From 7e0f021a9aec35fd8e6725e87e3313b101d26f5e Mon Sep 17 00:00:00 2001 From: Tobias Klauser Date: Sun, 27 Jan 2008 11:37:44 +0100 Subject: Initial import (2.0.2-6) --- reference/C/SYNTAX/storage_class.html | 187 ++++++++++++++++++++++++++++++++++ 1 file changed, 187 insertions(+) create mode 100644 reference/C/SYNTAX/storage_class.html (limited to 'reference/C/SYNTAX/storage_class.html') diff --git a/reference/C/SYNTAX/storage_class.html b/reference/C/SYNTAX/storage_class.html new file mode 100644 index 0000000..c12cbc8 --- /dev/null +++ b/reference/C/SYNTAX/storage_class.html @@ -0,0 +1,187 @@ +C Storage Classes + + + + + +
+
+

C Storage Classes.

+
+
+

+C has a concept of 'Storage classes' which are used to define the +scope (visability) and life time of variables and/or functions. +

+So what Storage Classes are available? +

+ + + +
auto +register +static +extern +typedef +
+

+

+

auto - storage class

+auto is the default storage class for local variables. +
+	{
+            int Count;
+            auto int Month;
+	}
+
+ +The example above defines two variables with the same storage class. +auto can only be used within functions, i.e. local variables.

+

+

register - Storage Class

+register 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). +
+	{
+            register int  Miles;
+	}
+
+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.

+

+

static - Storage Class

+static is the default storage class for global variables. The two +variables below (count and road) both have a static storage class. +
+	static int Count;
+        int Road;
+
+        {
+            printf("%d\n", Road);
+        }
+
+static variables can be 'seen' within all functions in this source file. At +link time, the static variables defined here will not be seen by the object +modules that are brought in.

+'static' can also be defined within a function! If this is done the variable +is initalised at run time but is not reinitalized when the function is called. +This is serious stuff - tread with care. + +

+	{
+	    static Count=1;
+        }
+
+Here is an example

+ + +There is one very important use for 'static'. Consider this bit of code. +

+        char *func(void);
+
+        main()
+        {
+            char *Text1;
+            Text1 = func();
+        }
+
+        char *func(void)
+        {
+            char Text2[10]="martin";
+            return(Text2);
+        }
+
+ +Now, 'func' returns a pointer to the memory location where 'text2' starts +BUT text2 has a storage class of 'auto' and will disappear when we exit the +function and could be overwritten but something else. The answer is to specify +
+	static char Text[10]="martin";
+
+The storage assigned to 'text2' will remain reserved for the duration if the +program. +

+

+ +C++ has overloaded static, here +are the details. + +
+

extern - storage Class

+extern 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. + + + + + + + + +
+Source 1 + +Source 2 +
+
+
+extern int count;			int count=5;
+
+write()					main()
+{					{
+    printf("count is %d\n", count);         write();
+}					}
+
+		 +test +
+ +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. +

+Source 1
+Source 2

+ +The compile command will look something like.

+

+	gcc source1.c source2.c -o program
+
+ +
+

See Also:

+Data types. + + +

+ +

+

+

+ + + + +
+ Top + + Master Index + + Keywords + + Functions +
+
+

+

+
Martin Leslie +

+ + -- cgit v1.2.3-54-g00ecf