foo.c: In function 'main':
foo.c:5:1: warning: 'gets' is deprecated [-Wdeprecated-declarations]
 gets(s1);
 ^~~~
In file included from foo.c:1:0:
/usr/include/stdio.h:640:14: note: declared here
 extern char *gets (char *__s) __wur __attribute_deprecated__;
              ^~~~
foo.c:8:14: warning: comparison between pointer and integer
 for(i=0;s1[i]!=NULL;i++)
              ^~
/tmp/ccRQFczi.o: In function `main':
foo.c:(.text.startup+0x11): warning: the `gets' function is dangerous and should not be used.

自豪的采用jd5进行评测(github.com/masnn/jd5)