单片机 red clock.c(118): error C267: 'write_com': requires ANSI-style prototype 求解

部分代码如下
void write_week(uchar hang,uchar add,uchar week)
{
if(hang==1)
write_com(0x80+add);
else
write_com(0x80+0x40+add);
switch(week)
{
case 1:write_data('M');
write_data('O');
write_data('N');
break;

    case 2:write_data('T');
           write_data('U');
           write_data('E');
           break;
    
    case 3:write_data('W');
           write_data('E');
           write_data('D');
           break;
    
    case 4:write_data('T');
           write_data('H');
           write_data('U');
           break;
    
    case 5:write_data('F');
           write_data('R');
           write_data('I');
           break;
    
    case 6:write_data('S');
           write_data('T');
           write_data('A');
           break;
    
    case 7:write_data('S');
           write_data('U');
           write_data('N');
           break;
}

}

write_com这函数调用之前没有声明