/in/foo.c: In function 'Concat':
/in/foo.c:7:9: warning: unused variable 'len2' [-Wunused-variable]
     int len2 = strlen(s2);  // 去掉第二个字符串的第一个字符
         ^~~~
/in/foo.c: In function 'main':
/in/foo.c:17:5: 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:18:5: 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__;
              ^~~~
/tmp/ccYmiMlT.o: In function `main':
foo.c:(.text.startup+0x15): warning: the `gets' function is dangerous and should not be used.