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Function prototype and declaring a function

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• Translation a function call
  • Suppose the C compiler need to translate the following program:

    int main() { int x, y; x = 4; y = SomeFunction(x); }

    Question:

    What information will the C compiler (or any compiler) need to translate the function call y = SomeFunction)x) correctly ???

    Answer:

    The C compiler needs to know whether it need to convert the int (x) represention to some other representation The C compiler also needs to know whether it need to convert the return value from some other representation to int (y) represention

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• Neccessary information on a function to perform data type matching in a function call.
  • In order for the C compiler to translate a function call correctly, it needs the following information on a function:

    The data type of each parameter variable used by the function The data type of the return value of the function

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• The function prototype information

  • 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 prototype information is contained in the function header !!!

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• Function declaration: specifying only the function prototype
  • Syntax to specify a function prototype in C:

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

    Example:

    extern int f( int ) ; char SomeFunc( double, int, float ) ; double sin( double x ) ;

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  • Note:

    The keyword extern is optional The parameter variable name is also optional

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  • Nomenclaure

    Function declaration = specifying the function protoctype of the function

    The function prototype provides information to the C compiler on whether it needs to perform conversions.

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• Solving the missing function prototype information in C
  • Recall the previous example where the function information was not available when the C compiler needs to translate a function call:

    int main( int argc, char *argv[] ) { double a, b; a = 4.0; b = square( a ); // <---- Function definition is not available // when C compiler translates this // function call.... } double square( double x ) { double r; // Define a local variable r = x * x; // Statement return ( r ); // Return statement }

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  • We can provide the function prototpe before the functionc all:

    double square( double ); // Function prototype (function declaration) int main( int argc, char *argv[] ) { double a, b; a = 4.0; b = square( a ); // <---- Function definition is NOW available !!! } 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 func1x-sol.c To run:        ./a.out

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• Postscript
  • Warning:

    The function declaration must contain the same types for the parameter variables and return value as the function definition

    Example: do not do the following

    int square( double ); // BAD declaration for the square( ) function !!! int main( int argc, char *argv[] ) { double a, b; a = 4.0; b = square( a ); // Make assumption: int square(int) printf("a = %lf, b (square) = %lf\n", a, b); } double square( double x ) { double r; // Define a local variable r = x * x; // Statement return ( r ); // Return statement }

    Result:

    cs255@aruba (6608)> gcc func1x-bad.c func1x-bad.c:18: error: conflicting types for 'square' func1x-bad.c:4: note: previous declaration of 'square' was here

    Demo:

    cd /home/cs255000/demo/c/set1 gcc func1x-bad.c