View Compilation Error

0_0_27392828_28931.c: In function 'is_prime':
0_0_27392828_28931.c:6:2: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
  for(int i=2;i<=sqrt(a);i++)
  ^
0_0_27392828_28931.c:6:2: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code