/in/foo.c: In function 'main':
/in/foo.c:22:1: 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:25:15: warning: variable 'p3' set but not used [-Wunused-but-set-variable]
 char *p1,*p2,*p3,*p4;
               ^~
/tmp/cc3XlKeh.o: In function `main':
foo.c:(.text.startup+0x34): warning: the `gets' function is dangerous and should not be used.