/in/foo.c: In function 'main':
/in/foo.c:6:5: 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:7:5: 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__;
              ^~~~
/in/foo.c: In function 'Concat':
/in/foo.c:15:8: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
     s1=strlen(a);
        ^~~~~~
/in/foo.c:15:8: warning: incompatible implicit declaration of built-in function 'strlen'
/in/foo.c:15:8: note: include '<string.h>' or provide a declaration of 'strlen'
/tmp/ccIyg6CG.o: In function `main':
foo.c:(.text.startup+0x15): warning: the `gets' function is dangerous and should not be used.