/in/foo.c: In function 'main':
/in/foo.c:7:2: 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:8:2: 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:9:2: warning: implicit declaration of function 'concat' [-Wimplicit-function-declaration]
  concat(s1,s2);
  ^~~~~~
/in/foo.c: At top level:
/in/foo.c:12:6: warning: conflicting types for 'concat'
 void concat(char s1[10000],char s2[10000])
      ^~~~~~
/in/foo.c:9:2: note: previous implicit declaration of 'concat' was here
  concat(s1,s2);
  ^~~~~~
/in/foo.c: In function 'concat':
/in/foo.c:14:6: warning: unused variable 'i' [-Wunused-variable]
  int i,j;
      ^
/tmp/cci4bSdD.o: In function `main':
foo.c:(.text.startup+0x15): warning: the `gets' function is dangerous and should not be used.