foo.c: In function ‘main’:
foo.c:4:1: error: implicit declaration of function ‘gets’ [-Wimplicit-function-declaration]
    4 | gets(num);
      | ^~~~
foo.c:7:11: error: implicit declaration of function ‘strlen’ [-Wimplicit-function-declaration]
    7 | int len = strlen(ch);
      |           ^~~~~~
foo.c:1:1: note: include ‘<string.h>’ or provide a declaration of ‘strlen’
  +++ |+#include <string.h>
    1 | int main()
foo.c:7:11: warning: incompatible implicit declaration of built-in function ‘strlen’ [-Wbuiltin-declaration-mismatch]
    7 | int len = strlen(ch);
      |           ^~~~~~
foo.c:7:11: note: include ‘<string.h>’ or provide a declaration of ‘strlen’
foo.c:16:1: error: implicit declaration of function ‘printf’ [-Wimplicit-function-declaration]
   16 | printf("%s", ch);
      | ^~~~~~
foo.c:1:1: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’
  +++ |+#include <stdio.h>
    1 | int main()
foo.c:16:1: warning: incompatible implicit declaration of built-in function ‘printf’ [-Wbuiltin-declaration-mismatch]
   16 | printf("%s", ch);
      | ^~~~~~
foo.c:16:1: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’