/in/foo.c:2:1: warning: return type defaults to 'int' [-Wimplicit-int]
 main()
 ^~~~
/in/foo.c: In function 'main':
/in/foo.c:5:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
  gets(s);
  ^~~~
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:6:8: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
  for(i=strlen(s)-1;i>=0;i--)printf("%c",s[i]);
        ^~~~~~
/in/foo.c:6:8: warning: incompatible implicit declaration of built-in function 'strlen'
/in/foo.c:6:8: note: include '<string.h>' or provide a declaration of 'strlen'
/in/foo.c:7:2: warning: implicit declaration of function 'system' [-Wimplicit-function-declaration]
  system("PAUSE");
  ^~~~~~
/tmp/ccuH8f51.o: In function `main':
foo.c:(.text.startup+0x10): warning: the `gets' function is dangerous and should not be used.