foo.c: In function 'main':
foo.c:6:5: error: 'for' loop initial declarations are only allowed in C99 mode
     for (int i = 0; i < n; i++)
     ^
foo.c:6:5: note: use option -std=c99 or -std=gnu99 to compile your code
foo.c:8:14: error: redefinition of 'i'
     for (int i = 0; i < n - 2; i++) {
              ^
foo.c:6:14: note: previous definition of 'i' was here
     for (int i = 0; i < n; i++)
              ^
foo.c:8:5: error: 'for' loop initial declarations are only allowed in C99 mode
     for (int i = 0; i < n - 2; i++) {
     ^
foo.c:9:9: error: 'for' loop initial declarations are only allowed in C99 mode
         for (int j = i + 2; j < n; j++) {
         ^
foo.c:11:17: error: 'for' loop initial declarations are only allowed in C99 mode
                 for (int k = i + 1; k < j; k++)
                 ^
foo.c:18:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^