/in/foo.c: In function 'main':
/in/foo.c:11:4: warning: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
  n=strlen(str);//字符串长度
    ^~~~~~
/in/foo.c:11:4: warning: incompatible implicit declaration of built-in function 'strlen'
/in/foo.c:11:4: note: include '<string.h>' or provide a declaration of 'strlen'
/in/foo.c:5:10: warning: unused variable 'x' [-Wunused-variable]
  int i,n,x;
          ^