hi,every one, i have to use udf on my research, i read many information about udf,but i still don't know what's the problem in my case. when i do iterate the case, it will shows the error.i think it is the udf problem. i hope someone can help me to tell me where the problem is ! here is my udf!

#include <stdio.h> #include "udf.h" # define UDF_FILENAME "piston.dat" # define PI 3.141592654 FILE *fp; //6 DEFINE_CG_MOTION(pistona, dt, cg_vel, cg_omega, time, dtime) { Thread *t=DT_THREAD (dt); face_t f; real force, loc; real velo, pressure; // real v1, v2, v3; fp= fopen("piston.dat","w"); NV_S(cg_vel,=,0.0); NV_S(cg_omega,=,0.0); if (!Data_Valid_P ())

return; force=0.0;

begin_f_loop(f,t) {

real *AA;

AA = F_AREA_CACHE (f, t);

force += F_P (f, t) * AA[0]; } end_f_loop (f, t) if (rp_axi)

force *= 2.0 * M_PI; pressure = force / 0.0099*0.0099*M_PI;

# define rp 0.0067 # define l1 0.079 # define rpm 500. / # define w 2.*PI*rpm/60. # define q w*dtime

velo = rp*w*sin(PI/2.-q)+1./sqrt(2.*rp*cos(PI/2.-q)* 2.*rp*cos(PI/2.-q)-4.*(rp*rp-l1*l1))*(2.*rp*cos(PI/2.-q)) *rp*sin(PI/2.-q)*(w);

loc = 0.01945+0.02+rp*cos(PI/2.-q)+0.5*sqrt ((2.*rp*cos(PI/2.-q))*(2.*rp*cos(PI/2.-q))-4.*(rp*rp-l1*l1));

fprintf (fp, "%e %e %e %e\n", time, loc, velo, dtime);

cg_vel[0]=velo;

fclose (fp);

message ("\nUDF valve: time = %f, loc(m)= %f,

velo = %f, dtime = %f\n", time, loc

,velo,dtime);

}

what is exactly the error message? (while solving? while compiling?)