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The Parameter passing mechanism of C

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• The most common parameter passing mechanisms
  • Recall that the 2 most commonly used forms of parameter passing mechanisms are:

    Pass-by-value The value of the actual parameter variable is passed (= copied) into the formal parameter variable Pass-by-reference The location (reference or address) of the actual parameter variable is passed (= copied) into the formal parameter variable

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  • If you need a refresher on the parameter passing mechanism, here are my CS170 material on these topics:

    CS170 notes on pass-by-value: click here CS170 notes on pass-by-reference: click here

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• The parameter passing mechanism in C
  • Parameter passing mechanism in C:

    The C programming language only has the pass-by-value parameter passing mechanism !!! (I.e., just like Java).

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  • However:

    The C language contains low level operators (reference operator & and de-reference operator * discussed later) that: Enable the C programmer to pass a parameter by reference !!!

    The low level operators (reference operator & and de-reference operator *) will be discussed later !!!

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• Experiment that shows that C uses the pass-by-value parameter mechanism
  • Example:

    void update( double a ) // a is passed by value { printf( "Inside update - before changing a, a = %lf\n", a); a = a + 1; printf( "Inside update - AFTER changing a, a = %lf\n", a); } int main( int argc, char* argv[] ) { double a = 3.14; printf( "Inside main - before calling update(a), a = %lf\n", a); update ( a ); printf( "Inside main - AFTER calling update(a), a = %lf\n", a); }

    Result:

    Inside main - before calling update(a), a = 3.140000 Inside update - before changing a, a = 3.140000 Inside update - AFTER changing a, a = 4.140000 Inside main - AFTER calling update(a), a = 3.140000 Unchanged !!!

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

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile:   gcc pass-by-value1.c To run:          ./a.out

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    Explanation:

    This is the situation when the program starts in the main() function: When execution reaches the variable definition, the local variable a will be created on the stack: When execution reaches the function invocation , a parameter variable (a) is created on the stack and the value of the actual parameter is copied (= passed) to the parameter variable: The execution continues inside the update() function.... When execution reaches the a = a + 1 statement, the parameter variable (a) is updated: After the function update() returns, the parameter variable (a) is destroyed: Hence: You can clearly see that statements after the update(a) call will use a = 3.14 (unchanged).