/in/foo.c: In function 'Concat':
/in/foo.c:6:2: warning: this 'while' clause does not guard... [-Wmisleading-indentation]
  while(str[i]!='\0')
  ^~~~~
/in/foo.c:8:3: note: ...this statement, but the latter is misleadingly indented as if it is guarded by the 'while'
   a[0]=str[i-2];
   ^
/in/foo.c: In function 'main':
/in/foo.c:15:2: 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:16:2: 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__;
              ^~~~
/in/foo.c: In function 'Concat':
/in/foo.c:11:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^
/in/foo.c:6:11: warning: 'i' is used uninitialized in this function [-Wuninitialized]
  while(str[i]!='\0')
           ^
/tmp/cc5TP5ab.o: In function `main':
foo.c:(.text.startup+0x15): warning: the `gets' function is dangerous and should not be used.