View Compilation Error

0_0_25159906_10392.c: In function 'isprimer':
0_0_25159906_10392.c:9:2: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
  for(int i=2; i<=sqrt(x); i++){
  ^
0_0_25159906_10392.c:9:2: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code