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  76. <a name="Qualifiers"></a>

  77. <h3 class="section">4.10 Qualifiers</h3>

  78. 

  79. <ul>

  80. <li> <cite>What constitutes an access to an object that has volatile-qualified

  81. type (C90 6.5.3, C99 and C11 6.7.3).</cite>

  82. 

  83. <p>Such an object is normally accessed by pointers and used for accessing

  84. hardware.  In most expressions, it is intuitively obvious what is a read

  85. and what is a write.  For example

  86. </p>

  87. <div class="smallexample">

  88. <pre class="smallexample">volatile int *dst = <var>somevalue</var>;

  89. volatile int *src = <var>someothervalue</var>;

  90. *dst = *src;

  91. </pre></div>

  92. 

  93. <p>will cause a read of the volatile object pointed to by <var>src</var> and store the

  94. value into the volatile object pointed to by <var>dst</var>.  There is no

  95. guarantee that these reads and writes are atomic, especially for objects

  96. larger than <code>int</code>.

  97. </p>

  98. <p>However, if the volatile storage is not being modified, and the value of

  99. the volatile storage is not used, then the situation is less obvious.

  100. For example

  101. </p>

  102. <div class="smallexample">

  103. <pre class="smallexample">volatile int *src = <var>somevalue</var>;

  104. *src;

  105. </pre></div>

  106. 

  107. <p>According to the C standard, such an expression is an rvalue whose type

  108. is the unqualified version of its original type, i.e. <code>int</code>.  Whether

  109. GCC interprets this as a read of the volatile object being pointed to or

  110. only as a request to evaluate the expression for its side effects depends

  111. on this type.

  112. </p>

  113. <p>If it is a scalar type, or on most targets an aggregate type whose only

  114. member object is of a scalar type, or a union type whose member objects

  115. are of scalar types, the expression is interpreted by GCC as a read of

  116. the volatile object; in the other cases, the expression is only evaluated

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  118. </p>

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