L4Re - L4 Runtime Environment
Interface Definition Language

An interface definition in L4Re is normally declared as a class derived from L4::Kobject_t.

For example, the simple calculator example declares its interface like that:

struct Calc : L4::Kobject_t<Calc, L4::Kobject>
{
L4_INLINE_RPC(int, neg, (l4_uint32_t a, l4_uint32_t *res));
};

The signature of each function is first declared using on of the RPC macros (see below) and then all the functions need to be listed in the Rpcs type.

Parameter types for RPC

Generally all value parameters, const reference parameters, and const pointer parameters to an RPC interface are considered as input parameters for the RPC and are transmitted from the client to the server.

Note
This means that char const * is treated as an input char and not as a zero terminated string value, for strings see L4::Ipc::String<>.

Parameters that are non-const references or non-const pointers are treated as output parameters going from the server to the client.

L4_RPC(long, test, (int arg1, char const *arg2, unsigned *ret1));

The example shows the declaration of a method called test with long as return type, arg1 is an int input, arg2 a char input, and ret1 an unsigned output parameter.

Type Direction Client-Type Server-Type
T Input T T
T const & Input T const & T const &
T const * Input T const * T const &
T & Output T & T &
T * Output T * T &
L4::Ipc::In_out<T &> In/Out T & T &
L4::Ipc::In_out<T *> In/Out T * T &
L4::Ipc::Cap<T> Input L4::Ipc::Cap<T> L4::Ipc::Snd_fpage
L4::Ipc::Out<L4::Cap<T> > Output L4::Cap<T> L4::Ipc::Cap<T> &

Array types can be used to transmit arrays of variable length. They can either be stored in a client-provided buffer (L4::Ipc::Array), copied into a server-provided buffer (L4::Ipc::Array_in_buf) or directly read and written into the UTCB (L4::Ipc::Array_ref). For latter type, the start position of an array type needs to be known in advance. That implies that only one such array can be transmitted per direction and it must be the last part in the message.

Type Direction Client-Type Server-Type
L4::Ipc::Array<const T> Input L4::Ipc::Array<const T> L4::Ipc::Array_ref<const T>
L4::Ipc::Array<const T> Input L4::Ipc::Array<const T> L4::Ipc::Array_in_buf<T> const &
L4::Ipc::Array<T> & Output L4::Ipc::Array<T> & L4::Ipc::Array_ref<T> &
L4::Ipc::Array_ref<T> & Output L4::Ipc::Array_ref<T> & L4::Ipc::Array_ref<T> &

Finally, there are some optional types where the sender can choose if the parameter should be included in the message. These types are for the implementation of some legacy message formats and should generally not be needed for the definition of ordinary interfaces.

Type Direction Client-Type Server-Type
L4::Ipc::Opt<T> Input L4::Ipc::Opt<T> T
L4::Ipc::Opt<const T*> Input L4::Ipc::Opt<const T*> T
L4::Ipc::Opt<T &> Output T & L4::Ipc::Opt<T> &
L4::Ipc::Opt<T *> Output T * L4::Ipc::Opt<T> &
L4::Ipc::Opt<Array_ref<T> &> Output Array_ref<T> & L4::Ipc::Opt<Array_ref<T>> &

RPC Return Types

On the server side, the return type of an RPC handling function is always long. The return value is transmitted via the label field in l4_msgtag_t and is therefore restricted to its length. Per convention, a negative return value is interpreted as an error condition. If the return value is negative, output parameters are not transmitted back to the client.

Attention
The client must never rely on the content of output parameters when the return value is negative.

On the client-side, the return value of the RPC is set as defined in the RPC macro. If l4_msgtag_t is given, then the client has access to the full message tag, otherwise the return type should be long. Note that the client might not only receive the server return value in response but also an IPC error code.

RPC Method Declaration

RPC member functions can be declared using one of the following C++ macros.

For inline RPC stubs, where the RPC stub code itself is inline:

  • L4_INLINE_RPC(res, name, (args...), flags)
    Define an inline RPC call (type and callable).
  • L4_INLINE_RPC_OP(op, res, name, (args...), flags)
    Define an inline RPC call with specific opcode (type and callable).
  • L4_INLINE_RPC_NF(res, name, (args...), flags)
    Define an inline RPC call type (the type only, no callable).
  • L4_INLINE_RPC_NF_OP(opcode, Ret_type, func_name, (args...), flags)
    Define an inline RPC call type with specific opcode (the type only, no callable).

For external RPC stubs, where the RPC stub code must be defined in a separate compile unit (usually a .cc file):

  • L4_RPC(Ret_type, func_name, (args...), flags)
    Define an RPC call (type and callable).
  • L4_RPC_OP(opcode, Ret_type, func_name, (args...), flags)
    Define an RPC call with specific opcode (type and callable).
  • L4_RPC_NF(Ret_type, func_name, (args...), flags)
    Define an RPC call type (the type only, no callable).
  • L4_RPC_NF_OP(opcode, Ret_type, func_name, (args...), flags)
    Define an RPC call type with specific opcode (the type only, no callable).

To generate the implementation of an external RPC stub:

  • L4_RPC_DEF(class_name::func_name)
    Generate the definition of an RPC stub.

The NF versions of the macro generally do not generate a callable member function named <name> but do only generate the type <name>_t. This data type can be used to call the RPC stub explicitly using <name>_t::call(L4::Cap<Iface_class> cap, args...).