foo.c: In function 'main':
foo.c:9:15: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
    9 |         len = strlen(s);
      |               ^~~~~~
foo.c:2:1: note: include '<string.h>' or provide a declaration of 'strlen'
    1 | #include <stdio.h>
  +++ |+#include <string.h>
    2 | typedef long long ll;
foo.c:9:15: warning: incompatible implicit declaration of built-in function 'strlen' [-Wbuiltin-declaration-mismatch]
    9 |         len = strlen(s);
      |               ^~~~~~
foo.c:9:15: note: include '<string.h>' or provide a declaration of 'strlen'

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