arg_val() returns NULL if no argument

With IBM® Informix® 4GL (I4GL), if the index passed to the function references an argument that does not exist, the arg_val() function returns a blank character. With Genero BDL, if the index passed to the function references an argument that does not exist, the arg_val() function returns NULL:

$ cat arg_val.4gl
MAIN
  DEFINE v VARCHAR(10)
  LET v = arg_val(1)
  IF v IS NULL THEN
     DISPLAY "IS NULL"
  ELSE
     DISPLAY "IS NOT NULL: [", v, "]"
  END IF
END MAIN

$ fglcomp arg_val.4gl && fglrun arg_val.42m
IS NULL

$ c4gl -o arg_val.bin arg_val.4gl && ./arg_val.bin
IS NOT NULL: [ ]

You must review arg_val() conditions using the != operator, and add NVL() to handle NULL arguments:

-- Default is "USER" mode when no argument is passed or when argument
-- is different from "ADMIN"
MAIN
  --IF arg_val(1)!="ADMIN" THEN
  IF NVL(arg_val(1),"?")!="ADMIN" THEN
     DISPLAY "User mode..."
  ELSE
     DISPLAY "Admin mode..."
  END IF
END MAIN