/in/foo.c: In function 'main':
/in/foo.c:9:10: warning: format '%s' expects argument of type 'char *', but argument 2 has type 'char (*)[100000]' [-Wformat=]
  scanf("%s",&a);
         ~^  ~~
/in/foo.c:10:10: warning: format '%s' expects argument of type 'char *', but argument 2 has type 'char (*)[100000]' [-Wformat=]
  scanf("%s",&b);
         ~^  ~~
/in/foo.c:13:3: warning: implicit declaration of function 'strcat' [-Wimplicit-function-declaration]
   strcat(b,a);
   ^~~~~~
/in/foo.c:13:3: warning: incompatible implicit declaration of built-in function 'strcat'
/in/foo.c:13:3: note: include '<string.h>' or provide a declaration of 'strcat'