0A storage class defines the scope (visibility) and life-time of variables and/or
functions within a C Program. They precede the type that they modify. We have
four different storage classes in a C program:
- auto
- register
- static
- extern
The auto Storage Class
The auto storage class is the default storage class for all local variables.
{
int mount;
auto int month;
}
The example above defines two variables within the same storage class. ‘auto’
can only be used within functions, i.e., local variables.
The register Storage Class
The register storage class 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 can't have the unary '&'
operator applied to it (as it does not have a memory location).
{
register int miles;
}
The register should only be used for variables that require quick access such as
counters. It should also be noted that defining 'register' does 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 implementation restrictions
The static Storage Class
The static storage class instructs the compiler to keep a local variable in
existence during the life-time of the program instead of creating and destroying
it each time it comes into and goes out of scope. Therefore, making local
variables static allows them to maintain their values between function calls.
The static modifier may also be applied to global variables. When this is done, it
causes that variable's scope to be restricted to the file in which it is declared.
In C programming, when static is used on a class data member, it causes only
one copy of that member to be shared by all the objects of its class.
The extern Storage Class
The extern storage class is used to give a reference of a global variable that is
visible to ALL the program files. When you use 'extern', the variable cannot be
initialized, however, it points the variable name at a storage location that has
been previously defined.
When you have multiple files and you define a global variable or function, which
will also be used in other files, then extern will be used in another file to provide
the reference of defined variable or function. Just for understanding, extern is
used to declare a global variable or function in another file.
The extern modifier is most commonly used when there are two or more files
sharing the same global variables or functions as explained below.
First File:
#include <stdio.h>
int count;
extern void write_extern();
main()
{
count = 5;
write_extern();
}
Second File:
#include <stdio.h>
extern int count;
void write_extern(void)
{
printf("count is %d\n", count);
}
