foo.c:2:1: warning: return type defaults to 'int' [-Wimplicit-int]
    2 | TongJi (char s[])
      | ^~~~~~
foo.c:8:1: warning: return type defaults to 'int' [-Wimplicit-int]
    8 | Concat(char s1[],char s2[])
      | ^~~~~~
foo.c: In function 'main':
foo.c:22:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
   22 |  gets(s1);
      |  ^~~~
In file included from /usr/include/stdio.h:867,
                 from foo.c:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:235:1: note: declared here
  235 | gets (char *__str)
      | ^~~~
foo.c:23:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
   23 |  gets(s2);
      |  ^~~~
In file included from /usr/include/stdio.h:867,
                 from foo.c:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:235:1: note: declared here
  235 | gets (char *__str)
      | ^~~~
foo.c: In function 'Concat':
foo.c:18:1: warning: control reaches end of non-void function [-Wreturn-type]
   18 | }
      | ^
foo.c: In function 'main':
foo.c:22:2: warning: ignoring return value of 'gets', declared with attribute warn_unused_result [-Wunused-result]
   22 |  gets(s1);
      |  ^~~~~~~~
foo.c:23:2: warning: ignoring return value of 'gets', declared with attribute warn_unused_result [-Wunused-result]
   23 |  gets(s2);
      |  ^~~~~~~~
/usr/bin/ld: /tmp/ccZbLf0c.o: in function `main':
foo.c:(.text.startup+0x4e): warning: the `gets' function is dangerous and should not be used.

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