foo.c:10:5: warning: built-in function 'index' declared as non-function [enabled by default]
 int index,n;
     ^
foo.c: In function 'spfa':
foo.c:30:2: error: 'for' loop initial declarations are only allowed in C99 mode
  for (int i=f[u];i!=0;i=a[i].next)
  ^
foo.c:30:2: note: use option -std=c99 or -std=gnu99 to compile your code