EQ2EMu/EQ2/source/depends/boost_1_72_0/libs/asio/doc/overview/sync_op.dot

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  // Program elements.
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    operating_system [ label="Operating System", shape=ellipse ];
    io_context [ label="io_context" ];
    io_object [ label="I/O Object\ne.g. socket" ];
    your_program [ label="Your Program" ];
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  // Owning relationships.
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    edge [ arrowtail="diamond" ];
    your_program:w -> io_object:nw;
    your_program:se -> io_context:ne;
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  // Non-owning relationships;
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    io_object:sw -> io_context:w;
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    // Forward actions.
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      your_program:sw -> io_object:n [ label="1" ];
      io_object:s -> io_context:nw [ label="2" ];
      io_context:sw -> operating_system:nw [ label="3" ];
    }

    // Reverse actions.
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      edge [ arrowhead="none", arrowtail="open" ];
      io_context:se -> operating_system:ne [ label="4" ];
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      your_program:s -> io_object:ne [ label="6" ];
    }
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