Working with the Unix OS

C Programming – Basics
main()
{
int x;
x = 1;
{ int x;
x = 2;
{
int x;
x = 3;
printf("x=%d\n",x);
}
printf("x=%d\n",x);
}
printf("x=%d\n",x);
}
Running this program will result in the following output:
x=3
x=2
x=l
Program Form
Files to include
Macro definitions
Global variable
declarations
Function definitions
Declarations of
formal parameters
Local variable
declarations
Function
Body
|#include ...
|#define ...
|--------------|
|func1(....) |
| |
|{ |
| |
|} |
|==============|
|==============|
|func2 (....) |
|{ |
| |
|} |
|==============|
|main(...) |
|{ |
|} |
|==============|
Storage Classes
As well as having a type, C variables have a class which describes how they are stored in memory. There are four
storage classes:
Automatic
This is the default storage class. Memory is allocated for an automatic variable when the block in which it
is declared is entered and this storage is deallocated when the block is exited. This is equivalent to the
normal Pascal local variable.
Register
This is the same as for automatic except that if possible the compiler will attempt to use a hardware register
for storing the variable making access faster. Most compilers find it too hard to do.
Static
Memory is allocated for a static variable at compile time and is never deallocated. Local static variables
retain their values between function calls. Similar to the SAVE facility in FORTRAN.
External
Equivalent to global variables in Pascal. They are not local to any function including main.
Variables declared outside the scope of any function are global variables. A function may access any global
variable declared above it in the source code without any further declarations.
If however the programmer chooses to declare the global variable again within a function as an external variable,
its global declaration may appear below the function in the source code.
Storage class descriptions appear before the type in a variable declaration.
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