foo.cc: In function 'int main()':
foo.cc:8:9: warning: 'char* gets(char*)' is deprecated [-Wdeprecated-declarations]
  gets(s1);
         ^
In file included from /usr/include/stdio.h:862:0,
                 from /usr/include/c++/7/cstdio:42,
                 from /usr/include/c++/7/ext/string_conversions.h:43,
                 from /usr/include/c++/7/bits/basic_string.h:6361,
                 from /usr/include/c++/7/string:52,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:232:1: note: declared here
 gets (char *__str)
 ^~~~
foo.cc:9:9: warning: 'char* gets(char*)' is deprecated [-Wdeprecated-declarations]
  gets(s2);
         ^
In file included from /usr/include/stdio.h:862:0,
                 from /usr/include/c++/7/cstdio:42,
                 from /usr/include/c++/7/ext/string_conversions.h:43,
                 from /usr/include/c++/7/bits/basic_string.h:6361,
                 from /usr/include/c++/7/string:52,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:232:1: note: declared here
 gets (char *__str)
 ^~~~
foo.cc:10:13: error: 'strlen' was not declared in this scope
  int len1 = strlen(s1);
             ^~~~~~
foo.cc:10:13: note: suggested alternative: 'mbrlen'
  int len1 = strlen(s1);
             ^~~~~~
             mbrlen
foo.cc:8:6: warning: ignoring return value of 'char* gets(char*)', declared with attribute warn_unused_result [-Wunused-result]
  gets(s1);
  ~~~~^~~~
foo.cc:9:6: warning: ignoring return value of 'char* gets(char*)', declared with attribute warn_unused_result [-Wunused-result]
  gets(s2);
  ~~~~^~~~

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