Thank you Amir and Ahmed for your help; i solved the problem but now i'm experiencing some other problems, take a look, please:

thread_loop_c(t,domain)

{

begin_c_loop(c,t)

{

C_CENTROID(xc,c,t);

dist=sqrt(pow((xc[0]-bar[0]),2)+pow((xc[1]-bar[1]),2)+pow((xc[2]-bar[2]),2));

if ( dist < d )

{

d=dist;

C_CENTROID(xc1,c,t);

cid=c;

}

}

end_c_loop(c,t)

}

printf("cid = %d\n", cid);

c_node_loop(cid,t,n) /*line 182*/

{

ny=NODE_Y(n);

d2=fabs(ny-bar[1]);

if (d2>d1)

{

d1=d2;

}

}

This is only the final part of my udf (too much big to post it all) but is where i have the error:

"C:\Fluent.Inc tbin tx86\cpp.exe -IC:\Fluent.Inc\fluent6.2.16/src -IC:\Fluent.Inc\fluent6.2.16/cortex/src -IC:\Fluent.Inc\fluent6.2.16/client/src -IC:\Fluent.Inc\fluent6.2.16/multiport/src -I. -DUDFCONFIG_H="" C:\Fluent.Inc tbin tx86\profili_source.c Error: C:\Fluent.Inc\ntbin\ntx86\profili_source.c: line 182: non-integer subscript expression: unsigned char."

But if i cut the c_node_loop he recognize the value for cid and the UDF works correctly writing it on the screen... oops, i forgot to say that it is a "DEFINE_ON_DEMAND" macro (i don't know if it is important).

Thank you

there is an old post about this, maybe it helps. http://www.cfd-online.com/Forum/flue....cgi/read/1480

Thanks a lot