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<a href="00304.html" target="topic">27.4 导入的任务和函数</a>, <a href="00979.html" target="topic">主题</a></div>
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27.4.1 Required properties of imported tasks and functions — semantic constraints</div>
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This section defines the semantic constraints imposed on imported tasks or functions. Some semantic restrictions are shared by all imported tasks or functions. Other restrictions depend on whether the special properties pure (see Section 27.4.2) or context (see Section 27.4.3) are specified for an imported task or function. A SystemVerilog compiler is not able to verify that those restrictions are observed and if those restrictions are not satisfied, the effects of such imported task or function calls can be unpredictable.</p></div>
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<a href="00306.html" target="topic">27.4.1.1 Instant completion of imported functions</a> </div></td><td class="Element206" valign="top" width="50%">
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Imported functions shall complete their execution instantly and consume zero-simulation time, similarly to native functions.<br><br>Note that imported tasks can consume time, similar to native SystemVerilog tasks. </div></td></tr><tr>
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<a href="00307.html" target="topic">27.4.1.2 input, output and inout arguments</a> </div></td><td class="Element206" valign="top" width="50%">
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Imported functions can have input, output and inout arguments. The formal input arguments shall not be modified. If such arguments are changed within a function, the changes shall not be visible outside the function; the actual arguments shall not be changed.<br><br>The imported function shall not assume anything about the initial values of formal output arguments. The initial values of output arguments are undetermined and implementation-dependent.<br><br>The imported function can access the initial value of a formal inout argument. Changes that the imported function makes to a formal inout argument shall be visible outside the function. </div></td></tr><tr>
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<a href="00308.html" target="topic">27.4.1.3 Special properties pure and context</a> </div></td><td class="Element206" valign="top" width="50%">
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Special properties can be specified for an imported task or function: as pure or as context (see also Section 27.4.2 or 27.4.3 ).<br><br>A function whose result depends solely on the values of its input arguments and with no side effects can be specified as pure. This can usually allow for more optimizations and thus can result in improved simulation performance. Section 27.4.2 details the rules that must be obeyed by pure functions. An imported task can never be declared pure.<br><br>An imported task or function that is intended to call exported tasks or functions or to access SystemVerilog data... <a href="00308.html" target="topic">more</a> </div></td></tr><tr>
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<a href="00309.html" target="topic">27.4.1.4 Memory management</a> </div></td><td class="Element206" valign="top" width="50%">
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The memory spaces owned and allocated by the foreign code and SystemVerilog code are disjoined. Each side is responsible for its own allocated memory. Specifically, an imported function shall not free the memory allocated by SystemVerilog code (or the SystemVerilog compiler) nor expect SystemVerilog code to free the memory allocated by the foreign code (or the foreign compiler). This does not exclude scenarios where foreign code allocates a block of memory, then passes a handle (i.e., a pointer) to that block to SystemVerilog code, which in turn calls an imported function (e.g. C standard function free) which directly or indirectly... <a href="00309.html" target="topic">more</a> </div></td></tr><tr>
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<a href="00310.html" target="topic">27.4.1.5 Reentrancy of imported tasks</a> </div></td><td class="Element206" valign="top" width="50%">
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Since imported tasks can block (consume time), it is possible for an imported task’s C code to be simultaneously active in multiple execution threads. Standard reentrancy considerations must be made by the C programmer. Some examples of such considerations include safe use of static variables, and ensuring that only thread-safe C standard library calls (MT safe) are used. </div></td></tr><tr>
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<a href="00311.html" target="topic">27.4.1.6 C++ exceptions</a> </div></td><td class="Element206" valign="top" width="50%">
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It is possible to implement DPI imported tasks and functions using C++, as long as C linkage conventions are observed at the language boundary. If C++ is used, exceptions must not propagate out of any imported task or function. Undefined behavior will result if an exception crosses the language boundary from C++ into SystemVerilog. </div></td></tr></table></div></div>
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