Pragma Export_Valued_Procedure (GNAT Reference Manual)

### 2.63 Pragma Export_Valued_Procedure ¶

Syntax:

    pragma Export_Valued_Procedure (
         [Internal        =>] LOCAL_NAME
      [, [External        =>] EXTERNAL_SYMBOL]
      [, [Parameter_Types =>] PARAMETER_TYPES]
      [, [Mechanism       =>] MECHANISM]);

    EXTERNAL_SYMBOL ::=
      IDENTIFIER
    | static_string_EXPRESSION
    | ""

    PARAMETER_TYPES ::=
      null
    | TYPE_DESIGNATOR {, TYPE_DESIGNATOR}

    TYPE_DESIGNATOR ::=
      subtype_NAME
    | subtype_Name ' Access

    MECHANISM ::=
      MECHANISM_NAME
    | (MECHANISM_ASSOCIATION {, MECHANISM_ASSOCIATION})

    MECHANISM_ASSOCIATION ::=
      [formal_parameter_NAME =>] MECHANISM_NAME

    MECHANISM_NAME ::= Value | Reference

This pragma is identical to `Export_Procedure` except that the first parameter of `LOCAL_NAME`, which must be present, must be of mode `out`, and externally the subprogram is treated as a function with this parameter as the result of the function. GNAT provides for this capability to allow the use of `out` and `in out` parameters in interfacing to external functions (which are not permitted in Ada functions). GNAT does not require a separate pragma `Export`, but if none is present, `Convention Ada` is assumed, which is almost certainly not what is wanted since the whole point of this pragma is to interface with foreign language functions, so it is usually appropriate to use this pragma in conjunction with a `Export` or `Convention` pragma that specifies the desired foreign convention.

Special treatment is given if the EXTERNAL is an explicit null string or a static string expressions that evaluates to the null string. In this case, no external name is generated. This form still allows the specification of parameter mechanisms.