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
     | Descriptor [([Class =>] CLASS_NAME)]
     | Short_Descriptor [([Class =>] CLASS_NAME)]
     
     CLASS_NAME ::= ubs | ubsb | uba | s | sb | a

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.

Passing by descriptor is supported only on the OpenVMS ports of GNAT. The default behavior for Export_Valued_Procedure is to accept either 64bit or 32bit descriptors unless short_descriptor is specified, then only 32bit descriptors are accepted.

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.