problem with sizeof

hi
i wrote this code which is written bellow when ever i check the value of u it appear to be the 3. while it is supposed to be 8.
Can any body tell me what is the problem with this code and how can i get the exact size of the arrgument of func.

void main(){
  func("fara.txt");
}
void func(unsigned char arr[]){
int u;
u=sizeof(arr);
}