The C language supports two kinds of memory allocation through the
variables in C programs:
Static allocation is what happens when you declare a static or
global variable. Each static or global variable defines one block of
space, of a fixed size. The space is allocated once, when your program
is started (part of the exec operation), and is never freed.
Automatic allocation happens when you declare an automatic
variable, such as a function argument or a local variable. The space
for an automatic variable is allocated when the compound statement
containing the declaration is entered, and is freed when that
compound statement is exited.
In GNU C, the size of the automatic storage can be an expression
that varies. In other C implementations, it must be a constant.
A third important kind of memory allocation, dynamic allocation,
is not supported by C variables but is available via GNU C library
18.104.22.168 Dynamic Memory Allocation
Dynamic memory allocation is a technique in which programs
determine as they are running where to store some information. You need
dynamic allocation when the amount of memory you need, or how long you
continue to need it, depends on factors that are not known before the
For example, you may need a block to store a line read from an input
file; since there is no limit to how long a line can be, you must
allocate the memory dynamically and make it dynamically larger as you
read more of the line.
Or, you may need a block for each record or each definition in the input
data; since you can't know in advance how many there will be, you must
allocate a new block for each record or definition as you read it.
When you use dynamic allocation, the allocation of a block of memory is
an action that the program requests explicitly. You call a function or
macro when you want to allocate space, and specify the size with an
argument. If you want to free the space, you do so by calling another
function or macro. You can do these things whenever you want, as often
as you want.
Dynamic allocation is not supported by C variables; there is no storage
class “dynamic”, and there can never be a C variable whose value is
stored in dynamically allocated space. The only way to get dynamically
allocated memory is via a system call (which is generally via a GNU C
library function call), and the only way to refer to dynamically
allocated space is through a pointer. Because it is less convenient,
and because the actual process of dynamic allocation requires more
computation time, programmers generally use dynamic allocation only when
neither static nor automatic allocation will serve.
For example, if you want to allocate dynamically some space to hold a
struct foobar, you cannot declare a variable of type struct
foobar whose contents are the dynamically allocated space. But you can
declare a variable of pointer type struct foobar * and assign it the
address of the space. Then you can use the operators `*' and
`->' on this pointer variable to refer to the contents of the space: