Introduction to C Programming for Embedded Programming

1
Introduction to C Programming for Embedded
Programming

• Data memory layout, Function and stack, and
  library functions

2
Memory Usage

• Common memory sections
• Code holds binary instructions of each function
• Static data stores static data accessible to all
  functions
• Stack stores run-time data local to each
  function
• Heap holds dynamically allocated data
• I/O does not really hold anything

3
Memory Usage
High end

• static char greeting Hello world!
• main()
• int i
• char bVal
• LCD_init()
• LCD_PutString(greeting)

I/O addresses
Stack(grows down)
Heap (grows up)
Static Data
Code
Low end
4
Variable Scope and Storage

• Global data
• Declared outside all C functions
• Visible to all C functions
• http//class.ece.iastate.edu/CprE211/labs/lectures
  /elevator.htm

5
Variable Scope and Storage

• Variables with static storage
• Storage space allocated in data sections
• Life time is from the start to the end of program
  execution

6
Variable Scope and Storage

• Variables in stack
• Local variables of a function go into stack
  storage
• Dynamically allocated and de-allocated every time
  the function is called
• Variable life time is from the allocation to the
  deallocation

7
Variable Scope and Storage

• scope Where a variable is visible
• int m
• int any_func()
• int m
• m n 5

8
Variable Scope and Storage

• C global variable (visible to all program files)
• int global_var
• C file-wide static variables (visible only in
  this file)
• static int static_var
• Local static variables
• any_func()
• static int static_var

9
Variable Scope and Storage

• Example How to define and use global variables
• In header file myvar.h
• extern int global_var
• In program file myvar.c
• include myvar.h
• int global_var
• In program file usevar.c
• include myvar.h
• / use myvar /

10
Data Memory Layout

• Alignment A scalar variable of n bytes must be
  aligned on n-byte boundaries
• Examples
• char no restriction
• short two-byte boundaries
• float four-byte boundaries

11
Data Memory Layout

• Example Whats the memory layout? Whats the
  address of each variable? Assume a starts at
  address 0x20000000.
• char a5
• short b
• int d
• double e
• short f
• char g

12
Data Memory Layout

• Byte-ordering Big-endian vs. Little-endian
• Big-endian Byte 0 of a n-byte variable is the
  Most Significant Byte (MSB)
• Little-endian Byte 0 of a n-byte variable is the
  Least Significant Byte (LSB)

13
Data Memory Layout

• Whats the value in string with the two endian
  types?
• static char string8
• (int) string 0x41424344
• string4 \0
• Note ASCII 0x41 is A.

14
Data Memory Layout

• Address Memory Contents________
• 30001FF8 25 24 5B 1A AC 57 9C 8D
• 30002000 00 FE 25 24 5B 1A EE 05
• 30002008 AC AE 81 83 45 67 89 25
• 30002010 01 23 55 3F AC CB F0 8D
• What are the variable contents on MPC555?
• char a5
• short b
• int d
• double e
• short f
• char g

15
Data Memory Layout

• What is the size of the following structure?
• Each component must be aligned
• Total size must be a multiple of eight
• struct mystruct
• int a
• short b
• double c
• char d

16
Function and Stack

• Stack LIFO structure (Last-in, First-out)
• Stack top and stack base to track usage
• Push operation add an item at the top and update
  the top
• Pop operation remove an item from the top and
  update the top

17
Function and Stack

• Conventional stack grows downwards

high address
stack top
stack growth
low address
18
Function and Stack

• Function Frame Local storage for a function
• Example 1. A is called 2. A calls B 3. B calls
  C 4. C returns

19
Function and Stack

• Why stack?
• The LIFO order matches perfectly with functions
  call/return order
• Efficient memory allocation and de-allocation

20
Function and Stack

• Determine frame layout
• Each variable is aligned
• Each frame is on eight-byte boundaries
• Example What is the frame size? (Add 8 bytes for
  return address and frame linking)
• int myfunc()
• char byVal
• long lVal
• char szName10
• / statements start here /

21
Function and Stack

• Function pointer How does it work?
• A function is translated into a block of machine
  instructions
• Calling the function is to transfer the program
  control to the starting address of the block
• At hardware level this is done by setting the
  program counter
• At assembly programming level this is to branch
  (jump) to that address
• A function pointer variable stores the pointer to
  the starting address of the block

22
Function and Stack

• Function pointer Use example
• int average(int x, int y)
• int main()
• int (pFunc)(int x, int y) average
• int a (pFunc) (10, 20)

23
C Library Functions

• In C many things are carried out by library
  functions
• Simple language, rich libraries
• Commonly used libraries
• File I/O (include user input/output)
• String manipulations
• Mathematical functions
• Process management
• Networking

24
C Library Functions

• Use standard file I/O
• / include the header file for I/O lib /
• include ltstdio.hgt
• main()
• / use the fprintf function /
• fprintf(stdout, s\n, Hello World\n)

25
C Library Functions

• Formatted output printf, fprintf, sprintf and
  more use conversion specifiers as follows
• s string
• d signed decimal
• u unsigned decimal
• x hex
• f floating point (float or double)
• How to output the following variables in format
• a , b , c , str in a single line?
• int a
• float b
• int c
• char str10

26
C Library Functions

• Formatted input scanf, fscanf, sscanf and more
• Use similar conversion specifiers as the printf
  family
• How to read the following variables in format
• a , b , c , str in a single line?
  (Use scanf)
• int a, b
• float c
• char str10

27
C Library Functions

• String operations copy, compare, parse strings
  and more
• include ltstring.hgt
• strcpy copy one string to another
• strcmp compare two strings
• strlen calculate the length of a string
• strstr search a string for the occurrence of
  another string

28
C Library Functions

• Error processing and reporting use exit function
• include ltstdio.hgt
• include ltstdlib.hgt
• ...
• void myfunc(int x)
• if (x lt 0)
• fprintf(stderr, s\n,
• x is out of range)
• exit(-1)

29
C Library Functions

• Math library functions
• include ltmath.hgt
• n round (x) / FP round function /
• To build
• gcc W o myprogram lm myprogram.c

30
C Library Functions

• How to find more?
• On Linux machines Use man
• man printf
• man string
• man string.h
• man math.h