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c++cscopestaticshadowing

Defining a variable and its static equivalent in the same function

I don't understand how the following code works:

#include "stdio.h"

int main(void) {
  int i = 3;
  while(i--) {
    static int i = 100;
    i--,
    printf("%d\n", i);
  }
  return 0;
}

The code compiled with either Clang or GCC prints the following output:

99
98
97

Can someone explain to me what happens here? It looks like two operations are achieved in a single instruction and more than once. Is it undefined behavior? I observe the same behavior in C++.

Solution

  • This is not undefined behavior.

    #include "stdio.h"

int main(void) {
  int i = 3; //first i
  while(i--) {
    static int i = 100; //second i
    i--,
    printf("%d\n", i);
  }
  return 0;
}

    In while loop body most local i (second i) is preferred. While checking condition in while loop it doesn't know what is there in body. So it has no problem choosing first i.