Passing an array as an argument to a function in C

Question

I wrote a function containing array as argument, and call it by passing value of array as follows.

void arraytest(int a[]) {     // changed the array a     a[0] = a[0] + a[1];     a[1] = a[0] - a[1];     a[0] = a[0] - a[1]; }  void main() {     int arr[] = {1, 2};     printf("%d \t %d", arr[0], arr[1]);     arraytest(arr);     printf("\n After calling fun arr contains: %d\t %d", arr[0], arr[1]); } 

What I found is though I am calling arraytest() function by passing values, the original copy of int arr[] is changed.

Can you please explain why?

Answer 1

When passing an array as a parameter, this

void arraytest(int a[]) 

means exactly the same as

void arraytest(int *a) 

so you are modifying the values in main.

For historical reasons, arrays are not first class citizens and cannot be passed by value.