Redeclaration of global variable vs local variable

Question

When I compile the code below

#include

int main()
{
  int a;
  int a = 10;
  printf(\"a is %d \\n\",a);
  return 0;
}

I get

Answer 1

You can't have two global variables with the same name in C program. C might allow multiple definitions in the same file scope through the tentative definition rule, but in any case all definitions will refer to the same variable.

Local Variable

In C, multiple local variables are not "merged" into one.

All the local variables with the same name will be referring to the different int-sized piece of memory.

 #include

 int main()
 {
  int a;
  int a = 10;
  printf("a is %d \n",a);  
  return 0;
 }

So when assigning the memory to the redeclaration of the same variable it gives an Error.

Global Variable

In C, multiple global variables are "merged" into one. So you have indeed just one global variable, declared multiple times. This goes back to a time when extern wasn't needed (or possibly didn't exist - not quite sure) in C.

In other words, all global variables with the same name will be converted to be one variable - so your

#include

int a;
int a = 10;
int main()
{
printf("a is %d \n",a);
return 0;
}

will be referring to the same int-sized piece of memory.