The C programming language (2nd) -- Notes -- 1.8

1.8. Parameters —— Value transfer call

         Get used to other languages （ especially Fortran） Programmers may be right C I feel strange about the way function parameters are passed in .C All function parameters of the language are “ Pass value ” Pass on . namely , The parameter values passed to the called function are stored in temporary instead of The original variable . This is different from some other languages , such as Fortran Such language is “ Call... By reference ”,Pascal Then Var Method of parameters , In these languages , The called function must access the original arguments , Instead of accessing a local copy of the parameter .

        The main difference is C In language , The called function cannot directly modify the value of the variable in the calling function , You can only modify the value of its private temporary copy .

        The advantages of calling by value outweigh the disadvantages . In the called function , Parameters can be considered as Easy to initialize local variables , So there are fewer extra variables to use . such , The program can be more compact and concise . as follows power Function takes advantage of this property ：

/* Power: Raise base to n-th power; n >= 0; Version 2*/
int power (int base, int n)
{
    int p;
    for (p = 1; n > 0; --n)
        p = p * base ;
    return p;
}

among , Parameters n Used as a temporary variable , And through the following for Loop statement decrement , Until its value is 0, There is no need to introduce extra variables i;power Function internal pair n Any operation of the does not affect the calling function n The original parameter value of .

        When necessary, , You can also let the function modify the variables in the main function . This requires the caller to provide Address of the variable whose value is to be set （ From a technical point of view , An address is a pointer to a variable ）, The called function needs Declare the corresponding parameter as a pointer type , And indirectly access variables through it . See the first 5 Chapter .

        If it's an array parameter , Things are different . When put Array name as parameter when , The value passed to the function is the location or address of the starting element of the array —— It does not copy the array elements themselves . In the called function , It can be done by The array subscript Access or modify the values of array elements . See the next section .