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| author | Tobias Klauser <tklauser@distanz.ch> | 2008-01-27 11:37:44 +0100 |
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| committer | Tobias Klauser <tklauser@xenon.tklauser.home> | 2008-01-27 11:37:44 +0100 |
| commit | 7e0f021a9aec35fd8e6725e87e3313b101d26f5e (patch) | |
| tree | b1cacc4b24393f517aeb4610e9e1021f954307a8 /reference/C/SYNTAX/volatile.html | |
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diff --git a/reference/C/SYNTAX/volatile.html b/reference/C/SYNTAX/volatile.html new file mode 100644 index 0000000..1de9b13 --- /dev/null +++ b/reference/C/SYNTAX/volatile.html @@ -0,0 +1,97 @@ +<html> +<head> +<title>The VOLATILE keyword.</title> +<script language="JavaScript"> +</script> +</head> +<body bgcolor="#ffffcc"> +<hr> +<center><h1>The VOLATILE keyword.</h1></center> +<hr> +I have had several sugestions on how to describe volatile, +If you have any input please mail me. +<ul> +<li>The <b>volatile</b> <a href="../glossary.html#keyword">keyword</a> acts as a +<a href="../CONCEPT/data_types.html">data type</a> qualifier. +The qualifier alters the default why in which +the compiler handles the variable and does not attempt to optimize the +storage referenced by it. <i>Martin Leslie</i> +<p> +<li><b>volatile</b> means the storage is likely to change at anytime +and be changed but something outside the control of the user program. +This means that if you reference the variable, +the program should always check the physical address (ie a mapped input fifo), +and not use it in a cashed way.<i>Stephen Hunt</i> +<p> +<li>Here is an example for the usage of the volatile keyword: +<pre> + /* Base address of the data input latch */ + + volatile unsigned char *baseAddr; + + /* read parts of output latch */ + + lsb = *handle->baseAddr; + middle = *handle->baseAddr; + msb = *handle->baseAddr; +</pre> +Between reads the bytes are changed in the latch. +<p> +Without the volatile, the compiler optimises this to a single assignment: +<pre> + lsb = middle = msb = *handle->baseAddr; +</pre> +<i>Tim Potter</i> +<p> +<li>A <b>volatile</b> variable is for dynamic use. E.G. for data that is to +be passed to an I/O port Here is an example. + +<pre> + #define TTYPORT 0x17755U + + volatile char *port17 = (char)*TTYPORT; + *port17 = 'o'; + *port17 = 'N'; +</pre> +Without the <b>volatile</b> modifier, the compiler would think that the +statement <code>*port17 = 'o';</code> is redundant and would remove it +from the object code. The volatile statement prevents the compiler +optimisation.<i>Alex Arzon</i>. +</ul> + +<hr> +<h2>Examples:</h2> +<img src="../../GRAPHICS/computer.gif" align=left alt="o"> No example:-( +<br clear=left> +<hr> +<h2>See also:</h2> +<ul> +<li><a href="../CONCEPT/data_types.html#qualifier">const qualifier</a> +</ul> + + +<p> + +<hr> +<p> +<center> +<table border=2 width="80%" bgcolor="ivory"> +<tr align=center> +<td width="25%"> +<a href="../cref.html"> Top</a> +</td><td width="25%"> +<a href="../master_index.html"> Master Index</a> +</td><td width="25%"> +<a href="keywords.html"> Keywords</a> +</td><td width="25%"> +<a href="../FUNCTIONS/funcref.htm"> Functions</a> +</td> +</tr> +</table> +</center> +<p> +<hr> +<address>Martin Leslie +</address><p> +</body> +</html> |