/in/foo.c: In function 'Concat':
/in/foo.c:21:13: error: stray '\' in program
  printf("%s"\n",s);
             ^
/in/foo.c:21:14: error: expected ')' before 'n'
  printf("%s"\n",s);
              ^
/in/foo.c:21:15: warning: missing terminating " character
  printf("%s"\n",s);
               ^
/in/foo.c:21:15: error: missing terminating " character
  printf("%s"\n",s);
               ^~~~~
/in/foo.c:21:11: warning: format '%s' expects a matching 'char *' argument [-Wformat=]
  printf("%s"\n",s);
           ^
/in/foo.c:22:1: error: expected ';' before '}' token
 }
 ^
/in/foo.c:15:7: warning: variable 's' set but not used [-Wunused-but-set-variable]
  char s[20000];
       ^
/in/foo.c: In function 'main':
/in/foo.c:25:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
  gets(s1);
  ^~~~
In file included from /in/foo.c:1:0:
/usr/include/stdio.h:640:14: note: declared here
 extern char *gets (char *__s) __wur __attribute_deprecated__;
              ^~~~
/in/foo.c:26:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
  gets(s2);
  ^~~~
In file included from /in/foo.c:1:0:
/usr/include/stdio.h:640:14: note: declared here
 extern char *gets (char *__s) __wur __attribute_deprecated__;
              ^~~~
/in/foo.c: In function 'Concat':
/in/foo.c:22:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^