/in/foo.c: In function 'concat':
/in/foo.c:7:5: error: 'i' undeclared (first use in this function)
   a[i+lena]=0;
     ^
/in/foo.c:7:5: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c: In function 'main':
/in/foo.c:14:4: warning: 'gets' is deprecated [-Wdeprecated-declarations]
    gets(a);
    ^~~~
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:15:4: warning: 'gets' is deprecated [-Wdeprecated-declarations]
    gets(b);
    ^~~~
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__;
              ^~~~