hello every one ,is there any function in allegro to set fill style or to set line style? (Just like the function in BGI)if not how to set style?

drawing_mode()
If that's what you mean...

firstly thanks for your advise,but not exactly I just want to draw a line with different styles,such as SOLID_LINE ,DOTTED_LINE,DASHED_LINE ,etc.Is there such function in allegro?

I'm affraid not. It wouldn't be too difficult to make though. You can use the do_line() function. It will call your custom callback for every pixel of the line that needs to be drawn and all you have to do is figure out whether to draw the pixel or not, depending on the position of the pixel and the line style. There are similar functions for circle and ellipse as well...