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  protocols.</para>
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  protocols.</para>
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  <section>
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  <section>
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    <title>Kernel Services</title>
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    <title>Kernel Services</title>
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    <para>Every message consists of 4 numeric arguments (32-bit and 64-bit on
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    <para>Every message consists of four numeric arguments (32-bit and 64-bit on
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    the corresponding platforms), from which the first one is considered a
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    the corresponding platforms), from which the first one is considered a
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    method number on message receipt and a return value on answer receipt. The
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    method number on message receipt and a return value on answer receipt. The
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    received message contains identification of the incoming connection, so
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    received message contains identification of the incoming connection, so
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    that the receiving application can distinguish the messages between
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    that the receiving application can distinguish the messages between
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    different senders. Internally the message contains pointer to the
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    different senders. Internally the message contains pointer to the
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    <para>Every message must be eventually answered. The system keeps track of
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    <para>Every message must be eventually answered. The system keeps track of
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    all messages, so that it can answer them with appropriate error code
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    all messages, so that it can answer them with appropriate error code
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    should one of the connection parties fail unexpectedly. To limit buffering
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    should one of the connection parties fail unexpectedly. To limit buffering
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    of the messages in the kernel, every task has a limit on the amount of
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    of the messages in the kernel, every task has a limit on the amount of
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    asynchronous messages it can send simultaneously. If the limit is reached,
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    asynchronous messages it can send simultaneously. If the limit is reached,
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    the kernel refuses to send any other message, until some active message is
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    the kernel refuses to send any other message until some active message is
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    answered.</para>
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    answered.</para>
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    <para>To facilitate kernel-to-user communication, the IPC subsystem
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    <para>To facilitate kernel-to-user communication, the IPC subsystem
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    provides notification messages. The applications can subscribe to a
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    provides notification messages. The applications can subscribe to a
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    notification channel and receive messages directed to this channel. Such
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    notification channel and receive messages directed to this channel. Such
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        <listitem>
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        <listitem>
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          <para>creating a callback connection,</para>
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          <para>creating a callback connection,</para>
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        </listitem>
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        </listitem>
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        <listitem>
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        <listitem>
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          <para>sending an address space area,</para>
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          <para>sending an address space area and</para>
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        </listitem>
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        </listitem>
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        <listitem>
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        <listitem>
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          <para>asking for an address space area.</para>
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          <para>asking for an address space area.</para>
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        </listitem>
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        </listitem>