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<title>C Storage Classes</title>
<head>
</head>
<body bgcolor="#ffffcc">
<center>
<hr>
<h1>C Storage Classes.</h1>
<hr>
</center>
<p>
C has a concept of '<i>Storage classes</i>' which are used to define the
scope (visability) and life time of variables and/or functions.
<p>
So what Storage Classes are available?
<p>
<table border=2 bgcolor=ivory>
<tr>
<td><a href="#auto">auto</a>
<td><a href="#register">register</a>
<td><a href="#static">static</a>
<td><a href="#extern">extern</a>
<td><A HREF="../SYNTAX/typedef.html">typedef</A>
</tr>
</table>
<p>
<hr>
<h2><a name="auto">auto - storage class</h2>
<b>auto</b> is the default storage class for local variables.
<pre>
{
int Count;
auto int Month;
}
</pre>
The example above defines two variables with the same storage class.
auto can only be used within functions, i.e. local variables. <p>
<hr>
<h2><a name="register">register - Storage Class</h2>
<b>register</b> is used to define local variables that should be stored
in a register instead of RAM. This means that the variable has a maximum size
equal to the register size (usually one word) and cant have the unary '&'
operator applied to it (as it does not have a memory location).
<pre>
{
register int Miles;
}
</pre>
Register should only be used for variables that require quick access - such
as counters. It should also be noted that defining 'register' goes not mean
that the variable will be stored in a register. It means that it MIGHT be stored
in a register - depending on hardware and implimentation restrictions.<p>
<hr>
<h2><a name="static">static - Storage Class</h2>
<a href="../SYNTAX/static.htm">Click here for static functions</a>
<p>
<b>static</b> is the default storage class for
<a href="../SYNTAX/glo_int_vars.html#global">global variables</a>. The two
variables below (<b>count</b> and <b>road</b>) both have a static storage class.
<p>
<center>
<table border=2 bgcolor=ivory width="50%">
<tr>
<td>
<pre>
static int Count;
int Road;
main()
{
printf("%d\n", Count);
printf("%d\n", Road);
}
</pre>
</td>
</tr>
</table>
</center>
<p>
'static' can also be defined within a function. If this is done, the variable
is initalised at compilation time and retains its value between calls.
Because it is initialsed at compilation time, the initalistation value
must be a constant.
This is serious stuff - tread with care.
<p>
<center>
<table border=2 bgcolor=ivory width="50%">
<tr>
<td>
<pre>
void Func(void)
{
static Count=1;
}
</pre>
</td>
</tr>
</table>
</center>
<p>
Here is an <a href="../EXAMPLES/static.c">example</a><p>
<a name="static2">
There is one very important use for 'static'. Consider this bit of code.
<p>
<center>
<table border=2 bgcolor=ivory width="50%">
<tr>
<td>
<pre>
char *Func(void);
main()
{
char *Text1;
Text1 = Func();
}
char *Func(void)
{
char Text2[10]="martin";
return(Text2);
}
</pre>
</td>
</tr>
</table>
</center>
<p>
'Func' returns a pointer to the memory location where 'Text2' starts
BUT Text2 has a storage class of <a href="#auto">auto</a> and will disappear
when we exit the function and could be overwritten by something else. The
answer is to specify:
<p>
<center>
<table border=2 bgcolor=ivory width="50%">
<tr>
<td>
<pre>
static char Text[10]="martin";
</pre>
</td>
</tr>
</table>
</center>
<p>
The storage assigned to 'Text2' will remain reserved for the duration if the
program.
<hr>
<h2><a name="extern">extern - storage Class</h2>
<b>extern</b> defines a global variable that is visable to ALL object
modules. When you use 'extern' the variable cannot be initalized as
all it does is point the variable name at a storage location that has
been previously defined.
<pre>
Source 1 Source 2
-------- --------
extern int count; int count=5;
write() main()
{ {
printf("count is %d\n", count); write();
} }
</pre>
Count in 'source 1' will have a value of 5. If source 1 changes the
value of count - source 2 will see the new value. Here are some example
source files.
<p>
<a href="../EXAMPLES/extern1.c">Source 1</a><br>
<a href="../EXAMPLES/extern2.c">Source 2</a><p>
The compile command will look something like.<p>
<pre>
gcc source1.c source2.c -o program
</pre>
<hr>
<h2>See Also:</h2>
<img src="../../GRAPHICS/whiteball.gif">
<a href="data_types.html">Data types.</a>
<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="../SYNTAX/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>
|
