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<title>13.9 volatile qualifier</title>
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<a name="1205194"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">13.9 volatile qualifier</font>
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<a name="1205195"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">An access to an object that has volatile-qualified type is a "read" from the memory location containing the object (if the object's value is required) or a "write" to the memory location containing the object (if the object is assigned to).</font>
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<a name="1205196"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">If the volatile object is an array, then the access is only to the appropriate element of the array.</font>
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<a name="1205197"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">If the volatile object is a structure and only a field of the structure is required, then the access is only to the appropriate field.</font>
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<a name="1205198"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">If the access is an indirect access through a pointer, then the access is equivalent to an access of an array element of that type.</font>
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<a name="1209572"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">If the object is not an array element or structure field and the access is not indirect, then the object occupies a whole number of words, and all the words are accessed. Otherwise, if the array element or structure field is shorter than a word, then only the appropriate bytes are accessed.</font>
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<a name="1205200"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">If the object is a bit-field, then in the case of read access, the entire word containing the bit-field is read; and in the case of write access, the entire word containing the bit-field is first read, and then written.</font>
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<p>
<a name="1205201"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">When the volatile qualifier is used, the compiler reduces the scope of its optimization. A typical use of volatile is where a variable is accessed from two separate threads of control, and compiler optimizations may otherwise prevent the correct functioning of the code.</font>
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<a name="1205202"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The following example shows the use of an interrupt handler which updates a shared synchronization variable, and the main program busy waits for the variable to be updated. Without the use of <font size=2 face=Courier><strong>volatile</strong></font> in this example, the compiler would be free to optimize away reading the <font size=2 face=Courier><strong>sync</strong></font> variable, because it knows that nothing in the <font size=2 face=Courier><strong>wait_for_isr()</strong></font> function can write to it:</font>
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<a name="1215510"> </a><font size=2 face=Courier><strong>void isr(unsigned int Areg, unsigned int Breg, unsigned int Creg);<br>#pragma ST_interrupt_handler(isr)<br><br></strong></font>
<a name="1215512"> </a><font size=2 face=Courier><strong>volatile int sync=0;<br><br></strong></font>
<a name="1205208"> </a><font size=2 face=Courier><strong>void isr(unsigned int Areg, unsign int Breg, unsigned int Creg)<br>{<br>⌨️ 快捷键说明
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