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Function prototyping (function declaration)

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• The need for conversion in function invocation (and return value)
  • Consider the following function call:

    float f(float a) { return a*a; } int main() { int x, y; x = 4; y = f(x); }

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  • Remember that:

    An int typed variable uses the two-complement encoding method to represent a value An float typed variable uses the IEEE 754 encoding method to represent a value

    Example:

    The int value 4 is represented as: 00000000 00000000 00000000 00000100 The float value 4.0 is represented as: 01000000 10000000 00000000 00000000 Because: 4.010 = 1002 = 1.0 × 22 The mantissa = 1.00000000000000000000000000 (leading 1 is omitted !) The exponent = 2, encoded in excess 01111111 ==> 10000001 Therefore: 4.0 is represented as: 01000000 10000000 00000000 00000000 ^^^^^^^^^------------------------- Exponent Mantissa

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  • How the function call f(x) is processed:

    x = 4; // x = 00000000 00000000 00000000 00000100 // because x is an int, use 2's complement encoding ! y = f(x); // 1. convert the 2's complement encoding: // // 00000000 00000000 00000000 00000100 // // into IEEE 754 encoding for the same value: // // 01000000 10000000 00000000 00000000 // // 2. pass the converted value as parameter // to the function f // // 3. Execute f // // 4. Receive the return value from f // // *** The return value is in the IEEE 754 encoding !!! // // 5. Before storing the return value in y, // we must: // // convert the IEEE 743 encoding into // 2's complement encoding !!!

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• The function prototype information

  • Fact:

    In order to pass the parameters in the correct encoding, the C compiler must know: The data type of each parameter variable In order to store the return value in the correct encoding (into soem receiving variable), the C compiler must know: The return data type of function

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  • Function prototype:

    Function prototype = the information of the data types of the parameter variables and the return value of a function/method

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• How programming languages provide the function prototype information
  • Recall from CS170: method header

    The method header is the first part of the method definition without the method body:

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  • The function prottype information is contained in the function header !!!

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• Missing function prototype information
  • When the function is called, the compiler will:

    Look up the function prototype information of the function

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  • When the function prototype information is missing, the compiler can do one of the following:

    Stop and report an error This is the choice made in the Java compiler Make an assumption on the data type of the parameters and the return value This is the choice made in the C compiler !!!!!!!!

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• Implicit assumptions made by the C compiler
  • Fact:

    When the C compiler encounter the use of a function for the first time, it will: Check whether the function has been defined or declared If the function has not been define or declared, the C compiler will assume: Every parameter variable has the type int the Return (data) type of the function is also int

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• Consequence of this practice....

  • Example 1:

    void print ( double x ) { printf("Number = %lf\n", x ); } double square( double x ) { double r; // Define a local variable r = x * x; // Statement return ( r ); // Return statement } int main( int argc, char *argv[] ) { double a, b; a = 4.0; b = square( a ); // <---- First use // square() was defined above print( a ); // <----- First use // print() has been defined above print( b ); // Call function print }

    Observation:

    The functions square() and print() have already been defined before they were used

    Therefore, the C compiler knows the data types of the parameters (and return value) and:

    The C compiler will not make the implcit assumptions on the data type of the parameters (and return value)

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  • Example 2:

    int main( int argc, char *argv[] ) { double a, b; a = 4.0; b = square( a ); // <---- First use // square() has not been defined yet !!! print( a ); // <----- First use // print() has not been defined yet !!! print( b ); // Call function print } void print ( double x ) { printf("Number = %lf\n", x ); } double square( double x ) { double r; // Define a local variable r = x * x; // Statement return ( r ); // Return statement }

    Observation:

    The functions square() and print() have not been defined before they were used (Their definitions appears after they were used !!!)

    Therefore, the C compiler does not know the data types of the parameters (and return value) and:

    The C compiler will make the implicit assumptions that the data type of the parameters (and return value) are integers !!!

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  • Consequence:

    When you compile the above program, you will get the following compiler error: func1x.c:16: warning: conflicting types for `print' func1x.c:12: note: previous implicit declaration of `print' was here func1x.c:21: error: conflicting types for `square' func1x.c:10: note: previous implicit declaration of `square' was here

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   gcc func1x.c To run:          java xxx

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• Defining and declaring a function
  • The definition of a function contains:

    The function header       and       The function body

    Example:

    double square( double x ) // Function header { // Function body double r; r = x * x; return ( r ); }

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  • The declaration of a function contains:

    The (optional) keyword extern The function header

    Example:

    extern double square( double x ) ; // Function declaration or: double square( double x ) ; // Function declaration

  • Note:

    The function declaration ends in a semi-colon (;) !!!

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• Declaring a function in C
  • The prevent the C compiler from making the implicit assumption about the parameters/return value of a function, you must:

    Define or Declare the function before the function it is used

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  • Example:

    void print ( double x ); // function declaration double square( double x ); // function declaration int main( int argc, char *argv[] ) { double a, b; a = 4.0; b = square( a ); // <---- First use // square() has been declared above !!! print( a ); // <----- First use // print() has been declared above !!! print( b ); } void print ( double x ) { printf("Number = %lf\n", x ); } double square( double x ) { double r; // Define a local variable r = x * x; // Statement return ( r ); // Return statement }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   gcc func1y.c To run:          java xxx

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• Postscript
  • What the C compiler will do when it detects the wrong implicit type assumption dependents on the implementation

    The recommended action would be: abort the compilation process (That is the case in gcc)

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  • There is an even more severe situation of wrong assumption made by the C compiler

    When the functions are located in separate C program files

    (The C compiler will have no way to knowing that the wrong type assumption had been made)

    We will discuss this topic next.....