/in/foo.c:2:1: warning: data definition has no type or storage class
 length(char a[],int n);
 ^~~~~~
/in/foo.c:2:1: warning: type defaults to 'int' in declaration of 'length' [-Wimplicit-int]
/in/foo.c: In function 'main':
/in/foo.c:6:2: 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:7:2: 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: At top level:
/in/foo.c:15:1: warning: return type defaults to 'int' [-Wimplicit-int]
 length(char a[],int n)
 ^~~~~~
/in/foo.c: In function 'main':
/in/foo.c:8:2: warning: 'n' is used uninitialized in this function [-Wuninitialized]
  length(s1,n);
  ^~~~~~~~~~~~
/tmp/ccT6bjyx.o: In function `main':
foo.c:(.text.startup+0xb): warning: the `gets' function is dangerous and should not be used.