Advanced C Language for Engineering Course

Advanced C Language for Engineering © V De Florio
ATHENS CourseATHENS Course
Advanced C Language
for Engineering
Vincenzo De Florio
17 – 21 November 2003
V.ppt-1.1
2003-11-10

2
Advanced C Language for Engineering ( V De Florio)
Overall structureOverall structure
• An introduction to C
• The FILE methodology. Classes. Data hiding.
Examples (class assoc, objalloc, vf. . . )
• Literate programming. The cweb tools
• The GNU autotools
• Other classes for embedded systems (class tom,
the DepAuDE framework)
• Linguistic support using C, Lex and YACC.
Examples (the icgi interpreter, class FN, PvmLinda,
the Ariel language...)
• Exercise sessions, case studies, project works

3
StructureStructure
• Introduction
• First examples
• Variables
• Fundamental data
types
• Loops
• Conditional statements
& other instructions
• The C preprocessor
• Functions
• Variables again
• Operators
• Standard I/O functions
• Derived types
• Pointers and arrays
• Use of pointers
• Type casts &
conversions
• Bitwise operators
• Standard streams, files
• Structures, bitfields,
unions
• LLP
• Data hiding
http://www.esat.kuleuven.ac.be/~deflorio/c/athens03.prn.gz

4
First examplesFirst examples
main ( )
{
printf(“hellon”);
}
Function name
Function with
no arguments
Program
starts here...
...and ends
here
Print string
“hello” and
character ‘n’

5
• Log into the system
login name
password
• Type in the hello program in file first.c
Activate an editor, e.g., xedit, pico, or vi
for instance xedit first.c
save the file at the end
• Compile the program
cc -o first first.c (or gcc -o first first.c)
• Execute the program
./first
• Modify the program so that
it prints another string / two strings / …
jump to “Compile the program”

6
First ExamplesFirst Examples
printf
followed
by “(“
means
“Call
function
printf”

7
main() { int inf, sup, step;
float fahr, celsius;
inf = 0; sup = 300;
step = 20;
fahr = inf;
while (fahr <= sup) {
celsius = (5.0/9.0) * (fahr-32.0);
printf("%4.0f %6.1fn", fahr, celsius);
fahr = fahr + step;
}
}
Integer and
real numbers
Loop
start,
loop
end
Format
string

8

9
• Type in the conversion program in file second.c
• Compile the program
• Execute the program
• Modify the program:
Change the format strings
Change steps
Downto vs. To
(5 / 9.0)
(5 / 9)
fahr - 32 vs. fahr - 32.0
fahr += fahr
jump to “Compile the program”

10
Definition of Variables (1)Definition of Variables (1)
• To define a variable or a list of variables, one has to
mention:
TYPE LIST ;
• For instance:
int a, b_b, c2, A;
double f, F=1.3, dimX;
• With LIST one can also initialize variables, like it has
been done for F in the above example
• One can define variables only at the beginning of a
compound instruction such as
{ TYPE LIST; … ; INSTRUCTIONS }

11
main() { int inf, sup, step;
float fahr, celsius;
inf = 0; sup = 300;
step = 20;
fahr = inf;
while (fahr <= sup) {
celsius = (5.0/9.0) * (fahr-32.0);
printf("%4.0f %6.1fn", fahr, celsius);
fahr = fahr + step;
}
}
init test
increment
for (fahr = inf; fahr <= sup; fahr += step) {...

12
• Fahr += step == Fahr = Fahr + step
Fahr++ == Fahr = Fahr + 1
• Compound statements in C:
 { s1; s2; …; sn; }
 s1, s2, …, sn;
• Relational operators
 <=, <, !=, ==, >, >= ...
 Return 0 when false, 1 when true
• “test” is any operation
• 0 = false, non-0 = true
• Both these statements are valid:
while ( x == y ) { …
while ( x = y ) { …

13
1. Let a be 0
2. Repeat
the loop
while
“a = 0”
is “true”
3. So the
while loop
is simply
ignored!

14
1. Let a be 1
(returns 1)
2. Repeat
the loop
while
“a = 1”
returns “true”
3. So the
while loop
never ends!

15
The initial
value of a
is undefined!
Here is -85899…
Here is 0 (cygwin)

16
Fundamental data typesFundamental data types
• Four types of integers:
char, short, int, long
• Two types of floating point numbers:
float, double
• Corresponding formats:
%c, %d, %d, %ld
%f, %lf
• Sizes:
sizeof(char) <= sizeof(short) <= sizeof(int) <=
sizeof(long)
sizeof(float) <= sizeof(double)
• Practically speaking:
char==1, short==2, int==2 or 4, long==4
float==4, double==8

17
• Check the actual size of type on your workstations
with function sizeof()

18
Same result on:
Linux / gcc
SunOS / (g)cc
HP-UX / cc
Win / m$ visual c
But this is not
100% guaranteed
on all systems!

19
TypeType ““charchar””
• A char in C is just an integer
• The only difference between a char and, e.g., an int
lies in the number of bytes that are used
• In C, we can write, e.g.
int i;
char c;
i = ‘A’;
c = 65;
if ( i != c ) printf(“not an “);
printf(“ASCII systemn”);
• A char is a small number that, when sent, e.g.,
to the screen, corresponds to a character
• In C there is no difference between
A character
The code of character

20
• Symbols such as ‘A’ are just symbolic constants
• Think of them as a #define statement:
#define ‘A’ 65
#define ‘B’ 66
. . .
• Every time you encounter a
APOSTROPHE CHARACTER APOSTROPHE
you are dealing with a symbolic constant that
defines a small integer, that when printed can be
interpreted as a character

21
• Example
• A = 66;
printf(“ As a number, A is t%dn”, A);
printf(“ As a character, A is t%cn”, A);
%c ==
“print as a
character”
%d ==
“print as
an integer”

22
Types: some formatting charactersTypes: some formatting characters
• %d : print as an integer
%c : print as a character
%ld : print as a long
%f : print as a float
%lf : print as a double

23

24

25
Types: an interpretation of a sameTypes: an interpretation of a same
““substancesubstance”” –– bytes!bytes!
(To be explained later on)
The first four bytes of
A are interpreted in
different ways

26
Loops in CLoops in C
While loops: while ( op1 ) op2;
• No initialisation
• Both op1 and op2 are operations
returning integers
• while op1 is non zero, do op2
• op2 can be a compound instruction, e.g.,
{ op21; op22; … ; op2N }
• relational operations return an integer!
• a <= b is 1 when a<=b and 0
otherwise

27
• Which of these loops are correct? And what do they
mean?
 while ( -25 ) a++ ;
 while ( read_next_character ( ) ) car = car + 1;
 while ( a + 1 ) { a = a + 1; b = 0; }
 while ( a ) a = b, b = tmp, tmp = a;

28
While loops: do op2 while ( op1 );
• No initialisation
• Both op1 and op2 are operations
returning integers
• Do op2; then, while op1 is not zero, do
op2 again
• op2 can again be a compound
instruction, e.g.,
{ op21; op22; … ; op2N }

29
• For loops: for ( op1; op2; op3 ) op4;
• Operation op1 is the initialisation
• Operation op2 is the condition
• Operation op3 is the conclusive part
• Operation op4 is the body of the loop,
• Both op1 and op3 can be compound (comma-
based!) instructions
• Usually used for iterations, e.g.
for (i=0; i<n; i++) { do_that(); }
• Exercise: modify second.c so to change the
while into a for.
• Exercise: modify the previous example so to
print the table in inverse order.

30
• Which of these loops are correct? And what do they
mean?
 for (;;) a++;
 for (; a<5; a++) ;
 for ( i=0; i<n; { i++; n--; } ) … /* do something */
 for ( i=0, j=n; i<n; i++, j-- ) … /* do sthg */

31
• Loops are an important source of performance for
the compiler and for optimizers
• Some computer architectures and compilers can
exploit them to achieve a very high speed-up
• This is called
“loop level parallelism (LLP) exploitation”

32
ConditionalConditional statementsstatements
• Statement “if”:
 if (condition) action1; else action2;
• “?:”
 (condition)? action1 : action2;
• Statement “switch”:
 switch(const integer expression) {
case value1: op1; break;
case value2: op2; break;
…
case valuen: opn; break;
default: opn+1;
}

33
Other instructionsOther instructions
• break;
• continue;
• goto label;
y:
while (a < b) {
break;
b--;
}
x:
…
a++;
…
break; continue;
goto x;
goto y;

34
The C PreprocessorThe C Preprocessor
• The C compiler was originally composed of four
components:
 cpp | cc | as | ld
 .c -> .i -> .s -> .o
• Component cpp is the C preprocessor
 cpp looks for preprocessor commands (#cmd’s)
#cmd’s start with “#” on column 1
 cpp converts a text file f into a text file f’
 All the valid “#”-statements are removed and
substituted with C strings or text files

35
• Examples:
• #define BEGIN {
• #define END }
• #define IF if(
• #define THEN )
• #define INT32 long
• #define MAX(A, B) ((A) > (B)? (A):(B))
• #define SQR(x) x * x
• IF SQR(x) == x THEN printf(“x==1n”);
is translated into
if( x * x ==x ) printf(“x==1n”);
Dangerous

36
• Inclusion of external files
1. #include <stdio.h>
2. #include “assoc.h”
• #ifdef … [ #else … ] #endif
• #ifndef … [ #else … ] #endif
• Differences between 1. and 2.
• Use of ifdef/ifndef.

37
• #include <stdio.h> is equivalent to
• #include “prefix/stdio.h”
• On many UNIX systems, prefix == /usr/include
• /usr/include contains the header files of the C
standard library
 stdio.h, string.h, time.h, ctype.h, math.h, …
• A header file is the user interface of a library or a
part of a library
• stdio.h defines the interface to a subset of the C
standard library
• stdio.h interfaces the standard I/O functions and
defines symbols thereof

38
FunctionsFunctions
• Functions are names that points to some memory
location where code has been stored by the
compiler
• They take arguments and return a value of some
type
E.g., double cos(double a);
This is a function prototype, with which we declare a
function, called cos, that expects and returns a double
• To call a function, we just name it and pass an
argument to it
cos (3.1415); /* cos (pi) */

39
FunctionsFunctions
• The name of a function is just a number – its
address in the code segment

40
FunctionsFunctions
• Functions can be
Declared
Defined
• A prototype declares a function
“There’s a function that looks like that and that…”
• Defining a function means specifying what the
function shall do
“The function does this and this…”
Memory is allocated in the code segment

41
FunctionsFunctions
• To define a function one has to supply the
compound instruction that it has to execute:
double addd (double a, double b);
double addd (double a, double b)
{
return a + b;
}
Prototype (declaration)
Definition

42
FunctionsFunctions
• Recursion is allowed
• C supports a single-level of functions that
can be compiled separately

43
FunctionsFunctions
• return closes the function and returns an output
value (default: integer) to the caller.
• Arguments are passed by value: this means that the
arguments are copied in temporary variables.
• The only way to let a function modify an argument is
by passing the address of the object to be modified.
• Operator & returns the address of a variable.

44
FunctionsFunctions
• Functions have the following structure:
• [ type ] name ( [ args ] ) {
[ declarations ] [ instructions ]
}
• int main() {
int i; int power (int, int);
for (i=0; i<10; ++i)
printf("%d %dn", i, power(2,i));
}
int power (int x, int n) { int i, p;
for (i = p = 1; i <= n; ++i)
p=p*x;
return (p);
}

45
• Two classes of variables
Dynamically allocated
Statically allocated
• A variable is dynamically allocated when it is local to
a function or a compound instruction and is not
declared as “static”
• Examples
main () {
int i;
for (i=0; i<n; i++) {
int j;
j = i * i;
}
}
Again, on VariablesAgain, on Variables
Automatic
variables
(hold undefined
value)

46
• If the variable is an automatic one, then the
initialisation is done each time the variable is
allocated (each time the function in which the
variable resides is called and the corresponding
compound instruction is executed).

47
• A variable is defined at compile time when
it is a global variable (I.e., a variable defined outside any
function)
It is a variable defined as “static”
• Example
int fd;
int open(int tmp) {
int j;
static int first;
}
Global
Automatic
Static

48
• C is case sensitive: int J; float j; is valid
• The scope of variables is such that a new name
overrides an old one:
int h = 3;
{ int h = 4;
printf(“h == %dn”, h);
}
printf(“h == %dn”, h);

49
OperatorsOperators
• binary +, -, *, /, %
• unary -
• precedences:
• (+,-) Ð (*,/,%) Ð (-) Ð (||)
Ð (&&) Ð (==, !=) Ð (> >= < <=)
• Note:pa Ð opb if opb has higher precedence than opa

50
OperatorsOperators
• Expressions such as:
i < lim && (c=getchar()) != ’n’ && c != EOF
do not require extra parentheses:
= Ð !=, hence parentheses are required around
c=getchar().
• Logic clauses are evaluated left-to-right. Evaluation
stops when the truth value of a logic expression is
found.

51
Standard functions for input/outputStandard functions for input/output
• Defined in stdio.h
• Require the stdio.h file to be included
• This defines functions such as:
int getchar();
int putchar(int c);
• getchar reads the next character in the standard
input stream or an end-of-file value (EOF)
• getchar returns the character as one byte stored
into a 2-byte or 4-byte integer (an int value)
• Putchar puts on the standard output stream the
character we pass as an argument

52
void main()
{ char c;
c = getchar();
while ( c != EOF ) {
putchar ( c );
c = getchar();
}
}
Are any
faults
present?
Which
ones?
c is declared
as a char
getchar either
returns a
char or EOF

53
void main()
{ char c;
c = getchar();
while ( c != EOF ) {
putchar ( c );
c = getchar();
}
}
c is a char
getchar
returns
an int!
This loop
may never be
verified

54
void main()
{ int c;
while ( c = getchar() != EOF ) {
putchar ( c );
}
}
Are any
faults
present?
Which
ones?
( )
implicit
parenthesesOK
If stdin = ‘h’ ‘e’ ‘l’ ‘l’ ‘o’, EOF
what goes to stdout?

55
ExercisesExercises
• Exercise: the just seen program can be easily
adapted to work, e.g., as a “word counter” (reports
the number of characters, words, and lines in the
input), or as a filter to remove blanks or tabs, and so
forth
• Input and output can be redirected with < and >.
Pipelines of programs can be built by chaining the
programs with |

56
Derived typesDerived types
• The following operators define complex types
derived from the basic types of C:
* operator pointer-to,
[] operator vector-of,
() operator function-pointer-to

57
• char *p; : p is of type “pointer-to-chars”
• float v[10]; : v is a vector, i.e., a pointer to the
beginning of a memory area allocated by the system
and consisting of 10 floats, i.e., v[0], . . . , v[9].
• int getX(); : getX is a pointer to a function
returning an int.

58
• Operator [] has higher priority with respect to
operator *. This means that
int *v[10]
declares v as a vector of ten pointers-to-int.
Parentheses are necessary to change the meaning:
int (*v)[10]
means that v is a pointer to a vector of ten integers
• What’s the difference in terms of sizes?
• What if short instead of int?

59
1. int *pi; pointer to integer;
2. char **argv; pointer to pointer-to-char;
3. float *(fvp)[5]; pointer to vectors-of-5-floats
4. long (*fp)(int); pointer to function reading an
int and returning a long.

60
• The address-of (&) operator returns the address of a
variable
char c; char *pc; pc = &c;
• Operator *, applied to a pointer, returns the pointed
object.
char c1 = ’a’; char *p = &c1;
char c2 = *p2; /* c2 == ’a’ */
void func(char *s) { printf("bye %sn", s) }
main() { void (*pfunc)(char*) = func;
*(pfunc)("hello");
}

61
void swap (int a, int b) { int t;
t = a; a = b; b = t;
}
void main() { int a, b;
a = 5, b = 6;
swap (a, b);
printf(“a = %d, b = %dn”, a, b);
}
Are any
faults
present?
Which
ones?
These are
not the same
variables!

62
Step Var Address Contents SP
int a, b; a 1024 ? 1028
b 1028 ? 1032
a = 5, b = 6; a 1024 5
b 1028 6
swap(a, b)
int a, int b a 1032 5 1036
b 1036 6 1040
int t; t 1040 ? 1044
t = a;a = b;b = t a 1032 6
b 1036 5
t 1040 5
} 1032
a 1024 5
b 1028 6

63
• Pointers solve the problem of the lack of “call-by-
reference” in C functions. For instance, in order to
realize a function that swaps its argument, one may
use the following strategy:
int swap(int *a, int *b) { int t;
t = *a, *a = *b, *b = t;
}
• The caller then needs to specify the addresses of
the operands it wants to swap, as in
swap(&i, &j).

64
void swap (int* a, int* b) { int t;
t = *a; *a = *b; *b = t;
}
void main() { int a, b;
a = 5, b = 6;
swap (&a, &b);
printf(“a = %d, b = %dn”, a, b);
}

65
Step Var Address Contents SP
a = 5, b = 6; a 1024 5 1028
b 1028 6 1032
swap(&a, &b)
int *a, int *b a 1032 1024 1036
b 1036 1028 1040
int t; t 1040 ? 1044
t=*a;*a=*b;*b=t *a 1024 6
*b 1028 5
t 1040 5
} 1032
a 1024 6
b 1028 5

66
Pointers and arraysPointers and arrays
• Note: declaring an array means
1. declaring a pointer
2. allocating memory for the pointed objects.
• The name of the array is indeed a pointer to the first
element of the array. In C lingo, this is
written as vect == &vect[0].
• Exercise: write a program, called report, that reports
the occurrences of each digit char in the input
stream. Use an array of ten elements corresponding
to the ten decimal digits. Produce a histogram at
end-of-input.

67
• Exercise: use two programs,
one that outputs the ten integer numbers that
count the occurrences of each digit char in the
input stream,
the other one that creates a histogram of its input
values.
• Then use a pipeline to hook the two programs:
report2 | histogram

68
• Arrays and pointers are strictly related to each
other: In particular, if int a[10]; then
a == &a[0], a+1 == &a[1], … and so forth.
• In other words, *(a+i) == a[i]
• Any indexed array is equivalent to a pointer plus
some offset, and vice-versa
• Big difference: the array is a constant, i.e., it can
never appear on the left of the = sign, as in
a = pa;
or in
a ++;

69
• When passing an array to a function we are indeed
passing the address of its first element; this address
is copied (call-by-value) in a temporary variable.
This variable may be a pointer.
• Example:
char s[7] = "hello!"; /* s is an array */
int i = strlen(s);
int strlen (char *x) { /* x is a pointer */
int n;
for (n=0; *x; x++) /* hence, can be modified */
n++;
return (n);
}

70
• If p is a pointer, p++ lets p point to the next item. It is
the language that takes care of moving p of the right
amount of memory.
• For instance (let us assume an int is 2 bytes and a
double is 8 bytes):
int *p; p == 1222 p+1 == 1224
double *p; p == 5644 p+1 == 5660
and so forth
• In other words: if p is a pointer to an object of type t,
then p+n points n objects further and p-n points n
objects before.
• The actual size of the object doesn’t matter.

71
Pointers to charsPointers to chars
• Strings are available in C as arrays of characters.
Any sentence enclosed between two quotes (“) is an
array of characters ending with character ’0’
(NULL).
• For instance, "hello" means ’h’, ’e’, ’l’, ’l’, ’o’, 0
• A very common mistake in C:
char s1[10] = "Hello ";
char s2[10] = "World";
s1=s2; /* ERROR */

72
Pointers to charsPointers to chars
• As strings are arrays, we can easily pass a string to
a function by passing its name, which points to its
characters:
char a[] = “Kennedy";
printf("Vote %s for president.n", a);
/* a = &a[0] */
• Variables defined within a function cannot be “seen”
from other functions.

73
Pointers to functionsPointers to functions
• Exercise: write a program that uses an array of
pointers-to-function
Input char == number i -> call array[i ]

74
Pointers and arrays (2)Pointers and arrays (2)
• Function strcpy(char *a, char *b);
• Assumption: NULL-terminated strings. Note that
NULL is (int)0, i.e., “false”
• Function strcmp(char *s, char *t): returns a negative
number if s < t, 0 if s == t, a positive number if s > t:
strcmp(char *s, char *t) {
for ( ; *s == *t; s++, t++)
if (*s == ’0’) return (0);
return (*s - *t);
}

75
• Is this #define OK?
• #define STRCPY(a,b) while (*a++ = *b++) ;

76
• To declare multidimensional arrays one declares
arrays of arrays. For instance,
static int day_of_month[2][13] = {
{ 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
{ 0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
};
int day_of_year(int day, int month, int year) {
int i, leap =
year%4 == 0 && year%100 != 0 || year%400 == 0;
for (i=1; i<month; i++)
day += day_in_month[leap][i];
return (day);
}

77
• int v[i][j] vs. int v[i,j]
• Entries are stored “by row”: the rightmost index is
the one that varies the most when entries are
referenced in the order they are stored.
• Initialisation: using curly brackets.
• When passing a bidimensional array to a function, it
is mandatory that the number of columns be
specified. For instance:
f(int day_in_month[2][13]), or
f(int day_in_month[][13]), or
f(int (*day_in_month)[13]),
• i.e., pointer to array-of-13 integer entries.

78
• “Entries are stored by rows” means that, e.g.,
int v[100][200];
• is allocated the same way as a
int v[100 × 200];
i.e., as if it were a mono-dimensional array of
20000 int’s.

79
• Fortran stores objects the opposite way with respect
to C:
for (i..) for (j..) a[i][j]
• is equivalent to
DO I.. DO J.. A(J,I)
• Accessing element (i, j) in a n × m matrix means
accessing element k = i × m + j in the associated
mono-dimensional array.
• The same applies for N-dimensional
matrices, N > 2.

80
• Note how, in order to compute value k for an N
dimensional matrix whose dimensions are
(d1, d2, . . . , dN), it is necessary to know the values
d2, . . . , dN:
k = f(d2, . . . , dN).
• Note also that when we need to access sequentially
all the elements of a multidimensional matrix, it is
preferable to use a pointer initialised to the first
entry of the matrix.

81
• #define N 500
#define M 500
main() { int a[N][M];
int i, j, c;
int *pa = & (a[0][0]);
int *pl = & (a[N-1][M-1]);
while (pa < pl) { for (i=0; i<N; i++)
*pa = 0; for (j=0; j<M; j++)
c = *pa + 1; { a[i][j] = 0;
*pa = c; c = a[i][j] +1;
pa++; a[i][j] = c;
} }HP9K 0.7–0.8s 1.2–1.3 s
SUN3 1.1 s 2.4 s

82
• Even when the access is not sequential, it is
possible to exploit specific access patterns (e.g.,
constant stride access).
• An interesting alternative with respect to
multidimensional arrays is by using pointers. For
instance, the computational cost to access
entry (i, j) in a n × m matrix is the one for computing
multiplication (i * m) and addition (i * m + j).

83
• If we change from
int a[100][100];
to
int** a;
and if we properly allocate the 100 pointers in the
row and, for each of them, the memory required for
storing 100 int’s, then
accessing entry (i, j) means executing *((*(a+i)+j))
that has a computational cost of only two additions.
• Furthermore, no dimension information is required
anymore to access any element of the array.

84
• Array of pointers – an example
• char *name_of_month (int n) {
static char *names[] = {
"illegal",
"January",
"February",
. . .
"December“
};
return ((n < 1 || n > 12)? names[0] : names[n] ;
}

85
• Argc and argv: a mechanism that allows a program
to inspect the strings on the command
• line.
• For instance, command echo:
main(int argc, char *argv[]) {
while (--argc > 0)
printf("%s%c", *++argv, (argc>1)?’ ’:’n’ );
}
• Exercise: compute 2 3 4 + * (in inverse Polish
notation).
• Exercise: write a function that associates user
functions to the command options.

86
• Exercise: write a function that sorts the entries of an
array of integers. Modify the function so that it
requires a pointer-to-function,
int (*confr)(int,int),
to realize sortings in increasing vs. decreasing
order.
• Exercise: “opaque” function, working with pointers to
object of unknown size.
• Exercise (array of functions): design a scanner of a
simple grammar. Tokens must correspond to the
entries in an array of functions.

87
Using pointersUsing pointers
• Using pointers in C may be described as a
connection-oriented service
• One has to open a connection to the memory
service, then use the service (read/write mode),
then disconnect (close) the service
• Example:
int *pi;
pi = malloc(4); /* open:memory-alloc:4 bytes */
*pi = 5; /* use:write-memory:store-in(p,5) */
j = 1 + *pi; /* use:read-memory:get-from(p) */
free(pi); /* close:free-memory-referred-in(p) */

88
1) Definition of a pointer
int *pi;
Note: by doing so, one allocates memory for the
pointer, not for the pointed
2) Memory allocation
pi = malloc(4); or better malloc(sizeof(int));
Note: this is a request for memory allocation.
malloc returns NULL (def:stdio.h) when the
request cannot be fulfilled, a value different from
NULL when the request can be fulfilled
3) Memory access
*pi = … or … = … *pi …
Note: the valid address returned by malloc points to
some memory location where the requested
memory resides

89
4) Memory release
free(pi);
Note: the memory pointed to by pi is freed
Note: pi is not “erased”! Still it holds the stale
reference (be careful…)

90
• A common mistake:
char *p;
*p = …
• This is faulty (use-before-connect fault)
• Weird consequences…

91
• A common mistake:
free(p);
*p = *p + 1;
• This is faulty (use-after-disconnect fault)
• Weird consequences…

92
void main() {
int *a, *b;
int c[10], d[10];
a = malloc(10*sizeof(int));
b = calloc(10, sizeof(int));
a = b;
a = c;
d = a;
}
No check
on return
values
The area
pointed by
a is lost
Illegal:
arrays are
const

93
Working with the debuggerWorking with the debugger
#include <stdio.h>
main()
{
char *p = NULL;
printf("dereferencing a NULL pointer!n");
*p = 'A';
printf("done.n");
}
Big, big mistake…

94

95
A useful tool!A useful tool!

96
Where did the fault take place?

97

98
Commands:
l=list
b=break-
point
r=run
s=step
(also
display var)

99
CommandCommand ““printprint””

100
CommandCommand ““printprint””

101

102
Command
“set”

103
ConstantsConstants
• scientific notation, e.g., 1.5e3 -> double
• postfix notation, e.g., 145L -> long
• prefix notation:
’0x44’ -> unsigned int, hexadecimal,
’044’ -> unsigned int, octal
• constant char: ’x’ = character x -> char
• special constants, e.g., n, t, 0, , " -> char
• “bit patterns”: ddd, ddd being an octal number.
• string constants, e.g., "string" or "".

104
Type casts and conversionsType casts and conversions
• Implicit type casts occur when, in expressions,
operands belong to different types.
• Conversions obey the following rules:
char and short Þ int , float Þ double
if an operand is double , the other becomes
double and the result is double
otherwise, if an operand is long , the other
becomes long and the result is a long
otherwise, if an operand is unsigned, the other
one becomes unsigned and the result is
unsigned.
otherwise, operands and result are int .

105
• Converting a string of digits into a number, and vice-
versa, requires specific support. Functions are
available for this, e.g., atoi():
int atoi(char s[]) { int i, n;
n = 0;
for (i=0; s[i]>=’0’ && s[i]<=’9’; ++i)
n=10*n + s[i] -’0’;
return (n);
}
• Note how expression s[i] - ’0’ converts numeric
character s[i] into the digit it represents.

106
• The following function can be used to convert an
uppercase character into its lowercase counterpart
int lower( int c)
{
if (c >=’A’ && c <= ’Z’)
return (c + ’a’ - ’A’);
else
return (c);
}
• Note: this program only works for code tables in
which ’a’ follows ’A’. This is true with ASCII and
false with, e.g., EBCDIC.

107
• Explicit cast:
• The cast operator changes the type of an object.
For instance, the following expression:
celsius = ( (double)5 /9) * (fahr-32.0);
is equivalent to
celsius = (5.0/9.0) * (fahr-32.0);
• Casting is invoked by specifying a type between
parentheses.

108
IIncrement/decrement operatorsncrement/decrement operators
• IN C: int i = 0; in Assembly:
i++; DATA segment
i DB 0
. . .
INC i
• Operators such as ++ or -- may have a direct
counterpart in the machine language.
• Operator ++ increments the contents of a variable.
x = n++ is not equivalent to x = ++n.
• Operator -- decrements the contents of a variable.
x = n-- is not equivalent to x = --n.

109
ExercisesExercises
• Exercise: function purge(char s[], int c) removes all
occurrences of c from s[].
• Exercise: functions strcpy() and strcat().

110
ExercisesExercises
• int strlen (char *p) { int i=0;
while (*p++) i++;
return i;
}
• *p returns the char pointed to by p. *p++ does the
same, but increments the pointer afterwards.
• When does the while loop ends?
• This is an alternative way to write function strlen:
int strlen (char *p) { char *q = p;
while (*p++) ;
return q - p - 1;
}

111
Bitwise operatorsBitwise operators
• The following six operands can be applied to any
integer expression:
& : bitwise AND
| : bitwise OR
ˆ : bitwise exclusive OR
<< : left shift
>> : right shift
˜ : unary complement.

112
• Bitwise AND can be used to set up “masks”, e.g.,
c = n & 0177;
which zeroes all the bits from bit 8 onward.
• Bitwise OR sets bits:
x = x | MASK
sets to 1 the bits that are set in MASK.
• << and >> are respectively arithmetical shifts to the
left and to the right (multiplication re: division by 2).
• Operator ˜ turns each 1 into 0 and vice-versa; it is
used in expressions like, e.g.,
x & ~ 077,
which zeroes the last bits of x. Note that this is
independent of the actual size of x, and hence it is
“more portable” than, e.g., x & 0177700.

113
• Let’s consider the following function:
unsigned int
MIDbit (unsigned x, unsigned start, unsigned num)
{
return((x >> (start+1-num)) & ~(~0 << num));
}
• For instance, MIDbit(x, 4, 3) returns the three bits at
position 4, 3, and 2.
• x >> (p+1-n) shifts the bits of interest on the
rightmost position in the word.
• ~ 0 is 11...1;
• n shifts to left lead to 11...1 00..0
• complementing this value we reach 00...0 11..1
n zeroes
n ones

114
• Binary operators
+ . / % << >> & ˆ and |
can use notation
e1 op= e2
instead of
e1 = (e1) op (e2).
• Note that x *= y+1 is equivalent to x = x*(y+1).
• An example based on botwise operators follows:
int bitcount(unsigned n) { int b;
for (b=0; n != 0; n >>= 1)
if (n & 01) b++;
return (b);
}

115
A curiosityA curiosity
• Let us consider the following two programs:
• main() { int n; main() { int n;
n = 0; n = 0;
n = n+2+n*n; n += 2+n*n;
} }

116
A curiosityA curiosity
• When compiled with option "-Qproduce .s" on a Sun
workstation, the output Assembly files differ in the
following lines:
16,19c16,17
< movl a6@(-0x4),d1
< addql #0x2,d1
< addl d1,d0
< movl d0,a6@(-0x4)
---
> addql #0x2,d0
> addl d0,a6@(-0x4)

117
The FILE classThe FILE class
• Using files in C may be described as a connection-
oriented service
• One has to open a connection to the file service,
then use the service (read/write mode), then
disconnect (close) the service
• Example:
FILE *pf;
pf = fopen(“readme”, “r”); /* openfile:(readme,rm) */
fprintf(pf, “hin”); /* usefile:store-chars(pf,”hin”) */
fread(buffer,1,1,pf); /*usefile:read-chars(pf,1,buf) */
fclose (pf); /* closefile(pf) */

118
StructuresStructures
• Keyword struct defines a “record.” An example
follows:
struct cd {
char author[30];
int year;
char producer[20];
};
• This is a declaration of a type: no element of this
type has been defined so far. No memory has been
allocated.
• It is now possible to declare objects of type struct cd
struct cd x, y, z;

119
• The members of a struct can be accessed via the
“dot-operator” (“.”). For instance,
struct cd d;
leap = d.year%4 == 0 &&
d.year%100 != 0 ||
d.year%400 == 0;
• Nesting of structures is allowed:
struct a { struct disco c; } d;
• Access: d.c.year.
• Typedefs.

120
• A pointer to a structure can be declared, e.g., as
follows:
struct disco *a;
• Access:
(*a).year;
• a->year is equivalent to (*a).year

121
TypedefTypedef
• What does, e.g., this statement do?
float complex[2];
• Consider now
typedef float complex[2];
• We have defined a new type
• Example:
complex a;
a[0] = 0.0, a[1] = 1.0;

122
TypedefTypedef
• General rule:
consider the definition of a complex object, called x
write “typedef x”
Now x is no more an identifier. It’s a type

123
TypedefTypedef
• Example:
int func_t (int, int, float);
This defines func_t, a pointer-to-function
typedef int func_t (int, int, float);
This defines a new type, called func_t. Now
func_t myF;
is equivalent to
int myF(int int, float);

124
TypedefTypedef
• There are two valid reasons for encouraging the use
of typedef:
parametrizing a program with its types may turn
into semplifying the solution of portability
problems: for instance,
typedef short integer;
Moving the code to a machine where the role of
short is played by int we only need to change
one line of code:
typedef int integer;
enhancing a program’s readability: a type called
LENGTH brings with its name also a hint at its
usage and meaning within the program.

125
• Arrays of structures:
• struct key {
char *keyword;
int keycount;
} keytab[100];
• Or
typedef struct key { … } key_t;
and then
key_t keytab[100];

126
• key_t keytab[100];
• Initialisation:
key_t Keywords[] = {
"break", 0,
"case", 0,
"char", 0,
…
"while", 0
};
• Exercise: Write a program that writes a static arrays
of struct’s to be used as look-up table for another
program.

127
• Structures can reference themselves:
struct tnode {
char *word; int count;
struct tnode *left;
struct tnode *right;
};
• Another example:
struct nlist {
char *name; char *def;
struct nlist *next;
};

128
• Exercise:
Write a set of functions dealing with linked lists
Structure the set of functions as a service
Open a connection to the file service, then use
the service (read/write/delete…), then disconnect
(close) the service

129
Bitfield structuresBitfield structures
• Flag registers:
#define KEYWORD 01
#define EXTERN 02
#define STATIC 04
#define AUTOMT 010
…
flags |= EXTERN | STATIC;
…
if ( flags & (EXTERN | STATIC) ) ...
• It is possible to store in a same variable, flags, a
series of conditions that can be “turned on” through
a bitwise OR (|) and can be tested via the bitwise
AND (&).
• Clearly the definitions must be powers of 2. Octal
implies unsigned.

130
• Structures can be used to specify ag registers via
so-called “bitfields”:
struct {
unsigned is_keyword : 1;
unsigned is_extern : 1;
unsigned is_static : 1;
unsigned is_regist : 1;
} flags;
• Accessing a field: standard way (“.” operator). For
instance: flags.is_extern.

131
• Bitfields can be used as lvalues and rvalues as any
other integer variable.
• This means that bitwise OR’s and AND’s are not
necessary:
flags.is extern = 0
if (flags.is extern == 0) ...

132
• Bitfields can only be unsigned or int
• Asking the address of a bitfield is illegal
• Bitfields can also be “unnamed,” e.g., for padding
• The standard doesn’t specify if MSB-first or LSB-
first is used.

133
UnionsUnions
• An union is a variable having many identifiers
associated to it.
• These identifiers may be of different type and hence
different sizeof’s. Each identifier refer to the same
amount of memory, i.e., as many bytes as the
“largest” type requires. For instance:
union point_x {
char *pc;
float *pf;
double *pd;
long regarded_as_an_int;
} object;

134
UnionsUnions
• Variable object has many “aliases”: it can be
regarded as a pointer-to-char, if referred to as
object.pc, or as pointer-to-double (object.pd), or as
a long (object.regarded as an int).
• The amount of memory is always the same, the one
that is enough in order to store the “largest” among
a (char*), a (float*), a (double*), or a (long).

135
UnionsUnions
• Typical use of unions:
struct {
int its_type;
union {
int ival;
float fval;
char *pval;
} uval;
} symbol_table[500];
• If we store in symbol table[i].its type a code that
represents the type of object i, then we can set up,
e.g., arrays of objects of different types, or linked
lists of different objects, and so forth.

136
UnionsUnions
• A switch on its type is the typical way to execute
diverse actions depending on the nature of the
current object, as it’s done in the following example:
for (i=0;i<500;++i)
switch(symbol_table[i].its_type) {
case ’i’: printf("%d",
symbol_table[i].uval.ival);
break;
…
}

137
Standard librariesStandard libraries
• In C many functions are defined in the so-called
“standard library”, libc.a. A set of headers refer to
the functions and definitions in the standard library.
The header file stdio.h contains the basic functions
and definitions for I/O:
#include <stdio.h>

138
Standard streamsStandard streams
• C and UNIX define three standard I/O streams:
stdin, i.e., the standard input stream, normally
given by the characters typed at the keyboard.
As in DOS, the redirection character < allows to
specify an alternative standard input stream as
follows:
prog < inputfile
stdout, the standard output stream, normally
given by the characters typed onto our display.
Character > redirects stdout on an other file, as
in
prog > outputfile.
stderr, the standard error stream, is again by
default our display. Is the stream where the
programmer does (should) send the error
messages. Can be redirected via 2 >, e.g.,
prog 2> /dev/null.

139
PipesPipes
• Besides the stream redirection facilities, UNIX
provides the concept of pipe: if we type
prog1 | prog2
the stdout of prog1 becomes the stdin of prog2.

140
PipesPipes
TASK 1 TASK 2
stdin stdoutstdin stdout
p
ipein pipe
out
int pipeline[2];
pipein = pipeline[STDIN], pipeout = pipeline[STDOUT];
pipe(pipeline);
• pipe() creates an I/O mechanism called “pipe” and returns
two file descriptors, fildes[0] and fildes[1]. The files
associated with fildes[0] and fildes[1] are streams and
are both opened for reading and writing.
• A read from pipeline[0] accesses the data written to
pipeline[1] and a read from pipeline[1] accesses the data
written to pipeline[0] (both on a FIFO basis.)

141
PipesPipes
• dup2() duplicates an open file descriptor
• dup2(int fildes, int fildes2)
• The dup2() function causes the file descriptor fildes2 to
refer to the same file as fildes. The fildes argument is a
file descriptor referring to an open file.
void main() {
dup2(1,2);
printf(“hello”);
fprintf(stderr, “ worldn”);
}
$ ./a.out
hello world

142
PipesPipes
TASK 1 TASK 2
pip
e
in pipe
out
dup2(pipeout, STDOUT); dup2(pipein, STDIN);
puts(”hello”); gets(msg); // msg is “hello”
h e l l o 0
• STDOUT == 1, STDIN ==0
• Task 1’s stdout is redirected to task 2’s stdin

143
PipesPipes
• Pipes are also available as FILE*: for instance,
FILE *popen(char *command, const char *type);
int pclose (FILE *stream);
• the popen() function creates a pipe between the
calling program and the command to be executed.
command consists of a shell command line. type is
an I/O mode, either r for reading or w for writing
• The value returned is a stream pointer such that one
can write to the standard input of the command, if
the I/O mode is w, by writing to the file stream; and
one can read from the standard output of the
command, if the I/O mode is r, by reading from the
file stream.

144
PipesPipes
1. FILE *f = popen(”date”, ”r”);
2. FILE *g=popen(”/bin/sort”, ”w”);
1. with the first one we can, e.g., read the output of
command date.
2. The second one connects to service /bin/sort so to
ask that external service (in this case, to sort a
stream)

145
The UNIX manThe UNIX man
• The UNIX command “man” is an important tool
• If you don’t remember how a certain functions is to
be invoked, or what is does, just type
man function
• Try for instance
 man printf
 man putc
 man strcpy
 man tolower

146
MakefilesMakefiles
• A makefile is a script that tells how to compile an
application
• It specifies the dependencies among a number of
resources (header and C files)
• An example follows:
all: myfile.exe
myfile.exe: myfile.o myobj.o
cc –o myfile.exe myfile.o
myobj.o
myfile.o: myfile.c common.h
cc –c myfile.c
myobj.o:myobj.c common.h myobj.h
cc –c myobj.c

147
Performance issuesPerformance issues
• How much can the choice of a data structure
influence a property such as locality in sequential
data accesses? In order to evaluate this we run a
simple experiment on a Win2000/Pentium3 PC

148
Performance issuesPerformance issues
• Experiment: the following structure:
typedef struct { int a;
char b[STRIDE];
} stru_t;
and the following access scheme:
for (i=0; i<itera-1; i++)
v[i].a = v[i+1].a + 1;
are compared with the following two data structures:
typedef int stru_ta;
typedef char stru_tb[STRIDE];
and access scheme:
for (i=0; i<itera-1; i++)
a[i] = a[i+1] + 1;

149
STRIDE == 4 (y axis: us; x axis: (x+1)*100000)

150
STRIDE == 8 (y axis: usecs; x axis: (x+1)*100000)

151
STRIDE == 12

152
STRIDE == 16

153
The following pictures plot a vertical line that
represent the gain in performance vs. the best “-O3”
cases for different values of STRIDE:
Stride = 4

154
Stride = 16

155
Stride = 32

156
Stride = 64

157
Stride = 128

158
ReferencesReferences
• Kernighan & Ritchie,The C programming Language,
Addison-Wesley

159
Optimisations in COptimisations in C
• Amdhal Law
• Applications in C: gprof

160
ExerciseExercise
• Write a program that renames a set of files by
substituting all the occurrences of a given string with
another one in the filename

161
ExercisesExercises
• Write a program that displays a Julia set of a given
function in a certain “window” of the Complex plane
• Write a program that zooms in the Julia set and
produces an mpeg animation
You need to find some resources in the Internet
• Project work: write a language that controls the
above
Functional input
Input parameters
Zoom parameters
Mpeg production parameters
Etc!

162
ExerciseExercise
• Problem: you want to organize a set of mp3 CDs
• Input: mp3 CDs containing files such as
/Pink Floyd/Meddle/One of these days.mp3
• Output: a web-based application that manages a DB
of mp3 CDs
Classifying their contents
One of these days PART-OF Meddle PART-OF Pink Floyd…
Building relationships among items of different CDs
Easy addition of new CDs
Search of items

CWEB : a system forCWEB : a system for
““structured documentationstructured documentation””
Axiom: programmers who want to provide the best
possible documentation for their program need two
things simultaneously:
• a language like TeX for formatting,
• a language like C or C++ for programming.

The ideaThe idea
A program can be thought of as a WEB that is made of
many interconnected pieces. CWEB makes the user able:
• to explain the local structure of each part,
• to explain how each entity relates to its neighbors,
• to attach documentation / code to each part.

CWEBCWEB
In essence, CWEB is a tool by means of which one
can use a style of programming that maximizes his/
her ability to perceive the structure of a complex
piece of software.
Involved actions and file types

As a Makefile:As a Makefile:
myprog : myprog.c
cc -o myprog myprog.c
myprog.c : myprog.w
ctangle myprog.w
myprog.ps : myprog.dvi
dvips -o myprog.ps myprog.dvi
myprog.dvi: myprog.tex
tex myprog
myprog.tex: myprog.w
cweave myprog.w

CWEB modulesCWEB modules
CWEB is made of two modules:
• cweave (w2tex translator), and
• ctangle (w2c or w2c++ translator.)

CTANGLECTANGLE
A WEB file is a collection of program/documentation pieces
that may appear in any order. This order reflects the idea the
programmer has of his/her program:
ctangle takes a given “web” and moves the sections
from their web structure into the order required by C.
<Header Files>
<Global Variables>
<Functions>
<The Main program>
<etc.>
<The Main...>=
<variables local to main>
<set up option selection>
<process all the files>
<print the grand totals if there were multiple files>
<etc.>= ...

CWEAVECWEAVE
cweave takes a given “web” and creates a
structured TeX document in which:
• all sections are numbered,
• every relationship between sections are highlighted,
• all places where something is defined are highlighted,
• the C program is “beautified.”

DrawbacksDrawbacks
• TeX is needed (a huge software system),
including fonts and tools like DVI viewers
and converters;
• some knowledge of TeX is needed!

How to define a SectionHow to define a Section
The basic part of a web document is the section.
A section can be named or unnamed. Unnamed
sections begin with “@*” (at-star) or “@ ”
(at-space):
@* An example of {tt CWEB}. This
unnamed section describes...
@ Another unnamed section.

How to define a Section (2)How to define a Section (2)
A named section has a different syntax based on
the characters @, <, >, and =
@<Variables local to main@>
@<Variables local to main@>=
int a, b;
int (*fptr)(void);

Using CWEAVEUsing CWEAVE

Using CTANGLEUsing CTANGLE

177
Deepenings: the TIRAN frameworkDeepenings: the TIRAN framework
• With “TIRAN framework” we mean:
a set of mechanisms fault masking, containment, error
detection, isolation and recovery
a software library
 a basic services library
a ‘backbone’, integrating and coordinating the
mechanisms in distributed environments
a distributed application
some configuration tools, with which the user fulfils the
requirements of her/his application
a “recovery language”, namely a language for programming
error recovery strategies (ARIEL)
a “configuration language”, with which the user defines,
e.g., some instances of the FT mechanisms

178
ArchitectureArchitecture
Run-time System
(Drivers, Kernel, …)
Userapplication
ARIEL
Detection
Masking
Backbone
Management of recovery
Containment
Reconfiguration
High level mechanisms
FaultInjection
Monitoring

179
MechanismsMechanisms
• Watchdog timer
• Voting system
• Task Synchronizer
• Distributed Memory
• Memory Stabilizer
• User-defined mechanisms
• Integrated or stand-alone

180
Basic services libraryBasic services library
• “Basic services library” (BSL)
intra-node, inter-node communication via group-based
communication primitives
task management
access to local clock
semaphores
node shutdown, restart...

181
BackboneBackbone
• The backbone (DB, or BB...): a distributed
application managing the following tasks:
management of a database of data describing the topology
and the current state of the application, plus error
notifications dispatched by
the mechanisms
the user application
the “basic service library”
filtering of error notification (via a-count)
identification of the fault class (transient vs.
permanent/intermittent)
this allows to set up fault-specific strategies
invocation of error recovery (executing the interpreter of the
recovery language)
DB
a-count
RINT

182
BackboneBackbone ® typical scenariotypical scenario
• A watchdog notifies its local BB agent via function
RaiseEvent
RaiseEvent(TIRAN_WD_KICK, TIRAN_WD,
TIRAN_ThisTaskUniqueId(), 0)
• The local component of the BB receives that
notification and:
stores it in its copy of the DB
feeds an alpha-count filter
forwards the notification to the remote agents
(if it's an error notification) initiates recovery... (described
later)

183
BackboneBackbone
• Recovery may ask specific services to basic tools
for isolation or recovery
• Prototypal implementation in C and the BSL for NT
and C/EPX for Parsytec PowerXplorer

184
BackboneBackbone
• Backbone: distributed application
made of “agents”: (task D, task I, task R)
" node i in { 0,..,n-1 } : D[i], I[i], R[i] run on node i.
D : database / a-count,
I: “I’m Alive” task (watchdog)
R: recovery interpreter
D-I-R => “DIR-net”
• Agents: one manager and a set of assistants in
hierarchical order
• Based on an algorithm for tolerating node and task
crashes

185
BackboneBackbone
• BB is compliant to the timed-asynchronous
distributed system model (Cristian & Fetzer)
• In particular, BB tolerates system partitioning during
periods of instability
• Partitions are merged back at the end of a period of
instability
• As a special case, when a crashed node is
rebooted, it is re-entered in the BB

186
BackboneBackbone
• These features are provided by a distributed
algorithm, called “algorithm of mutual suspicion”
because agents mutually question their state
• This algorithm has been designed by means of the
TOM class

187
BackboneBackbone ® AMSAMS
• Twofold error detection strategy:
LOCAL PART: " i : task I[i] watches task D[i]
D[i] periodically clears an “I’m Alive” flag, I[i] periodically
checks and sets it.
If I[i] finds a set flag, it broadcasts a TEIF (“this entity is
faulty”)
REMOTE PART:
manager expects TAIA messages (“this assistant is alive”)
assistants expect MIA messages (“manager is alive”)
WHEN A MESSAGE DOES NOT COME IN WITHIN A
CERTAIN DEADLINE, A SUSPICION PERIOD IS ENTERED
• In absence of faults:
LOCAL PART: flag is set and cleared
REMOTE PART: MIA’s and TAIA’s are sent around resp.
by the manager and the assistants

188
BackboneBackbone ® ““Mutual SuspicionMutual Suspicion””
• When a message does not
come in within a certain
deadline, a suspicion period
is entered
• A suspicion period ends
either returning to the
normal situation or by
applying a graceful
degradation or a recovery
strategy

189
BackboneBackbone ® time-out managementtime-out management
• Based on application-level time-outs
Objects that schedule an event (called alarm) a given
amount of seconds in the future
Cyclic or non-cyclic
In the BB, alarms send a message to task D with a
notification “a time-out has occurred”
So it turns time-related transitions into message arrivals
A special task, task A, runs on each node as time-out
manager
Task A just need to monitor the top entry in the time-out list
Class of C functions for NT and EPX

190
BackboneBackbone ® time-out managementtime-out management

191
• Four classes of time-outs
IA_SET_TIMEOUT, IA_CLEAR_TIMEOUT
“It’s time to set/clear the I’m Alive flag!”
Alarm: send D[this node] message IA_SET or IA_CLEAR
MIA_SEND_TIMEOUT, MIA_RECV_TIMEOUT
“It’s time to send a MIA message / to check whether a MIA
message has come in”
Alarm: send D[this node] message MIA_SEND or
MIA_RECV
TAIA_SEND_TIMEOUT, TAIA_RECV_TIMEOUT
“Time to send a TAIA / to check whether a TAIA has come in”
Alarm: send D[this node] message TAIA_SEND or
TAIA_RECV
TEIF_TIMEOUT
“Time to check whether a TEIF message has come in”
Alarm: send D[this node] message TEIF_CHECK

192
MIA: Manager Is Alive: sent by the manager to all its
assistants
TAIA: This Assistant Is Alive: sent by each assistant to the
manager
TEIF: This Entity Is Faulty: sent by a task I to signal that its
task D is faulty
MIA_SEND, TAIA_RECV: sent from task A[m] to task D[m]
(the manager)
TAIA_SEND, MIA_RECV: sent from A[i] to D[i] (an
assistant)
TEIF_CHECK: sent from A[i] to D[i] (either the manager or
an assistant)
IA_SET (resp. IA_CLEAR): sent from A[i] to D[i] (resp. from
A[i] to I[i])

193
BackboneBackbone ® AMSAMS ® coordinatorcoordinator

194
BackboneBackbone ® AMSAMS ® assistantassistant

195
• When a crash fault affects D[i]:
I[i] detects this as a set flag Þ broadcasts a TEIF
The others detect this as a lacking TAIA or MIA and a
coming TEIF
• When a crash fault affects node i:
The others detect this as a lacking TAIA or MIA and a
lacking TEIF

196
Pseudo-code of the coordinatorPseudo-code of the coordinator
coordinator()
{
/* the alarm of these alarms simply sends a message of id
<alarm-type>, subid = <subid> to the coordinator
*/
insert alarms (IA-flag-alarm, MIA_SEND-alarms, TAIA_RECV-alarms)
clear all entries of sus[] /* suspicion periods are all off */
set IA-flag /* ``I'm alive'' flag */
activate IAT /* I'm Alive Task */
loop (wait for incoming messages)
{
switch (message.type)
{
/* time to set the IA-flag! */
case IA-flag-alarm:
set IA-flag,
break;

197
/* time to send a MIA to an assistant */
case MIA_SEND-alarm:
send MIA to assistant <subid>
break;
/* a message which modifies the db */
case DB:
if (local)
{
renew MIA_SEND alarm on all <subid>s
broadcast modifications to db
break;
}
else /* if it's a remote message, it's also
a piggybacked TAIA */
{
update your copy of the db
/* don't break */
}

198
/* if a TAIA comes in or a remote DB message comes in... */
case TAIA:
if ( ! ispresent(TAIA_RECV-alarm, <subid> ) {
insert TAIA_RECV-alarm, <subid>
broadcast NIUA /* node is up again! */
}
renew TAIA_RECV-alarm, <subid>
/* if you get a TAIA while expecting a TEIF, then
no problem, simply get out of the suspicion period */
if (sus[ <subid> ] == TRUE)
sus[ <subid> ] = FALSE;
break;
/* no heartbeat from a remote component... enter a suspicion period */
case TAIA_RECV-alarm:
sus[ <subid> ] = TRUE
insert alarm (TEIF_RECV-alarm, NON_CYCLIC,
IMALIVE_RECV_TIMEOUT, <subid>)
delete alarm (TAIA_RECV-alarm, <subid>);
break;

199
/* a TEIF message has been sent from a IAT: as its last action,
the IAT went to sleep */
case TEIF:
if (sus[ <subid> ] == TRUE) {
delete alarm (TEIF_RECV-alarm, <subid>)
sus[ <subid> ] = FALSE
/* agent recovery will spawn a clone of the
assistant <subid>. If no assistant clones
are available, the entire node will
be rebooted. */
Agent-Recovery( <subid> )
} else {
if (local) {
set IA-flag
enable IAT
} else send ENIA (i.e., ``ENable IAt'') to <subid>
}
break;

200
case TEIF_RECV-alarm:
if (sus[ <subid> ] == TRUE) {
delete alarm (TAIA_RECV-alarm, <subid>)
sus[ <subid> ] = FALSE
/* the entire node will be rebooted. */
Node-Recovery( <subid> )
}
break;
case ENIA:
set IA-flag
activate IAT
/* if an IAT gets an “activate” message while it's
active, the message is ignored */
break;
default: deal with other messages
} /* end switch (message.type) */
set IA-flag
renew IA-flag-alarm
} /* end loop */
} /* end coordinator */

201
Rendering via a hypermedia monitorRendering via a hypermedia monitor
• Non-parsing header CGI script (written in C)
• Remotely controlled Netscape browser

202
BackboneBackbone ® MonitorMonitor

203
SimulationsSimulations

204
Simulations (2)Simulations (2)

205

206

207
Deepenings: RDeepenings: ReeL: adoption of aL: adoption of a
configuration languageconfiguration language
• A configuration language is a linguistic framework to
define system, middleware, and application parameters
define tasks, groups, nodes
configure the basic tools
configuration of parameters (e.g., for alpha-count)

208
RReeLL ® ConfigConfig ® ExampleExample
# Defines are importable from C header
# files via the INCLUDE statement
INCLUDE "phases.h"
INCLUDE ”../backbone.h"
# Definitions
NPROCS = 4
Define 0 = MANAGER
Define 1-3 = ASSISTANTS
MIA_SEND_TIMEOUT = 800000 # Manager Is
# Alive -- manager side
TAIA_RECV_TIMEOUT = 1500000 # This
# Agent Is Alive timeout--manager side

209
ReLReL ® ConfigConfig ® ExampleExample
TASK 1 = "Backbone0" IS NODE 1, TASKID
{BACKBONE_TASKID}
{BACKBONE_TASKID}
{BACKBONE_TASKID}
{BACKBONE_TASKID}
LOGICAL 1 = "LBack0" IS TASK 1 END
LOGICAL 2 = "LBack1" IS TASK 2 END
LOGICAL {ALL} ="all" IS TASK 1,2,3,4 END

210
ALPHA-COUNT 1 IS threshold = 3.0,
factor = 0.4 END ALPHA-COUNT
# Tool Configuration
WATCHDOG TASK 13 WATCHES TASK 14
HEARTBEATS EVERY 100 MS
ON ERROR REBOOT # or WARN TASK x...
END WATCHDOG
output: a configured instance of a watchdog in file
TIRAN_Task10.c

211
• A linguistic framework to
configure replicated tasks, retry blocks, multiple-version
software fault tolerance means (NVP, CRB, …)
using multicasts to replicate stdin
using pipelining and stream redirection for transparent
forward of the output values
using voting for de-multiplexing output values
ReLReL ® ConfigConfig

212
N-VERSION TASK 15
VERSION 1 IS TASK 16 TIMEOUT 100 ms
VOTING ALGORITHM IS MAJORITY
METRIC "task20_cmp"
ON SUCCESS TASK 19
ON ERROR TASK 20
END N-VERSION
• Output: source files TIRAN_Task1[678].c

213
#include "TIRAN.h"
/* Task 18 of NVersion Task 15
Version 3 / 3
*/
int TIRAN_task_18(void) {
TIRAN_Voting_t *dv;
size_t size;
double task20_cmp(const void*, const void*);
dv = TIRAN_VotingOpen(task20_cmp);
if (dv == NULL) {
TIRAN_exit(TIRAN_ERROR_VOTING_CANTOPEN);
} ...

214
ReLReL ® ConfigConfig ® Support towards NVPSupport towards NVP

215
ReLReL ® ConfigConfig ® Pipelines+redirectionPipelines+redirection
TASK 1 TASK 2
p
ipein pipe
out
int pipeline[2];
pipein = pipeline[STDIN], pipeout = pipeline[STDOUT];
pipe(pipeline);
TASK 1 TASK 2
pip
e
in pipe
out
dup2(pipeout, STDOUT); dup2(pipein, STDIN);
puts(”hello”); gets(msg); // msg is “hello”
h e l l o 0

216
bash-2.02$ art -s -i .ariel
Ariel translator, v2.0f 03-Mar-2000, (c)
K.U.Leuven 1998, 1999, 2000.
Parsing file .ariel...
...done
Output written in file .rcode.
Watchdogs configured.
N-version tasks configured.
Logicals written in file LogicalTable.csv.
Tasks written in file TaskTable.csv.
Preloaded r-codes written in file trl.h.
Time-outs written in file timeouts.h.
Identifiers written in file identifiers.h.
Alpha-count parameters written in file
../alphacount.h.

217
• Translator written in C and Lex & YACC
• Still work to be done
C output files
Proper syntax
Testing…
Þ DepAuDE

218
ReLReL ® RecoveryRecovery ® ExampleExample
INCLUDE "my_definitions.h"
TASK {VOTER1} IS NODE {NODE1}, TASKID {VOTER1}
TASK {SPARE} IS NODE {NODE4}, TASKID {SPARE}
IF [ PHASE (T{VOTER1}) == {HAS_FAILED} ]
THEN
STOP T{VOTER1}
SEND {WAKEUP} T{SPARE}
SEND {VOTER1} T{SPARE}
SEND {SPARE} T{VOTER2}
SEND {SPARE} T{VOTER3}
FI

219
rcode_t rcodes[] = { /* HEADER FILE PRODUCED BY THE ARIEL TRANSLATOR */
• /*line#*/ /* opcode */ /* operand 1 */ /* operand 2 */
• /*0*/ { R_SET_ROLE, 1, 2000 },
• /*1*/ { R_SET_ROLE, 2, 2000 },
• /*2*/ { R_SET_ROLE, 3, 2000 },
• /*3*/ { R_SET_ROLE, 1, 3000 },
• /*4*/ { R_SET_DEF_ACT, 666, -1 },
• /*5*/ { R_INC_NEST, -1, -1 },
• /*6*/ { R_STRPHASE, 0, -1 },
• /*7*/ { R_COMPARE, 1, 9999 },
• /*8*/ { R_FALSE, 10, -1 },
• /*9*/ { R_KILL, 18, 0 },
• /*10*/ { R_PUSH, 18, -1 },
• /*11*/ { R_SEND, 18, 3 },
• /*12*/ { R_PUSH, 0, -1 },
• /*13*/ { R_SEND, 18, 3 },
• /*14*/ { R_PUSH, 3, -1 },
• /*15*/ { R_SEND, 18, 1 },
• /*16*/ { R_PUSH, 3, -1 },
• /*17*/ { R_SEND, 18, 2 },
• /*18*/ { R_DEC_NEST, -1, -1 },

220
Art translated Ariel strategy file: .... prova
into rcode object file : ............... .rcode
line rcode opn1 opn2
-----------------------------------------------
00000 SET_ROLE 1 Assistant
00003 SET_ROLE 1 Manager
00004 SET_DEFAULT_ACTION 666
00005 IF
00006 STORE_PHASE... Thread 0
00007 ...COMPARE == 9999
00008 FALSE 10
00009 KILL Thread 0
00010 PUSH... 18
00011 ...SEND Thread 3
00012 PUSH... 0
00014 PUSH... 3
00016 PUSH... 3
00018 FI

223
ReLReL ® RecoveryRecovery ® Run-timeRun-time
• The backbone awakes a recovery interpreter (RINT)
• Virtual machine executing r-codes
• Stack machine
• Currently, r-codes are read either from a file or from
memory (static version, rcodes[] array)
while ( ! End of r-codes )
{
get next r-code;
goto? Skip r-codes;
does it deal with backbone?
Push( query (backbone) );
execute generic r-code;
}

224
ExampleExample
• NVP + spares + fault-specific recovery
• IF [ FAULTY TASK10 ]
THEN IF [ TRANSIENT TASK 10 ]
THEN RESTART TASK 10 …
ELSE … reconfiguration
ENDIF
ENDIF

225
ExampleExample

226
ExampleExample

227
ExampleExample

228
ResultsResults ® Time to detect a crash failureTime to detect a crash failure

229
ResultsResults ® recovery timesrecovery times

230
Deepening: the redundant watchdogDeepening: the redundant watchdog
• Past: automation solutions for electricity were based
on
Hardware FT tools and approaches
Ad-hoc developed platforms and architectures
• Today: need for higher flexibility
flexible FT solutions
portable
based on standard architectures and heterogeneous
platforms
• TIRAN addressed this problem

231
IntroductionIntroduction
• Example domain: electrical substation automations
• Auto-exclusion functionality
requirement: in case of a failure, one must guarantee that
the equipment be left in an “acceptable” state, e.g., forcing
a “safe” output value, alerting the operators and so forth
(for instance: traffic light in safe mode)
The auto-exclusion functionality must guarantee high
availability and high integrity
• Typical solution: watchdog timer

232
WatchdogsWatchdogs
• Aim: watching a user task
• The user task is requested to send periodically
a “sign of life” (a heartbeat msg) to the watchdog
• The watchdog resets a counter each time a
heartbeat is received
• The counter is incremented linearly with time
• When the counter reaches a threshold, an alarm
is issued
watchdogUser task
HB
clear
HB
clear
Alarm!

233
Redundant watchdogRedundant watchdog
• Traditional solution: hardware watchdog
detects crash/performance failures
typical availability and integrity of hardware systems
low degree of flexibility
often a non-standard, non-portable solution
• TIRAN solution: software, though redundant,
watchdog
replication of tasks on several nodes
user-defined replication level
higher flexibility
exploiting the inherent redundancy of standard distributed
architectures (e.g., nodes in a cluster of workstations)
higher portability

234
• ReL strategy and the ariel language to specify the
involved parameters:
frequency of sending heartbeat messages
parameters of the a-count filter
number of nodes to use
task-to-node mapping
recovery strategy

235
The TIRAN redundant watchdogThe TIRAN redundant watchdog
• TIRAN tools that we have used:
TIRAN framework
Library of mechanisms (WD)
The backbone
ariel and the ReL methodology
Basic services library (communication,
management of tasks, synchronization…)
In particular:
configuration of a mechanism (WD)
composition of a sophisticated mechanisms
through the use of a simpler one

236
The RThe ReeL methodology in TIRANL methodology in TIRAN
Recovery
pseudo-code
Application
description
System
description
D-tool
description
Configuration code
Recovery
description
Recovery code
Configured
instances
Application
source code
Application executable Backbone/RINT
ariel ariel
art art
cc cc

237
Problem: how to improve the availabilityProblem: how to improve the availability
and integrity of the TIRAN watchdogand integrity of the TIRAN watchdog
Watchdog
User task
Controller
alarm!
Watchdog
User task
Controller
no alarm
• Problem: what if a WD “forgets” to fire?
• Solution: redundant WD with voting in ariel

238
• Problem: what if a WD fires without reason?
Watchdog
User task
Controller
alarm!
• Solution: redundant WD and voting in ariel
Watchdog
ControllerWatchdog
Watchdog

239
Demo: redundant watchdogDemo: redundant watchdog
• Aim of the demo: showing how to use
the TIRAN framework
the recovery language approach
in order to realize, at low costs, a prototype of a
redundant watchdog characterized by
Þ either a higher availability
Þ or a higher integrity

240
RWD: Demo: step 1RWD: Demo: step 1
• Configuration
System composed of 3 “nodes” (node 1, 2 and 3)
A client process connects to “watchdog W”
W is indeed a “logical”: W = { W1, W2, W3 }
The cyclic heartbeat is sent to W in multicast mode
For each task t in W:
if the heartbeat does not show up in time, t fires
RaiseEvent(… FIRED!…)
The source codes of W1, W2 e W3 are created by the ariel
translator (TIRAN_task_16.c, TIRAN_task17.c, …18.c)
These source codes are automatically imported in the C++
user workspace

241

242
Demo: step 2Demo: step 2
• Compiling
• W1, W2 and W3 are compiled
• Three executables are produced: watchdog16.exe,
watchdog17.exe and watchdog18.exe

243

244
Demo: step 3Demo: step 3
• Execution
The TIRAN backbone, the user client and the three
watchdogs are launched
The three watchdogs wait for an activation message, after
which they expect heartbeats
As soon as a heartbeat is not received =>
RaiseEvent(… FIRED…)
that is, the alarm condition is triggered

245
The RThe ReeL methodology in TIRANL methodology in TIRAN
Recovery
pseudo-code
Application
description
System
description
D-tool
description
Configuration code
Recovery
description
Recovery code
Configured
instances
Application
source code
Application executable Backbone/RINT
ariel ariel
art art
cc cc

246
Scheme of execution of the recoveryScheme of execution of the recovery
actionsactions
DBUser application
Recovery
application
Error
Detection
Store
Recovery starts
Query
Skip/execute
actions
Result
Recovery endsOK

247
Demo: recovery strategiesDemo: recovery strategies
• In our demo, error recovery is the execution of a
system-level alarm
• This alarm is triggered when a certain condition
occurs:
• Possible strategies:
OR strategy:
IF [ FIRED W1 OR FIRED W2 OR FIRED W3 ]
The alarm triggers when any of the watchdogs fires
This compensates up to two watchdog crashes
It lowers the probability that a crashed task goes undetected
because its WD had failed (enhances error detection
coverage)
in other words: increases INTEGRITY

248
Demo: recovery strategiesDemo: recovery strategies
• AND strategy
IF [ FIRED W1 AND FIRED W2 AND FIRED W3 ]
The system alarm triggers when all the WD’s agree
Tolerates up to two faulty WD’s
Lowers the probability that the system alarm is raised
without a very good reason (client still OK)
Useful to reduce the probability that the system be
erroneously stopped due to faulty watchdog
Þ Better availability

249
W1
W2
W3
BB
client
activation
watchdogFIRE!
Recovery interpretation
System alarm is raised

250
Redundant watchdog: lessons learned andRedundant watchdog: lessons learned and
conclusionsconclusions
• AND Þ higher availability
• OR Þ higher integrity
• 2-out-of-3 : trade-off
• The development is simplified
• Aspects related to error recovery are specified
outside the user application
• Aspects related to the configuration of the error
detection provisions (in this case, WDs) are
specified outside the user application

Advanced C Language for Engineering Course

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (20)

Similar to Advanced C Language for Engineering Course

Similar to Advanced C Language for Engineering Course (20)

More from Vincenzo De Florio

More from Vincenzo De Florio (20)

Recently uploaded

Recently uploaded (20)

Advanced C Language for Engineering Course