foo.c: In function 'main':
foo.c:6:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
  gets(s1);
  ^~~~
In file included from /usr/include/stdio.h:862:0,
                 from foo.c:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:232:1: note: declared here
 gets (char *__str)
 ^~~~
foo.c:7:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
  gets(s2);
  ^~~~
In file included from /usr/include/stdio.h:862:0,
                 from foo.c:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:232:1: note: declared here
 gets (char *__str)
 ^~~~
foo.c: In function 'Concat':
foo.c:15:12: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
  for(i=0;i<strlen(s1)-1;i++,j++)
            ^~~~~~
foo.c:15:12: warning: incompatible implicit declaration of built-in function 'strlen'
foo.c:15:12: note: include '<string.h>' or provide a declaration of 'strlen'
foo.c:17:12: warning: incompatible implicit declaration of built-in function 'strlen'
  for(i=1;i<strlen(s2);i++,j++)
            ^~~~~~
foo.c:17:12: note: include '<string.h>' or provide a declaration of 'strlen'
foo.c: In function 'main':
foo.c:6:2: warning: ignoring return value of 'gets', declared with attribute warn_unused_result [-Wunused-result]
  gets(s1);
  ^~~~~~~~
foo.c:7:2: warning: ignoring return value of 'gets', declared with attribute warn_unused_result [-Wunused-result]
  gets(s2);
  ^~~~~~~~
foo.c: In function 'Concat':
foo.c:22:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^
/tmp/ccM8TQLk.o: In function `main':
foo.c:(.text.startup+0x30): warning: the `gets' function is dangerous and should not be used.