/in/foo.c: In function 'main':
/in/foo.c:9:5: warning: implicit declaration of function 'fan' [-Wimplicit-function-declaration]
  sa=fan(fw,zi);
     ^~~
/in/foo.c: In function 'fan':
/in/foo.c:20:3: error: 'yu' undeclared (first use in this function)
   yu=shu1%shu2;
   ^~
/in/foo.c:20:3: note: each undeclared identifier is reported only once for each function it appears in