/in/foo.c: In function 'main':
/in/foo.c:25:7: error: 's' undeclared (first use in this function)
    if(s[i] != s[len-i-1]) a = 0;
       ^
/in/foo.c:25:7: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:17:9: warning: variable 'a' set but not used [-Wunused-but-set-variable]
     int a = 1, b = 1, len = 0;
         ^