!!

    ---

  • Example Program: (Demo above code)                                                
    • Prog file func5a.c: click here
    • Prog file func5b.c: click here
    • Header file func5b.h: click here

    How to run the program:

    Right click on links and save in a scratch directory To compile:   gcc func5a.c func5b.c To compile:   a.out You will now get the correct result !!!

    Result:

    Number = 4.000000 Number = 16.000000 (correct !)