Embedded C - Lecture 2

Prepared by: Mohamed AbdAllah
Embedded C-Programming
Lecture 2
1

Agenda
 Preprocessor directives (continue).
 Data types (Primitive, derived, user defined).
 Qualifiers (Size, Sign and volatile modifiers and Storage classes).
 Scope and lifetime.
 Makefile introduction.
 Task 2.
2

Preprocessor directives (continue)
4

 Examples on preprocessor directives (continue):
• Function-like Macro:
#define MAX(a,b) ((a)>(b)) ? (a):(b))
Define macro to be used like a function call, so typing:
int maximum_num = MAX(2,7);
will be replaced with:
int maximum_num = ((2)>(7)) ? (2):(7));
Which will make maximum_num value be 7
5

#define OUTPUT 1
#define LOGIC_HIGH 1
#define LED_INIT(PORT,PIN)
portInit(PORT,PIN,OUTPUT);
portState(PORT,PIN,LOGIC_HIGH)
Define macro to be used like a function call, so typing:
LED_INIT(‘A’,3);
will be the same as typing:
portInit(‘A’,3,OUTPUT);
portState(‘A’,3,LOGIC_HIGH);
Note: Function-like Macro is like normal macro, it is just text replacement.
6

 Pros and Cons of using Function-like Macro
• Function-like macro usage is just copy and paste, so no function call
actually occurs which makes it faster than function call.
• It is more difficult to debug in function-like macro code than normal
function code.
• Macros are error-prone because they rely on textual substitution:
#define square(a) a*a
/* works fine when used with an integer */
square(5); /* --> 5*5 --> 25 */
/* but does very strange things when used with expressions */
square(1+2); /* --> 1+2*1+2 --> 1+2+2 --> 5 */
square(x++); /* --> x++*x++ --> increments x twice */
7

 Pros and Cons of using Function-like Macro
Using parentheses around all your variables inside macro will help in first
problem but not the second:
#define square(a) ((a)*(a))
square(1+2); /* --> ((1+2)*(1+2)) --> 3*3 --> 9 */
square(x++);/*((x++)*(x++))-->still increments x twice*/
• Macros do not perform type-checking:
#define MAX(A,B) (((A)>(B)) ? (A) : (B))
Then doing the following will not produce any error or warning:
printf("%dn", MAX("abc","def"));
But if the same was done with usal functions then the compiler will
generate warning during compilation.
8

• Stringification:
#define LED_INIT(PORT,PIN)
printf("Initializing PORT: "#PORT" n");
portInit(PORT,PIN,OUTPUT);
portState(PORT,PIN,LOGIC_HIGH)
When we want to convert a macro argument into a string constant.
So typing:
LED_INIT(‘A’,3);
will be the same as typing:
printf("Initializing PORT: A n");
portInit(‘A’,3,OUTPUT);
portState(‘A’,3,LOGIC_HIGH);
9

• Concatenation:
It is used to merge two tokens into one while expanding macros. This
is called token pasting or token concatenation.
Suppose you have a function with 3 different implementations:
portInit_implementation1(void);
Then you can define:
#define PORT_INIT(NUM)
portInit_implementation ## NUM ()
Then to choose which function of the 3 to call using only PORT_INIT:
PORT_INIT(2);
it is the same as typing:
portInit_implementation2(); 10

 Primitive Data Types
• The primitive data types in c language are the inbuilt data types
provided by the c language itself. Thus, all c compilers provide support
for these data types.
• The following primitive data types in c are available:
 Integer Data Type, int: Integer data type is used to declare a
variable that can store numbers without a decimal. The keyword
used to declare a variable of integer type is “int”. Thus, to declare
integer data type following syntax should be followed:
int variable_name;
 Float data Type, float: Float data type declares a variable that can
store numbers containing a decimal number:
float variable_name;
13
Data types

 Primitive Data Types
 Double Data Type, double: Double data type also declares variable
that can store floating point numbers but gives precision double
than that provided by float data type. Thus, double data type are
also referred to as double precision data type:
double variable_name;
 Character Data Type, char: Character data type declares a variable
that can store a character constant. Thus, the variables declared as
char data type can only store one single character:
char variable_name;
 Void Data Type, void: Unlike other primitive data types in c, void
data type does not create any variable but returns an empty set of
values. Thus, we can say that it stores null:
void myFunc(); /* Function returns void data type */
14
Data types

 Derived Data Types
• A derived data type is a complex classification of a new data type that is
made up of simpler primitive data type. Derived data types have
advanced properties and uses far beyond those of the basic primitive
data types that operate as their essential building blocks.
• The following are examples for derived data types:
 Array: a kind of data structure that can store a fixed-size sequential
collection of elements of the same type. So instead of typing:
int var1, var2, var3, var4;
We can simply make them in array of 4 integers:
int Arr[4]; /* Uninitialized */
OR
int Arr[4] = {3,2,4,6}; /* Initialized */
Array consists of contiguous memory locations. The lowest index
Arr[0] corresponds to the first element and the highest index Arr[3]
to the last element. 15
Data types

 Derived Data Types (Array continue)
Array can be given value at declaration (Initialized array) or can have
no initial values at declaration.
Array should be fixed size, and size should equal number of
initialization elements:
int Arr[4]; /* Valid*/
int Arr[] = {3,2,4,6}; /* Valid, compiler will put size
= 4 as it is initialized */
int Arr[]; /* Invalid, compiler error */
int Arr[2] = {5,6,7,8}; /* Invalid, compiler error */
Accessing array is through array name and element index:
Arr[2] = 10; /* Change third element */
Exceeding array limits:
int Arr[10]; /* Declare array of 10 integers*/
Arr[15] = 10; /* Will not produce compilation error,
but at runtime it may cause a lot of problems (Data corruption,
program termination, …) */ 16
Data types

String doesn’t exist as a primitive data type in C language, but arrays
can be used to represent string in an array of characters:
char Arr[6] = ‚Hello‛;
Note that array size is number of string characters + 1, this extra
character is used to store the NULL ‘0’ terminator which marks the
end of string, so the following will lead to same result:
char Arr[6] = {‘H’,’e’,’l’,’l’,’o’,’0’};
This NULL terminator is used to know where string ends, for example
when required to print the string:
printf(‚%s‛, Arr) /* Will print Hello*/
Or by doing:
int i = 0;
while(Arr[i] != ‘0’)
printf(‚%c‛, Arr[i++]); /* Will print Hello*/
17
Data types

Till now this is called 1-D (1 dimensional) array.
In memory it looks like that:
18
Data types
Arr[0]
Arr[1]
Arr[2]
Arr[3]

C language also provides a multi-dimensional array, for example we
can create a 2-D array:
int Arr[3][4];
Which can be considered as a table with 3 rows and 4 columns:
19
Data types
column 0 column 1 column 2 column 3
row 0 Arr[0][0] Arr[0][1] Arr[0][2] Arr[0][3]
Generally we can any number of dimensions:
int Arr[size1][size2]………..[sizen];

2-D array initialization:
int Arr[3][4] = {
{1,22,13,40}, /* row 0 */
{7,4,45,7}, /* row 1 */
{23,34,83,47} /* row 2 */
};
It can be also initialized like that, but this is not recommended for
readability:
int Arr[3][4] = {1,22,13,40,7,4,45,7,23,34,83,47};
20
Data types

 Function: A function type describes a function with specified
return type. A function type is characterized by its return type and
the number and types of its parameters.
A function type is said to be derived from its return type, and if its
return type is int , the function type is sometimes called function
returning int.
/* Function that returns integer, and takes 2 parameters
first is integer and second is character*/
int myFunc(int x, char y);
This is called function declaration, or function signature.
21
Data types

 Pointers: A pointer is a variable whose value is the address of
another variable, i.e., direct address of the memory location:
int Var1 = 10; /* Var1 is integer contains value 10 */
int *ptr = &Var1; /* ptr is pointer to integer, its
value is the address of Var1 in
memory*/
22
Data types
Memory
Var1 = 10 0xFE00
Ptr = 0xFE00 0xFF04

 Derived Data Types (Pointers continue)
We can make the pointer points to another variable during runtime:
int Var1 = 10, Var2 = 30;
int *ptr = NULL; /* NULL Pointer points to nothing */
ptr = &Var1; /* Pointer now holds Var1 address */
*ptr = 20; /* Change Var1 value through pointer */
ptr = &Var2; /* Pointer now holds Var2 address */
*ptr = 50; /* Var2 will change to be 50 */
23
Data types

Pointer size: Pointer size depends on a lot of factors (hardware,
operating system, compiler, etc.), and not all pointer types on the
same platform may have the same size.
For example, there are embedded processors that use a Harvard
architecture, where code and data are in separate memory areas, and
each may have a different address bus size (e.g., 8 bits for data, 16 bits
for code). This means that object pointers (int *, char *, double *) may
be 8 bits wide, but function pointers (int (*)()) may be 16 bits wide.
So on 32-bits address bus controller:
char *ptr;
printf(‚%d‛, sizeof(ptr)); /* Print 4 bytes */
24
Data types

Pointer arithmetic: A pointer in c is an address, which is a numeric
value. Therefore, you can perform arithmetic operations on a pointer
just as you can on a numeric value but the result is different than
normal variables. There are four arithmetic operators that can be
used on pointers: ++, --, +, and - .
When we type:
long int Var1 = 10;
long int *ptr = &Var1;
ptr ++;
If Var1 address is for example 1000, then pointer value at the start is
the same as Var1 address which is 1000.
After ptr++ the value of ptr will become 1004, which means it is
incremented by 4 not just 1.
Pointer increment by 1 will increment pointer value by the size of the
variable it points at which is 4 bytes in our case. 25
Data types

Another example:
long int Var1 = 10;
long int *ptr = &Var1;
ptr += 3;
If Var1 address is for example 1000, then pointer value at the start is
the same as Var1 address which is 1000.
Adding 3 to ptr will make ptr value becomes 1012, which means it is
incremented by 12 (3 * 4 bytes = 12).
The same for other increment and decrement operations.
26
Data types

Pointer can point to any data type, for example pointer to array:
long int Arr[5] = {10,20,30,40,50};
long int *ptr = &Arr[0];
This will make ptr points at the first element in the array so:
printf(‚%d‛, *ptr); /* Will print 10 */
printf(‚%d‛, *(ptr+1)); /* Will print 20*/
printf(‚%d‛, *(ptr+2)); /* Will print 30*/
*(ptr+4) = 0; /*Will change last element in Arr to 0*/
Pointer can also be used like array with index:
printf(‚%d‛, ptr[2]); /* Will print 30 */
ptr[4] = 3; /*Will change last element in Arr to 3*/
27
Data types

Pointer can be used to represent string like arrays used:
char *ptr = ‚Hello‛;
printf(‚%s‛, ptr); /* Will print Hello */
This is the same as typing:
char ptr[] = ‚Hello‛;
Note: Array name is itself a pointer to the first element in the array, but it is a
constant pointer so Arr value can’t be changed by any means:
char Arr[6] = ‚Hello‛;
Arr++; /* Compilation error */
28
Data types

We can create an array of pointers:
long Var1 = 1, Var2 = 2, Var3 = 3;
long int *ptr[3] = {&Var1, &Var2, &Var3};
/* Now each element in ptr array is a pointer to variable */
printf(‚%d‛, *ptr[0]); /* Will print 1*/
29
Data types

We can have a pointer to pointer, which means that a pointer 2 holds
the address of pointer 1, and pointer 1 points to any variable:
long Var1 = 1;
long int *ptr1 = &Var1; /* ptr1 points to Var1 */
long int **ptr2 = &ptr1; /* ptr2 points to ptr1 */
30
Data types
*ptr1 = 10; /* Will change Var1 value to 10 */
**ptr2 = 20; /* Will change Var1 value to 20 */
*ptr2 = NULL; /* Will change value of ptr1 to NULL */
Var1 = 1
Address
0xFF00
ptr1 = 0xFF00
Address
0xFFE4ptr2 = 0xFFE4
Address
0xFFF8
Var1 = 20
Address
0xFF00
ptr1 = NULL
Address
0xFFE4ptr2 = 0xFFE4
Address
0xFFF8

We can make a pointer to function: A function pointer is a variable
that stores the address of a function that can later be called through
that function pointer.
int myFunc(char myChar)
{
printf(‚Character is %c‛, myChar);
return 0;
}
/* inside any code */
int (*ptr)(char) = myFunc;
/* Now ptr holds the address of function myFunc */
ptr(‘A’); /* Will call myFunc which will print A*/
(*ptr)(‘A’); /* The same as previous line*/
31
Data types

Pointers examples: what is the difference between all of this?
int a;
int *a;
int **a;
int a[10];
int *a[10];
int (*a)[10];
int (*a)(int);
int (*a[10])(int);
32
Data types

Pointers examples:
int a; /* An integer */
int *a; /* A pointer to an integer */
int **a; /* A pointer to a pointer to an integer */
int a[10]; /* An array of 10 integers */
int *a[10]; /* An array of 10 pointers to integers */
int (*a)[10]; /* A pointer to an array of 10 integers */
int (*a)(int); /* A pointer to a function a that takes an
integer argument and returns an integer */
int (*a[10])(int); /* An array of 10 pointers to
functions that take an integer argument
and return an integer */
33
Data types

 User Defined Data Types
Sometimes, the basic set of data types defined in the C language such
as int, float etc. may be insufficient for your application. In
circumstances such as these, you can create your own data types
which are based on the standard ones.
 Structure: a kind of data structure that can store a collection of
elements that are not of the same type. For example:
struct carInfo {
int speed;
float batteryLevel;
double kilometers;
char bodyTemperature;
};
This is called structure carInfo data type declaration, but till now
there is no variable of that type exists.
34
Data types

 User Defined Data Types (Structure continue)
To declare a variable of type carInfo:
struct carInfo myCar; /* Uninitialized */
OR
struct carInfo myCar = {0, 0, 0, 0}; /*Initialized*/
To edit any value of variable structure members:
myCar.speed = readCarSpeed();
myCar.batteryLevel = readBatteryLevel();
myCar.kilometers = readCarKilometers();
myCar.bodyTemperature = readBodyTemperature();
35
Data types

 User Defined Data Types (Structure continue)
We can define a pointer to structure:
struct carInfo *ptr = &myCar; /*ptr points to myCar*/
Editing structure members through pointer is achieved through arrow
operator ->
ptr->speed = readCarSpeed();
36
Data types

 Union: A union is a special data type available in C that allows to
store different data types in the same memory location. You can
define a union with many members, but only one member can
contain a valid value at any given time. Unions provide an efficient
way of using the same memory location for multiple-purpose:
union U_data {
int Var1;
double Var2;
char Var3;
};
To declare a variable of type U_data :
union U_data test; /* Uninitialized */
OR
union U_data test = {0}; /*Initialized*/
37
Data types

 User Defined Data Types (Union continue)
Union members share the same memory location, so editing any
member’s value will corrupt other members’ values:
test.Var1 = 32; /* all other members are now 32*/
test.Var2 = 10;/* first 2 bytes of the union
memory contains value 10, other
2 bytes are not changed*/
Size of Union variable test is the size of the biggest union member
(double in our example).
Using union saves memory, but a big caution should be taken to track
which member contains valid data.
38
Data types

 Enum: An enumeration is a user-defined data type consists of
integral constants and each integral constant is give a name:
enum week {sunday, monday, tuesday, wednesday, thursday,
friday, saturday};
First element in enum by default takes value 0 if not initialized
explicitly, so sunday will have value 0, monday will have value 1 and so
on.
Usage:
enum week today; /*Declare variable of type enum week*/
today = Wednesday; /* today value now is 3*/
39
Data types

 User Defined Data Types (Enum continue)
enum members can be initialized with default values:
enum week {sunday=5,monday,tuesday,Wednesday=9,thursday,
friday, saturday};
Now sunday will have value 5, monday 6, tuesday 7, but wednesday
will be 9 not 8, then thursday 10 and so on.
Note: Multiple members can have the same value, but member name
can’t exist twice.
We can add boolean data type to C language using enum:
enum boolean{ false, true};
enum boolean check; /* Now we can use check as boolean */
40
Data types

 typedef: We can define any data type we want from another data
type using typdef keyword:
typedef unsigned char uint8;
typedef unsigned short uint16;
typedef unsigned long uint32;
typedef unsigned long long uint64;
Now we can use the new defined data types:
uint8 Var1; /*Var1 is unsigned char*/
41
Data types

 User Defined Data Types (typedef continue)
typedef with structure:
typedef struct carInfo {
unsigned long speed;
unsigned short batteryLevel;
unsigned long kilometers;
unsigned char bodyTemperature;
} Car_Type;
Now instead of typing:
struct carInfo myCar;
We can type:
Car_Type myCar;
42
Data types

 User Defined Data Types (typedef continue)
We can simulate the behavior of typedef using #define like that:
#define uint8 unsigned char
uint8 Var1, Var2;/*Var1 and Var2 are now unsigned char*/
But the problem arises when trying something like that:
typedef unsigned char* u8_ptr_Type;
#define u8_ptr_Define unsigned char*
u8_ptr_Type ptr1, ptr2; /* Both will be pointers */
u8_ptr_Define ptr3, ptr4;
It is expected that both ptr3 and ptr4 are of type pointers to unsigned
char, but because #define is just text replacement then what really
happens is that previous line is replaced with:
unsigned char* ptr3, ptr4;
Which will make ptr3 successfully a pointer, but ptr4 will be normal
variable.
43
Data types

 Declaration vs. Definition
Declaration: A declaration tells the compiler the type of a variable,
object or function, but doesn’t allocates memory for it or know all
function details.
extern uint8 Var1; /* Variable extern */
void myFunc(uint8 Var2); /* Function prototype */
This is particularly useful if you are working with multiple source files,
and you need to use a function or variable in multiple files. You don't
want to put the body of the function or same variable in multiple files
statically, but you do need to provide a declaration for it.
Then in only one file you can put the definition.
44
Data types

 Declaration vs. Definition
Definition: A definition allocates memory for a variable or object and
is the implementation of a function.
uint8 Var1 = 10;
void myFunc(uint8 Var2) /* Function Definition*/
{
printf(‚myFunc definitionn‛);
}
Multiple declarations are allowed, but only one definition.
45
Data types

 Overflow vs. Underflow
Overflow: It is the condition that occurs when a calculation produces
a result that is greater in magnitude than the maximum value that a
given register or storage location can store or represent.
Unsigned overflow:
unsigned char Var1 = 255;
Var1++; /* Overflow */
printf(‚%d‛, Var1); /* Prints Zero */
Signed overflow:
signed char Var2 = 127;
Var2++; /* Overflow */
printf(‚%d‛, Var2); /* Prints -128*/
46
Data types

 Overflow vs. Underflow
Underflow: It is the condition that occurs when a calculation produces
a result that is smaller than the minimum value that a can be stored.
Unsigned underflow:
unsigned char Var1 = 0;
Var1 -= 8; /* Underflow */
printf(‚%d‛, Var1); /* Prints 248*/
Signed underflow:
signed char Var2 = -128;
Var2 -= 3; /* Underflow */
printf(‚%d‛, Var1); /* Prints 125*/
47
Data types

 Type Casting
Type casting is a way to convert a variable from one data type to
another data type. For example, if you want to store a 'long' value into
a simple integer then you can type cast 'long' to 'int'.
Explicit casting: You can convert the values from one type to another
explicitly using the cast operator as follows:
(type_name) expression
Example:
int Var1 = 8, Var2 = 20;
float Var3 = (float) Var2 / Var1;
Implicit casting: Implicit casting doesn't require a casting operator.
This casting is normally used when converting data from smaller
integral types to larger or derived types to the base type:
Var3 = Var2; /* Var2 value is implicitly converted to
float and put inside Var3*/
48
Data types

 Data Type Qualifiers
• Apart from the mentioned primitive data types, there are certain data
type qualifiers that can be applied to them in order to alter their range
and storage space and other properties to fit in various situations as per
the requirement.
• The data type qualifiers available in c are:
 Size modifier: affects size (number of bytes):
short int variable_name; /* 2 bytes integer */
long int variable_name; /* 4 bytes integer */
 Sign modifier: affects values range:
signed int variable_name; /* can have negative range */
unsigned int variable_name; /*can’t have negative range*/
 Constant modifier: affects ability to change it during runtime:
const char Var = ‘A’; /* can’t change at runtime */
50
Data Type Qualifiers

 Primitive Data Types Summary with size and sign modifiers
51
Data type Size Value range
char 1 -128 to 127 or 0 to 255
unsigned char 1 0 to 255
signed char 1 -128 to 127
int 2 or 4 -32,768 to 32,767 or
-2,147,483,648 to
2,147,483,647
unsigned int 2 or 4 0 to 65,535 or
0 to 4,294,967,295
short 2 -32,768 to 32,767
unsigned short 2 0 to 65,535
long 4 -2,147,483,648 to
2,147,483,647
unsigned long 4 0 to 4,294,967,295

 Primitive Data Types Summary with size and sign modifiers
52
Data type Size Value range Precision
float 4 1.2E-38 to
3.4E+38
6 decimal places
double 8 2.3E-308 to
1.7E+308
15 decimal
places
long double 10 3.4E-4932 to
1.1E+4932
19 decimal
places

It should be noted that the size and sign modifiers cannot be applied to
float and can only be applied to integer and character data types.
 Volatile modifier: tells the compiler that this variable can change
outside of the code control (ex. HW Register value):
volatile char Var = 10;
if (Var == 10)
{
/* Any code*/
/* If variable was not declared volatile, the compiler
may optimize this part by considering the condition
always True and remove the if condition check ! */
}
When a variable is declared volatile, any reference for that variable makes
the compiler reload its value from RAM again to make sure it is the last
updated version of that variable. 53

 Storage classes: A storage class defines the scope (visibility) and
life-time of variables and/or functions within a C Program.
o auto: The auto storage class is the default storage class for all
local variables.
int variable_name = 20;
o register: 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 and can't have the unary '&' operator applied to it
(as it does not have a memory location).
/* Compiler will try to put this variable inside
processor registers */
register int variable_name = 10;
54

 Storage classes:
o static: 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 or
functions. When this is done, it causes that variable or function’s
scope to be restricted to the file in which it is declared.
o extern: The extern storage class is used to give a reference of
a global variable or function that is visible to ALL the program
files. When you use 'extern', the variable cannot be initialized
and function can’t have a body however, it points to variable
or function name that has been previously defined in another
file.
55

 Storage classes:
56
File1.c
static int variable_1 = 10;
int variable_2 = 20;
File2.c
static int variable_1 = 50;
extern int variable_2;
/* Then inside any part of code */
printf(‚variable_1=%d, variable_2=%d‛, variable_1, variable_2);
/* It will print variable_1=50, variable_2=20 */

 Scope
58
Scope and Lifetime
Local scope
File scope
Global scope

 Scope
• The area of our program where we can actually access our variable is
the scope of that variable.
 Local scope: visible within function or statement block from point
of declaration until the end of the block.
void myFunc()
{
uint8 Var1 = 10;
{
uint8 Var2 = 11;
printf(‚%d, %d‛, Var1, Var2);
}
printf(‚%d‛, Var2);
}
Var1 is visible inside all myFunc, but Var2 is only visible inside its inner
block { } which is a local inner scope. 59
Scope and Lifetime

 Scope
 File scope: visible within current file only, but its is visible to all
functions inside this file, by using static keyword with variables or
functions.
60
Scope and Lifetime
File1.c
static int Var1 = 50;
void myFunc1()
{
}
void myFunc2()
{
}
File2.c
void myFunc3()
{
}
File3.c
static int Var1 = 20;
/* Same name, but totally
different variable */

 Scope
 Global scope: visible everywhere, it is defined in one file, and all
other files can use it using extern keyword.
61
Scope and Lifetime
File1.c
int Var1 = 50;
void myFunc1()
{
}
void myFunc2()
{
}
File2.c
extern int Var1;
void myFunc3()
{
}

 Lifetime
• Life time of any variable is the time for which the particular variable
outlives in memory during running of the program.
 Automatic: An automatic variable has a lifetime that begins when
program execution enters the function or statement block or
compound and ends when execution leaves the function or block.
Automatic variables are stored in a "function call stack".
void myFunc()
{
uint8 Var1 = 10;
{
uint8 Var2 = 11;
/* Var2 scope and life time ends here */
}
/* Var1 scope and life time ends here */
}
62
Scope and Lifetime

 Lifetime
 Dynamic: The lifetime of a dynamic data begins when memory is
allocated (e.g., by a call to malloc()) and ends when memory is
deallocated (e.g., by a call to free()). Dynamic data are stored in
"the heap".
/* Allocate 1000 bytes in RAM (Heap part) */
uint8 *myData = (uint8*) malloc(1000);
/* Any code */
free(myData); /* Data is deallocated, so its
life time ends here */
63
Scope and Lifetime

 Lifetime
 Static: A static variable is stored in the data segment of the "object
file" of a program. Its lifetime is the entire duration of the
program's execution.
Static variable is initialized only the first time function is entered, and
its value remains in memory between function calls.
void myFunc()
{
static uint8 Var1 = 10;
Var1++;
}
First call to myFunc will print 10, second call 11, third call 12 and so on.
64
Scope and Lifetime

 What is Make (GNU Make as an example)
• GNU Make is a tool which controls the generation of executables and
other non-source files of a program from the program's source files.
• Make gets its knowledge of how to build your program from a file called
the makefile, which lists each of the non-source files and how to
compute it from other files. When you write a program, you should
write a makefile for it, so that it is possible to use Make to build and
install the program.
66
Makefile introduction

 Capabilities of Make
• Make enables the end user to build and install your package (in case
you provided them with source code) without knowing the details of
how that is done because these details are recorded in the makefile
that you supply.
• Make figures out automatically which files it needs to update, based on
which source files have changed. It also automatically determines the
proper order for updating files, in case one non-source file depends on
another non-source file.
As a result, if you change a few source files and then run Make, it does
not need to recompile all of your program. It updates only those non-
source files that depend directly or indirectly on the source files that you
changed.
• Make is not limited to any particular language. For each non-source file
in the program, the makefile specifies the shell commands to compute
it. These shell commands can run a compiler to produce an object file,
the linker to produce an executable, ar to update a library..etc. 67

 makefile structure
• Make searches for a file called makefile without any extension to do its
work.
• Basic building elements of make file are:
 Target: What is the output of current step.
 Dependencies: What should be already available to start making
current target.
 Commands: What should be done to make current target.
All of them are called “Rule”. A rule in the makefile tells Make how to
execute a series of commands in order to build a target file from source
files. It also specifies a list of dependencies of the target file. This list
should include all files (whether source files or other targets) which are
used as inputs to the commands in the rule.
68

• Rule structure:
69
• Target name is typed at start of line, then followed by a colon : then
followed by list of all dependencies that should be available to start
executing commands, then on each new line write a new command to
be executed, each command should have a Tab space before it or the
make will not work.
• Dependencies can be on one line, or can be divided on multiple lines:
Target: dependency1 dependency2 ...
commands
...
Target: dependency1 dependency2
dependency3 ...
commands
...

• Rule example:
70
D:newProject>make file.o
file.o: file.c
gcc -c file.c -o file.o
Create any C code file with the name file.c and put it in the same directory
with the makefile.
Then by running Make through command line:
Create a text file with the name makefile without any extension and type
the following inside it:
 The Make utility will search for the file makefile, then it will search
inside it for the required target file.o .
 It will search what are file.o dependencies, it will find that it only
depends on file.c.

 It will check “is the required dependency file.c available ?”, the Make
file will find it in the same directory.
 After the Make utility finds file.c, it will execute the commands under
file.o target which is here compiling file.c, so the output of this
command will be:
71
D:newProject>make file.o
gcc -c file.c -o file.o
 Now you will find in the same directory a file created called file.o which
is the compiled output file of file.c .

• If the Make didn’t find the required dependency file, it will search if this
dependency is another required target then do it first:
72
app.exe: file1.o file2.o
gcc file1.o file2.o -o app.exe
file1.o: file1.c
gcc -c file1.c -o file1.o
file2.o: file2.c
Create a C code file with the name file1.c which contains the main
function, create another C code file file2.c with any code, and put them in
the same directory with the makefile.

73
Run the Make and the output will be:
D:newProject>make app.exe
Now you will find in the same directory a file called app.exe which is the
compilation output.

• If the Make didn’t find the required dependency file in the directory or
as any other target then it will generate this error and stop:
74
D:newProject>make newFile.c
make: *** No rule to make target `newFile.c'. Stop.
• If Tab space doesn’t exist before any command then Make will generate
this error and stop (note that 5 is number of first line containing error):
D:newProject>make app.exe
makefile:5: *** missing separator. Stop.
• When you run Make, you can specify particular targets to update;
otherwise, Make updates the first target listed in the makefile. Of
course, any other target files needed as input for generating these
targets must be updated first.

• Usually makefile contains 2 targets called all and clean to be used by
any one without knowing the details of our makefile content:
75
# all target is usually put as the first target
all: app.exe
clean:
rm file1.o file2.o app.exe
app.exe: file1.o file2.o
file1.o: file1.c
file2.o: file2.c

• So typing all target will output the same result as typing app.exe, and
typing clean target will delete file1.o, file2.o and app.exe .
76
D:newProject>make clean
• clean and all targets are called “Phony Targets”. A phony target is one
that is not really the name of a file, rather it is just a name for a recipe
to be executed when you make an explicit request, it may have
dependencies like all target and may not have any dependencies like
clean target.

 Make variables
• A variable begins with a $ and is enclosed within parentheses (...) or
braces {...}. Single character variables do not need the parentheses.
Example:
77
CC = gcc
file1.o: file1.c
$(CC) -c file1.c -o file1.o

 Make variables
• Automatic Variables:
Automatic variables are set by make after a rule is matched. They include:
$@: the target filename.
$*: the target filename without the file extension.
$<: the first prerequisite filename.
$^: the filenames of all the prerequisites, separated by spaces, discard
duplicates.
$+: similar to $^, but includes duplicates.
$?: the names of all prerequisites that are newer than the target,
separated by spaces.
78

 Make variables
Example 1:
79
file1.o: file1.c
$(CC) -c $< -o $@
# It is the same as typing $(CC) -c file1.c -o file1.o
Example 2:
OBJ_FILES = file1.o file2.o
app.exe: $(OBJ_FILES)
$(CC) $^ -o $@
# It is the same as typing:
# $(CC) file1.o file2.o -o app.exe

 Make variables
Example 3:
80
OBJ_FILES = file1.o file2.o
LINK_TARGET = app.exe
CLEAN_TARGET = $(LINK_TARGET) $(OBJ_FILES)
all: $(LINK_TARGET)
# all: app.exe
clean:
rm $(CLEAN_TARGET)
# clean:

 Make implicit rules
Implicit rules are a set of generalized instructions for doing certain tasks,
where the instructions are provided as default.
For example, an implicit rule can tell Make utility how to construct a .o file
from a .c file without explicitly typing files names.
Example 1:
81
%.o: %.c
$(CC) -c $< -o $@
# It means ‚whenever you find a required target with
#any name but with extention .o and there is no
#explicit rule for how to make this target, then
#execute the next commands on this target.

 Make implicit rules
Example 2:
82
%.o: %.c
$(CC) -c $< -o $@
file1.o: file1.c
$(CC) -c file1.c -o explicit_file1.o
app.exe: file1.o file2.o file3.o
$(CC) $^ -o $@
# For file2.o and file3.o it will use the implicit
#rule, so their output names will be file2.o and
#file3.o but for file1.o it will find an explicit rule
#for it then it will execute it, so its output name
#will be explicit_file1.o

 Creating project hierarchy
Most of the time the project contains different folders, not just all source
and headers files in the same place, for example if we need to make one
folder for source code files with name “src” and another folder for header
files with name “inc”, then in the makefile we should do the following
(assuming that the makefile is in the same directory as both folders):
83
# Tell makefile Where to find source files
vpath %.c ./src
# Header files path
INCLUDE_PATH = ./inc
%.o: %.c
$(CC) -c -I$(INCLUDE_PATH) $< -o $@
# -I$(INCLUDE_PATH) tells the compiler where to find
#header files

 Header files issue
After running make all, if we edited any source file the Make utility will
recompile this file only and targets that depends on it.
But if we changed any header file the make utility will not even notice and
will not make any recompilation for the source code that depends on this
file which we don’t want. This is because we didn’t tell the Make utility any
thing about header files dependencies inside the makefile.
That’s why we should make dependencies generation, which makes the
compiler enters all source files, extract information about what header
files that each source file depends on, generate a corresponding
dependency file for this source file and include this information in our
makefile.
After making these steps, whenever we change a header file, any source
file that depends on this header file will be recompiled and any target that
depends on this source file will be made again.
84

 Header files issue (solution)
85
# Generate dependencies list
# Dependencies output path
DEPS_PATH = ./dep/
# Source files path
SRC_PATH = ./src/
# Header files path
INCLUDE_PATH = ./inc/
# Get source files names with path
SRC_FILES_ABSOLUTE = $(wildcard $(SRC_PATH)*.c)
# Get source files names without path
SRC_FILES = $(patsubst $(SRC_PATH)%.c,%.c,$(SRC_FILES_ABSOLUTE))

86
# Make dependencies names the same as sources names but with extension .d
DEPS_FILES = $(patsubst %.c,%.d,$(SRC_FILES))
# Get dependencies files names with their path
DEPS_FILES_ABSOLUTE = $(patsubst %.d,$(DEPS_PATH)%.d,$(DEPS_FILES))
# Rule to make dependencies files
$(DEPS_PATH)%.d : %.c
# Generate dependencies files (Small make file for each c file)
$(CC) -MM -MP -MT$@ -I$(INCLUDE_PATH) $< -o $@

87
# Include all dependencies make files inside current make to be
#used in #linking target to detect when any header file changes
-include $(DEPS_FILES_ABSOLUTE)
# Add dependencies to our target
app.exe: $(OBJ_FILES) $(DEPS_FILES_ABSOLUTE)
$(CC) $^ -o $@

 Make simple complete example
Create 3 different folders:
• src: contains all your source files.
• inc: contains all your header files.
• dep: empty, will be the output for dependencies files.
Create empty text file with the name makefile without any extension and
put the following inside the file:
88

89
# Tell makefile Where to find source files
vpath %.c ./src
# Compiler used
CC = gcc
# Dependencies output path
DEPS_PATH = ./dep/
# Source files path
SRC_PATH = ./src/
# Header files path
INCLUDE_PATH = ./inc/

90
# What is our target name
LINK_TARGET = app.exe
# Objects used later to be linked to generate our link target
OBJ = main.o
Func1.o
Func2.o
Func3.o
# What will be used at clean target
CLEAN_TARGET = $(LINK_TARGET) $(OBJ)
# Used with > make all
all:$(LINK_TARGET)
echo Bulding done !

91
# Used with > make clean
clean:
rm $(CLEAN_TARGET)
echo Cleaning done !
# What will happen when requiring $(LINK_TARGET)
$(LINK_TARGET): $(OBJ) $(DEPS_FILES_ABSOLUTE)
$(CC) $(OBJ) -o $@
echo Linking done !
# What will happening when requiring any target with any file name with
#extension .o
%.o: %.c
$(CC) -c -I$(INCLUDE_PATH) $< -o $@
echo File $< compilation done !

92
# Generate dependencies list
# Get source files names with path
SRC_FILES_ABSOLUTE = $(wildcard $(SRC_PATH)*.c)
# Get source files names without path
SRC_FILES = $(patsubst $(SRC_PATH)%.c,%.c,$(SRC_FILES_ABSOLUTE))
# Make dependencies names the same as sources names but with extension .d
DEPS_FILES = $(patsubst %.c,%.d,$(SRC_FILES))
# Get dependencies files names with their path
DEPS_FILES_ABSOLUTE = $(patsubst %.d,$(DEPS_PATH)%.d,$(DEPS_FILES))

93
# Rule to make dependencies files
$(DEPS_PATH)%.d : %.c
#Generate dependencies files (Small make file for each c file)
$(CC) -MM -MP -MT$@ -I$(INCLUDE_PATH) $< -o $@
# Include all dependencies files inside current make to be used in
#linking target to detect when any header file changes
-include $(DEPS_FILES_ABSOLUTE)

Mohamed AbdAllah
Embedded Systems Engineer
mohabdallah8@gmail.com
95

Embedded C - Lecture 2

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