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Storage Classes. Storage Classes. Scope and linkage Storage classes Automatic memory Dynamic memory Memory Model. Storage Classes. There are five storage classes That specify the lifetime and visibility of a variable This does not include dynamic memory - PowerPoint PPT Presentation
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Storage Classes
Storage Classes
Scope and linkage Storage classes
Automatic memory Dynamic memory
Memory Model
Storage Classes
There are five storage classes That specify the lifetime and visibility of
a variable This does not include dynamic memory
The different storage classes vary in combinations of three categories Scope Linkage Duration
Scope and Linkage
Scope
The scope of a variable is the region of the program in which it is visible Block scope Function prototype scope File scope
Block Scope
A block is a region of a program enclosed in opening and closing (curly) brackets Function definitions If and switch statements Loops ...
A variable defined in a block is visible from where it is defined to the end of the block
More About Block Scope
Two variables with the same name cannot be declared in the same block But the scope of variables with the same
name may overlap▪ Only the variable with the least scope is visible
Function parameters have the same scope as the function block Even though they are declared outside
the function’s enclosing brackets
Loops and C99 Prior to the C99 standard, variables had to
be declared at the start of a block Compilers compliant with C99 allow
variables to be declared anywhere in a block Including in the initialization statement of a for
loop If a variable is declared inside a for loop its scope
is the loop To use a C99 compliant compiler in gcc use the std=c99 switch▪ gcc -o foo foo.c -std=c99
Function Prototype Scope Applies to variable names used in
function prototypes The scope only applies to the function
prototype Names given to formal parameters may
differ from the names used in the prototype
File Scope
A variable with its definition outside any function has file scope It is visible from the point it is defined to
the end of the file containing its definition
It may also be visible to other files
Linkage Linkage refers to the file visibility of variables Variables with block scope have no linkage
They are private to the block or prototype in which they are defined
Variables with file scope have either internal linkage or external linkage A variable with internal linkage can be used
anywhere inside the file in which it is declared A variable with external linkage can be used
anywhere in a multi-file program (i.e. in other files)
Linkage and static
By default, a variable with file scope has external linkage If the variable is to be visible only within
one file it should be specified as static static int answer = 42;
Storage Classes
Duration
A variable has one of three storage durations Static Automatic Dynamic
The duration of a variable determines its lifetime within a program
Static Storage
Statically stored variables last for the lifetime of a program
The number of static variables does not change as a program runs So no special system is required to
maintain them They are usually allocated space
sequentially in an area of main memory They are initialized to default values
Types of Static Variable
Static variables can have one of three types of linkage External Internal None
External Linkage
A variable declared outside any function has external linkage It can be used by other files that are part
of the same program▪ int global = 1213; //outside main
A variable with external linkage can only be defined in one file
Using External Variables
External linkage raises the spectre of name ambiguity A file could define a variable with the same
name as another file’s external variable Variables from other files should be
declared with the extern specifier extern int global; This specifies that the variable has been
declared in another file
Internal Linkage
Variables declared globally are external by default
It may be desirable to limit their scope to a single file This can be achieved by declaring the
variable with the static specifier▪ static int just_in_this_file = 211;
No Linkage
A variable declared inside a function can also be specified as static This affects its lifetime not its visibility▪ The lifetime of the variable is that of the
program It is stored in the same region of main
memory as other static variables The values of block scope static
variables are preserved between function calls Their scope is still block scope
Block Scope and Static
void f1(int x){ static int count = 0; int y = 0; … count++;}
defined once and initialized to 0
defined each time that f1 runs
adds one to count each time that f1 runs
x, count, and y all have scope only within f1
Automatic StorageThe Call Stack
Automatic Storage
Function variables and parameters use automatic storage by default And have local scope and no linkage
Automatic variables are typically managed on a call stack A section of allocated memory that
grows and shrinks as functions are called Automatic variables are not
initialized Unless done so explicitly
Functions and Memory
A variable defined in a function block only persists for the lifetime of that function call Unless it is declared as static
Consider what memory might be allocated when a function is running Memory required for the function’s data
and only required during the function call
Memory that is to persist beyond the lifetime of the function
Allocating Memory for a Call During a function call, memory can be
allocated to the function's variables But not to variables of other functions▪ Ignoring any dynamic memory allocation
Memory allocation should be fast and simple That is, the process of allocating main
memory space to a variable▪ Not the process of accessing a variable
(although that should be fast too)
Main Memory Allocation
When allocating main memory to a function it is desirable to Waste as little space as possible Minimize the amount of administration
required to track free space It is relatively easy to allocate space
to function calls sequentially If function a calls function b, then
function a is not returned to until function b is terminated
Call Stack
Automatic variables are controlled by the call stack A stack is a last-in-first-out (LIFO) data
structure A program keeps track of the stack
with two pointers One points to the base of the stack The other points to the top of the stack▪ The next free memory location▪ Stack memory is allocated sequentially
Stack Memory – Simple Example
Let's look at a simple example of allocating memory on a stack as described previously
int x = 12;x = x * 3;double d = 23.567;
Notice that this example ignores all sorts of complicating issues
… 3600 3601 3602 3603 3604 3605 3606 3607 3608 3609 3610 3611 3612 …
Why start at byte 3600? No reason, it's just an arbitrary value
1236 23.567
Stack and Functions
Let's look at a more complex example of allocating memory That includes a main function and two
other function calls To make the example a bit simpler
the byte values are not shown Coloured boxes represent the memory
allocated to variables
Another Exampleint main(){ int r = 3; double area = circleArea(r);
double circleArea(double radius){ double pi = 3.1415; double sq_r = square(radius); return sq_r * pi;}
double square(double x){ return x * x;}
main memory
3 3 3.1415
start of circleArea's memory
square's memory3
r radius pi
x
Another Example
main memory
3 3 3.1415
start of circleArea's memory
9sq_rr radius pi
int main(){ int r = 3; double area = circleArea(r);
double circleArea(double radius){ double pi = 3.1415; double sq_r = square(radius); return sq_r * pi;}
double square(double x){ return x * x;}
Another Example
main memory
3 28.274r area
int main(){ int r = 3; double area = circleArea(r);
double circleArea(double radius){ double pi = 3.1415; double sq_r = square(radius); return sq_r * pi;}
double square(double x){ return x * x;}
Pushing and Popping
Function parameters and function variables are pushed onto the stack And the pointer to the top of the stack is
moved up by the size of each variable When a function terminates, the top
of the stack is reset to its original position Releasing memory assigned to the
function
Register Variables
C supports the register specifier for declaring variables Register variables are automatic so have▪ Lifetime over the life of the block,▪ Local scope, and▪ No linkage
Register is a hint to the compiler to assign the variable to a register Allowing the CPU to access it quickly
Compiler Hints
Specifying a variable as register is a request to the compiler so may be ignored If the registers are full Or the variable type does not fit in a
register Modern compilers may be able to
assign variables to registers on their own e.g. for loop indexes ▪ As they are required frequently
Stack Overflow
A compiler sets aside a fixed portion of main memory for the call stack If this is all used up by function calls
then a stack overflow occurs Resulting in termination of the
application Some recursive algorithms are prone
to stack overflow
Functions and Linkage
By default functions have external linkage Functions cannot be declared inside one
another so exist over the life of a program
Functions can be declared static to specify that they have internal linkage And can only be called in that file Allowing another function with the same
name to be used in another file
Scope Quiz
int main(){
int i;int x = 1;printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);printf("\n.. and into loop (y += i, x += i) ...\n\n");for(i=0; i < 4; i++){
int x = 1000;y = y + i; //aargh! (global)x = x * i;printf("%9s%04d %12s%04d ", "loop i = ", i, "loop x = ", x);printf("%12s%04d\n", "global y = ", y);foo(x);
}printf("\n.. and out of loop ...\n\n");printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);foo(x);return 0;
}
void foo(int y){
static int foo_count = 0;foo_count++;y++;x++; //aargh! (global)printf("%9s%04d %12s%04d ", "foo y = ", y, "foo_count = ", foo_count);printf("%12s%04d\n", "global x = ", x);
}
// Global Variablesint x = 10;static int y = 100;
Scope Quiz
int main(){
int i;int x = 1;printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);printf("\n.. and into loop (y += i, x += i) ...\n\n");for(i=0; i < 4; i++){
int x = 1000;y = y + i; //aargh! (global)x = x * i;printf("%9s%04d %12s%04d ", "loop i = ", i, "loop x = ", x);printf("%12s%04d\n", "global y = ", y);foo(x);
}printf("\n.. and out of loop ...\n\n");printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);foo(x);return 0;
}
void foo(int y){
static int foo_count = 0;foo_count++;y++;x++; //aargh! (global)printf("%9s%04d %12s%04d ", "foo y = ", y, "foo_count = ", foo_count);printf("%12s%04d\n", "global x = ", x);
}
// Global Variablesint x = 10;static int y = 100;
where is global x visible?
just in foo
Scope Quiz
int main(){
int i;int x = 1;printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);printf("\n.. and into loop (y += i, x += i) ...\n\n");for(i=0; i < 4; i++){
int x = 1000;y = y + i; //aargh! (global)x = x * i;printf("%9s%04d %12s%04d ", "loop i = ", i, "loop x = ", x);printf("%12s%04d\n", "global y = ", y);foo(x);
}printf("\n.. and out of loop ...\n\n");printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);foo(x);return 0;
}
void foo(int y){
static int foo_count = 0;foo_count++;y++;x++; //aargh! (global)printf("%9s%04d %12s%04d ", "foo y = ", y, "foo_count = ", foo_count);printf("%12s%04d\n", "global x = ", x);
}
// Global Variablesint x = 10;static int y = 100;
where is global y visible?
just in main
Scope Quiz
int main(){
int i;int x = 1;printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);printf("\n.. and into loop (y += i, x += i) ...\n\n");for(i=0; i < 4; i++){
int x = 1000;y = y + i; //aargh!x = x * i;printf("%9s%04d %12s%04d ", "loop i = ", i, "loop x = ", x);printf("%12s%04d\n", "global y = ", y);foo(x);
}printf("\n.. and out of loop ...\n\n");printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);foo(x);return 0;
}
void foo(int y){
static int foo_count = 0;foo_count++;y++;x++; //aargh!printf("%9s%04d %12s%04d ", "foo y = ", y, "foo_count = ", foo_count);printf("%12s%04d\n", "global x = ", x);
}
// Global Variablesint x = 10;static int y = 100;
where is main x visible?
Scope Quiz
int main(){
int i;int x = 1;printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);printf("\n.. and into loop (y += i, x += i) ...\n\n");for(i=0; i < 4; i++){
int x = 1000;y = y + i; //aargh!x = x * i;printf("%9s%04d %12s%04d ", "loop i = ", i, "loop x = ", x);printf("%12s%04d\n", "global y = ", y);foo(x);
}printf("\n.. and out of loop ...\n\n");printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);foo(x);return 0;
}
void foo(int y){
static int foo_count = 0;foo_count++;y++;x++; //aargh!printf("%9s%04d %12s%04d ", "foo y = ", y, "foo_count = ", foo_count);printf("%12s%04d\n", "global x = ", x);
}
// Global Variablesint x = 10;static int y = 100;
where is loop x visible?
Scope Quiz
int main(){
int i;int x = 1;printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);printf("\n.. and into loop (y += i, x += i) ...\n\n");for(i=0; i < 4; i++){
int x = 1000;y = y + i; //aargh!x = x * i;printf("%9s%04d %12s%04d ", "loop i = ", i, "loop x = ", x);printf("%12s%04d\n", "global y = ", y);foo(x);
}printf("\n.. and out of loop ...\n\n");printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);foo(x);return 0;
}
void foo(int y){
static int foo_count = 0;foo_count++;y++;x++; //aargh!printf("%9s%04d %12s%04d ", "foo y = ", y, "foo_count = ", foo_count);printf("%12s%04d\n", "global x = ", x);
}
// Global Variablesint x = 10;static int y = 100;
what is the effect of making y static?
it is not visible in other files
Scope Quiz
int main(){
int i;int x = 1;printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);printf("\n.. and into loop (y += i, x += i) ...\n\n");for(i=0; i < 4; i++){
int x = 1000;y = y + i; //aargh!x = x * i;printf("%9s%04d %12s%04d ", "loop i = ", i, "loop x = ", x);printf("%12s%04d\n", "global y = ", y);foo(x);
}printf("\n.. and out of loop ...\n\n");printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);foo(x);return 0;
}
void foo(int y){
static int foo_count = 0;foo_count++;y++;x++; //aargh!printf("%9s%04d %12s%04d ", "foo y = ", y, "foo_count = ", foo_count);printf("%12s%04d\n", "global x = ", x);
}
// Global Variablesint x = 10;static int y = 100;
what is the effect of making foo_count static?
it's lifetime is the lifetime of the program
Scope Quiz
int main(){
int i;int x = 1;printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);printf("\n.. and into loop (y += i, x += i) ...\n\n");for(i=0; i < 4; i++){
int x = 1000;y = y + i; //aargh!x = x * i;printf("%9s%04d %12s%04d ", "loop i = ", i, "loop x = ", x);printf("%12s%04d\n", "global y = ", y);foo(x);
}printf("\n.. and out of loop ...\n\n");printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);foo(x);return 0;
}
void foo(int y){
static int foo_count = 0;foo_count++;y++;x++; //aargh!printf("%9s%04d %12s%04d ", "foo y = ", y, "foo_count = ", foo_count);printf("%12s%04d\n", "global x = ", x);
}
// Global Variablesint x = 10;static int y = 100;
what is printed?
Scope Quiz
// Global Variablesint x = 10;static int y = 100;
int main(){
int i;int x = 1;printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);printf("\n.. and into loop (y += i, x += i) ...\n\n");for(i=0; i < 4; i++){
int x = 1000;y = y + i; //aargh!x = x * i;printf("%9s%04d %12s%04d ", "loop i = ", i, "loop x = ", x);printf("%12s%04d\n", "global y = ", y);foo(x);
}printf("\n.. and out of loop ...\n\n");printf("%9s%04d %12s%04d\n", "main x = ", x, "global y = ", y);foo(x);return 0;
}
void foo(int y){
static int foo_count = 0;foo_count++;y++;x++; //aargh!printf("%9s%04d %12s%04d ", "foo y = ", y, "foo_count = ", foo_count);printf("%12s%04d\n", "global x = ", x);
}
what is printed ...
Storage Class Combinations
ignoring dynamic memory ...
Storage Class
Duration
Scope
Linkage
Declared
automatic automatic
block none in a block
register automatic
block none in a block with register
static external
static file external
outside all functions
static internal static file internal
outside all functions with static
static no linkage
static block none in a block with static
Dynamic Memory
Dynamic Memory Introduction The duration of variables allocated in
automatic or static memory is fixed The size of such variables is dependent
on type and is also fixed It may be useful to determine the
space to be allocated to a variable at run-time To avoid wasting space where the space
requirements may vary considerably
Dynamic Memory
Dynamic memory is allocated from a different area of memory than the stack This area of memory is referred to as the
free store or the heap Like stack memory the size is fixed, and
is (of course) finite
Allocating Dynamic Memory Dynamic memory can be allocated at
runtime Using malloc to request memory of a given size The malloc function takes a single argument▪ The number of bytes to be allocated
Memory allocated with malloc remains allocated until it is released It is not limited to the lifetime of the block in
which it is allocated Memory is released by calling free
More About Pointers
In addition to allocating memory, malloc returns the address of this memory The address should be assigned to a
pointer variable Dynamic memory is often used to
create dynamic arrays For example create an array of ten
integers▪ int* arr = malloc(10 * sizeof(int));▪ The address returned by malloc is assigned to
arr
Dynamic Arrays
Arrays assigned to pointers may be used just like regular (static) arrays The elements can be accessed using an
index There is one major difference
between dynamic arrays and static arrays A pointer can be assigned a new array But what happens to the old array?
Creating New Arraysint* arr = malloc(10 * sizeof(int));// ... do stuff with arr// ... and make a new, larger arrayarr = malloc(100 * sizeof(int));
allocates 40 bytes in the heap
allocates another 400 bytes in the heap, it does not re-use the 40 bytes that were previously allocatedthe original 40 bytes cannot be accessed, since the program no longer records the addresshowever, they are also unavailable for reuse, thereby causing a memory leak
Memory Leaks
A memory leak occurs when dynamic memory is allocated but is not de-allocated When it is no longer required
Dynamic memory that is not de-allocated is unavailable until the program terminates Or even longer in some (usually older)
operating systems Large memory leaks may affect the
performance of applications
Freeing Dynamic Memory Any dynamic memory that is
allocated by a program should be de-allocated By calling free▪ The free function takes a single argument,
the pointer to the memory that is to be de-allocated
Every use of malloc should be matched by a call to free When the allocated dynamic memory is
no longer required
Lifetime of Dynamic Memory Unlike static or automatic memory
the lifetime of dynamic memory is not fixed It is dependent on when the memory is
first allocated and when it is released The lifetime of a variable created in
dynamic memory is between the calls to malloc and free
Allocating Memory with calloc The calloc function can also be used
to allocate dynamic memory It takes two arguments, the size of the
array and the size of the array type▪ int* arr = calloc(10, sizeof(int));
Unlike malloc, calloc also sets all of the bits in the allocated memory to 0
The free function is used to de-allocate memory allocated with calloc
Out of Memory
Dynamic memory can be exhausted To make memory allocation safe it is
good practice to ensure that allocation succeeds Both malloc and calloc will return the
NULL pointer if allocation failsint* p;int n = 10000000;p = (int*) malloc(n * sizeof(int));if(p == NULL){
puts("Memory allocation failed.");exit(EXIT_FAILURE);
}exit terminates the application
ANSI C Standard and malloc Under the ANSI C standard both
calloc and malloc return pointers to void A generic pointer
It is therefore good practice to cast the pointer returned by malloc to the correct typeint* arr = (int *) malloc(10 * sizeof(int));
casts (converts) the pointer to void to an int pointer
Memory Model
static
Storage
code storage
automatic
dynamic
storage space for program instructions
global variables and variables declared as static
function calls, local variables and function parameters
dynamically allocated variables, addresses are assigned to pointers
Static Memory
Used for global variables and variables that are explicitly declared as static With the keyword static Exist for the lifetime of the program
Local, static variables are declared and initialized once Should be used for variables that are to
persist over the life of the program
Automatic Memory
Used for most local variables declared in a block or function header Exist only for the lifetime of the block The most common method of variable
storage Variables can be explicitly declared as
automatic by using the auto keyword▪ To indicate that the variable should not be
changed to some other storage class Typically allocated on a call stack
Dynamic Memory
Used for variables whose size or lifetime can only be determined at run-time Allows arrays of varying size to be assigned
to one pointer variable (over time) The allocation of dynamic memory is
controlled by function calls A dynamic variable’s lifetime starts with a
call to malloc (or calloc) and ends with a call to free▪ Or when the program terminates
Memory Model Summary
Static, automatic and dynamic memory are different regions of RAM Variables in static memory require the
least amount of overhead Automatic variables require more
overhead▪ Since information about function calls needs
to be maintained Variables in dynamic memory require
the most overhead
Memory TestThis function demonstrates that static, automatic and dynamic variables are allocated space in different areas of main memoryvoid memoryTest(){
static int x;int y = 0;int* p = malloc(4 * sizeof(int));double* q = calloc(10, sizeof(double));char* s = "albatross";
printf("static (s:albatross): %x\n", s);printf("static (&x): %x\n", &x);printf("automatic (&y): %x\n", &y);printf("automatic (&p): %x\n", &p);printf("automatic (&q): %x\n", &q);printf("automatic (&s): %x\n", &s);printf("dynamic (p): %x\n", p);printf("dynamic (q): %x\n", q);
}
string literals are stored in static memory