Get warning when a variable is shadowed

Question

I generally want to avoid code like this:

#include <stdio.h>

int main(int argc, char *argv[]){

  int n = 3;

  for (int n = 1; n <= 10; n++){
    printf("%d\n", n);
  }

  printf("%d\n", n);
}

How can I find such usage of variables? That means, that in the same function a "more local" variable has the same name as a more global variable?

C-Standard : C 99

Solution

Both gcc and clang support the -Wshadow flag which will warn about variables that shadow one another. For example the warning I receive from gcc for your code is the following:

warning: declaration of ‘n’ shadows a previous local [-Wshadow]
for (int n = 1; n <= 10; n++){
         ^
warning: shadowed declaration is here [-Wshadow]
int n = 3;
    ^

gcc documents the flag here and says:

Warn whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C) or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum.

In Visual Studio this looks like it was not possible before but seems to be fixed in recent versions.