foo.c: In function 'main':
foo.c:8:10: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
  int n = strlen(s);
          ^~~~~~
foo.c:8:10: warning: incompatible implicit declaration of built-in function 'strlen'
foo.c:8:10: note: include '<string.h>' or provide a declaration of 'strlen'

自豪的采用jd5进行评测(github.com/masnn/jd5)