/in/foo.c: In function 'main':
/in/foo.c:7:11: warning: format '%s' expects argument of type 'char *', but argument 2 has type 'char (*)[20]' [-Wformat=]
   scanf("%s", &a);
           ^
/in/foo.c:8:23: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
   for (int i = 0; i < strlen(a); ++i)
                       ^~~~~~
/in/foo.c:8:23: warning: incompatible implicit declaration of built-in function 'strlen'
/in/foo.c:8:23: note: include '<string.h>' or provide a declaration of 'strlen'