udf question

[Dan Schwarzkopf]

given that my UDF executable is called 'ttestudf_linux' in the udf
function table, with 4 arguments, and
I write 'd ttestudf_linux('var1','var2','var3','var4')'
and get a message
Syntax Error: Invalid Operand
'ttestudf_linux' not a variable or function
 Error ocurred at column 1
DISPLAY error:  Invalid expression
  Expression = ttestudf_linux(100,annc1,100,annf161)

what am I doing wrong?