Ada Reference Manual (Ada 2022) — Legal Information

# E.4 Remote Subprogram Calls

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A remote subprogram call is a subprogram call that invokes the execution of a subprogram in another (active) partition. The partition that originates the remote subprogram call is the calling partition, and the partition that executes the corresponding subprogram body is the called partition. Some remote procedure calls are allowed to return prior to the completion of subprogram execution. These are called asynchronous remote procedure calls.
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There are three different ways of performing a remote subprogram call:
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As a direct call on a (remote) subprogram explicitly declared in a remote call interface;
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As an indirect call through a value of a remote access-to-subprogram type;
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As a dispatching call with a controlling operand designated by a value of a remote access-to-class-wide type.
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The first way of calling corresponds to a static binding between the calling and the called partition. The latter two ways correspond to a dynamic binding between the calling and the called partition.
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Remote types library units (see E.2.2) and remote call interface library units (see E.2.3) define the remote subprograms or remote access types used for remote subprogram calls.

#### Legality Rules

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In a dispatching call with two or more controlling operands, if one controlling operand is designated by a value of a remote access-to-class-wide type, then all shall be.
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A nonblocking program unit shall not contain, other than within nested units with Nonblocking specified as statically False, a dispatching call with a controlling operand designated by a value of a remote access-to-class-wide type.

#### Dynamic Semantics

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For the execution of a remote subprogram call, subprogram parameters (and later the results, if any) are passed using a stream-oriented representation (see 13.13.1) which is suitable for transmission between partitions. This action is called marshalling. Unmarshalling is the reverse action of reconstructing the parameters or results from the stream-oriented representation. Marshalling is performed initially as part of the remote subprogram call in the calling partition; unmarshalling is done in the called partition. After the remote subprogram completes, marshalling is performed in the called partition, and finally unmarshalling is done in the calling partition.
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A calling stub is the sequence of code that replaces the subprogram body of a remotely called subprogram in the calling partition. A receiving stub is the sequence of code (the “wrapper”) that receives a remote subprogram call on the called partition and invokes the appropriate subprogram body.
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Remote subprogram calls are executed at most once, that is, if the subprogram call returns normally, then the called subprogram's body was executed exactly once.
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The task executing a remote subprogram call blocks until the subprogram in the called partition returns, unless the call is asynchronous. For an asynchronous remote procedure call, the calling task can become ready before the procedure in the called partition returns.
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If a construct containing a remote call is aborted, the remote subprogram call is cancelled. Whether the execution of the remote subprogram is immediately aborted as a result of the cancellation is implementation defined.
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If a remote subprogram call is received by a called partition before the partition has completed its elaboration, the call is kept pending until the called partition completes its elaboration (unless the call is cancelled by the calling partition prior to that).
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If an exception is propagated by a remotely called subprogram, and the call is not an asynchronous call, the corresponding exception is reraised at the point of the remote subprogram call. For an asynchronous call, if the remote procedure call returns prior to the completion of the remotely called subprogram, any exception is lost.
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The exception Communication_Error (see E.5) is raised if a remote call cannot be completed due to difficulties in communicating with the called partition.
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All forms of remote subprogram calls are potentially blocking operations (see 9.5).
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In a remote subprogram call with a formal parameter of a class-wide type, a check is made that the tag of the actual parameter identifies a tagged type declared in a declared-pure or shared passive library unit, or in the visible part of a remote types or remote call interface library unit. Program_Error is raised if this check fails. In a remote function call which returns a class-wide type, the same check is made on the function result.
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In a dispatching call with two or more controlling operands that are designated by values of a remote access-to-class-wide type, a check is made (in addition to the normal Tag_Check — see 11.5) that all the remote access-to-class-wide values originated from Access attribute_references that were evaluated by tasks of the same active partition. Constraint_Error is raised if this check fails.

#### Implementation Requirements

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The implementation of remote subprogram calls shall conform to the PCS interface as defined by the specification of the language-defined package System.RPC (see E.5). The calling stub shall use the Do_RPC procedure unless the remote procedure call is asynchronous in which case Do_APC shall be used. On the receiving side, the corresponding receiving stub shall be invoked by the RPC-receiver.
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With respect to shared variables in shared passive library units, the execution of the corresponding subprogram body of a synchronous remote procedure call is considered to be part of the execution of the calling task. The execution of the corresponding subprogram body of an asynchronous remote procedure call proceeds in parallel with the calling task and does not signal the next action of the calling task (see 9.10).
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NOTE 1 A given active partition can both make and receive remote subprogram calls. Thus, an active partition can act as both a client and a server.
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NOTE 2 If a given exception is propagated by a remote subprogram call, but the exception does not exist in the calling partition, the exception can be handled by an others choice or be propagated to and handled by a third partition.

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