如何让用户创建一个功能？ [ 图书馆 ？ ]

Is there any free library to let a user easily build a C mathematical expression, which can be used like any other function? I mean c expression/function which could be as quick as 'inline' mathematical expression and could be used many times in program. I think I can be done in C somehow, but does anybody can tell if it could be real if it have to be a CUDA dveice function?Is there any free library to let a user easily