View Compilation Error

0_0_20703791_12860.c:3:1: error: unknown type name 'using'
 using namespace std;
 ^
0_0_20703791_12860.c:3:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
 using namespace std;
                 ^
0_0_20703791_12860.c:5:6: error: variably modified 'str' at file scope
 char str[maxn];
      ^
0_0_20703791_12860.c: In function 'main':
0_0_20703791_12860.c:15:3: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
   for (int i = 1;i <= len && i <= len/4;i++)
   ^
0_0_20703791_12860.c:15:3: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code
0_0_20703791_12860.c:17:4: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
    for (int j = 2;i*j*j <= len;j++)
    ^