CITS2002 Systems Programming  
← prev
next → CITS2002 CITS2002 schedule  

Very common mistakes with parameter passing

Some common misunderstandings about how parameters work often result in incorrect code
(even a number of textbooks make these mistakes!):

  • The order of evaluation of parameters is not defined in C. For example, in the code: 

    
int square( int a )
{
    printf("calculating the square of %i\n", a);
    return a * a;
}

void sum( int x, int y )
{
    printf("sum = %i\n", x + y );
}

....

    ....
    sum( square(3), square(4) );

    are we hoping the output to be: 

        calculating the square of 3     // the output on PowerPC Macs
    calculating the square of 4
    sum = 25

or

    calculating the square of 4     // the output on Intel Macs
    calculating the square of 3
    sum = 25

    Do not assume that function parameters are evaluated left-to-right. The compiler will probably choose the order of evaluation which produces the most efficient code, and this will vary on different processor architectures.

    (A common mistake is to place auto-incrementing of variables in parameters.)

 

CITS2002 Systems Programming, Lecture 4, p7, 31st July 2024.