/in/foo.c: In function 'main':
/in/foo.c:4:2: warning: implicit declaration of function 'gets' [-Wimplicit-function-declaration]
  gets(num);
  ^~~~
/in/foo.c:7:12: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
  int len = strlen(ch);
            ^~~~~~
/in/foo.c:7:12: warning: incompatible implicit declaration of built-in function 'strlen'
/in/foo.c:7:12: note: include '<string.h>' or provide a declaration of 'strlen'
/in/foo.c:16:2: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
  printf("%s", ch);
  ^~~~~~
/in/foo.c:16:2: warning: incompatible implicit declaration of built-in function 'printf'
/in/foo.c:16:2: note: include '<stdio.h>' or provide a declaration of 'printf'
/tmp/ccMzZFxN.o: In function `main':
foo.c:(.text.startup+0x13): warning: the `gets' function is dangerous and should not be used.