foo.c: In function 'main':
foo.c:24:13: error: 'for' loop initial declarations are only allowed in C99 mode
             for (int k=j+1; k<=n; k++) {
             ^
foo.c:24:13: note: use option -std=c99 or -std=gnu99 to compile your code
foo.c:33:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^