foo.cc: In function 'void revstr(const char*, char*)':
foo.cc:6:9: error: 'strlen' was not declared in this scope
 int len=strlen(s),i;
         ^~~~~~
foo.cc:6:9: note: suggested alternative: 'mbrlen'
 int len=strlen(s),i;
         ^~~~~~
         mbrlen
foo.cc:7:6: error: 'i' was not declared in this scope
  for(i=0;i<len;++i)
      ^
foo.cc:9:6: error: 'i' was not declared in this scope
  res[i]=0;
      ^
foo.cc: In function 'int main()':
foo.cc:25:8: error: 'strcmp' was not declared in this scope
    if(!strcmp(str,res))
        ^~~~~~
foo.cc:25:8: note: suggested alternative: 'str'
    if(!strcmp(str,res))
        ^~~~~~
        str

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