Assembly Language Help (alang.hlp) (Table of Contents; Topic list)
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Generate Error for Missing Argument
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  Syntax:   .ERRB <textitem> [,message]
 
            .ERRNB <textitem> [,message]
 
  See also: IFB, IFNB, .ERR, .ERRE, .ERRDEF, .ERRDIF, MACRO, PROC
 
  Description:
 
     The .ERRB directive generates an A2057 error if <textitem> is
     blank. The .ERRNB directive generates an A2058 error if <textitem>
     is not blank.
 
     These directives are usually used within macro definitions; a
     macro parameter is replaced by an empty string if you call the
     macro and do not give a corresponding argument. These directives
     let you write macros that generate an assembly-time error if the
     wrong number of arguments is used in the macro call. However, the
     :REQ parameter attribute or the variable argument feature often
     provides the same functionality in a more straightforward way.
 
     Parameter      Description
 
     textitem       Item to be tested, any text item.
                    See: textitem
 
     message        Programmer-supplied error message, any text item.
                    See: textitem
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