/in/foo.c: In function 'main':
/in/foo.c:6:2: warning: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
  scanf("%d",&n);
  ^~~~~
/in/foo.c:6:2: warning: incompatible implicit declaration of built-in function 'scanf'
/in/foo.c:6:2: note: include '<stdio.h>' or provide a declaration of 'scanf'
/in/foo.c:11:3: warning: implicit declaration of function 'strcat' [-Wimplicit-function-declaration]
   strcat(b,a);
   ^~~~~~
/in/foo.c:11:3: warning: incompatible implicit declaration of built-in function 'strcat'
/in/foo.c:11:3: note: include '<string.h>' or provide a declaration of 'strcat'
/in/foo.c:14:2: warning: implicit declaration of function 'puts' [-Wimplicit-function-declaration]
  puts(a);
  ^~~~