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E.9. Signal Handling

An unexpected event, or signal, can terminate a program prematurely. Some unexpected events include interrupts (pressing Ctrl+C on a UNIX, LINUX, Mac OS X or Windows system), illegal instructions, segmentation violations, termination orders from the operating system and floating-point exceptions (division by zero or multiplying large floating-point values). The signal-handling library provides function signal to trap unexpected events. Function signal receives two argumentsan integer signal number and a pointer to the signal-handling function. Signals can be generated by function raise, which takes an integer signal number as an argument. Figure E.5 summarizes the standard signals defined in header file . The next example demonstrates functions signal and raise.


Figure E.5. Signals defined in header .

Signal

Explanation

SIGABRT

Abnormal termination of the program (such as a call to abort).

SIGFPE

An erroneous arithmetic operation, such as a divide by zero or an operation resulting in overflow.

SIGILL

Detection of an illegal instruction.

SIGINT

Receipt of an interactive attention signal.

SIGSEGV

An invalid access to storage.

SIGTERM

A termination request sent to the program.

Figure E.6 traps an interactive signal (SIGINT) with function signal. The program calls signal with SIGINT and a pointer to function signalHandler. (Remember that the name of a function is a pointer to the function.) Now, when a signal of type SIGINT occurs, function signalHandler is called, a message is printed and the user is given the option to continue normal execution of the program. If the user wishes to continue execution, the signal handler is reinitialized by calling signal again (some systems require the signal handler to be reinitialized), and control returns to the point in the program at which the signal was detected. In this program, function raise is used to simulate an interactive signal. A random number between 1 and 50 is chosen. If the number is 25, then raise is called to generate the signal. Normally, interactive signals are initiated outside the program. For example, pressing Ctrl+C during program execution on a UNIX, LINUX, Mac OS X or Windows system generates an interactive signal that terminates program execution. Signal handling can be used to trap the interactive signal and prevent the program from terminating.

Figure E.6. Using signal handling.

(This item is displayed on pages 1258 - 1260 in the print version)

 1 // Fig. E.6: figE_06.cpp
 2 // Using signal handling
 3 #include 
 4 using std::cout;
 5 using std::cin;
 6 using std::endl;
 7
 8 #include 
 9 using std::setw;
10
11 #include 
12 using std::raise;
13 using std::signal;
14
15 #include 
16 using std::exit;
17 using std::rand;
18 using std::srand;
19
20 #include 
21 using std::time;
22
23 void signalHandler( int );
24
25 int main()
26 {
27 signal( SIGINT, signalHandler );
28 srand( time( 0 ) );
29
30 // create and output random numbers
31 for ( int i = 1; i <= 100; i++ )
32 {
33 int x = 1 + rand() % 50;
34
35 if ( x == 25 )
36 raise( SIGINT ); // raise SIGINT when x is 25
37
38 cout << setw( 4 ) << i;
39
40 if ( i % 10 == 0 )
41 cout << endl; // output endl when i is a multiple of 10
42 } // end for
43
44 return 0;
45 } // end main
46
47 // handles signal
48 void signalHandler( int signalValue )
49 {
50 cout << "
Interrupt signal (" << signalValue
51 << ") received.
"
52 << "Do you wish to continue (1 = yes or 2 = no)? ";
53
54 int response;
55
56 cin >> response;
57
58 // check for invalid responses
59 while ( response != 1 && response != 2 )
60 {
61 cout << "(1 = yes or 2 = no)? ";
62 cin >> response;
63 } // end while
64
65 // determine if it is time to exit
66 if ( response != 1 )
67 exit( EXIT_SUCCESS );
68
69 // call signal and pass it SIGINT and address of signalHandler
70 signal( SIGINT, signalHandler ); 
71 } // end function signalHandler
 
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 Interrupt signal (2) received.
 Do you wish to continue (1 = yes or 2 = no)? 1
 100
 
 
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 Interrupt signal (2) received.
 Do you wish to continue (1 = yes or 2 = no)? 2
 

Introduction to Computers, the Internet and World Wide Web

Introduction to C++ Programming

Introduction to Classes and Objects

Control Statements: Part 1

Control Statements: Part 2

Functions and an Introduction to Recursion

Arrays and Vectors

Pointers and Pointer-Based Strings

Classes: A Deeper Look, Part 1

Classes: A Deeper Look, Part 2

Operator Overloading; String and Array Objects

Object-Oriented Programming: Inheritance

Object-Oriented Programming: Polymorphism

Templates

Stream Input/Output

Exception Handling

File Processing

Class string and String Stream Processing

Web Programming

Searching and Sorting

Data Structures

Bits, Characters, C-Strings and structs

Standard Template Library (STL)

Other Topics

Appendix A. Operator Precedence and Associativity Chart

Appendix B. ASCII Character Set

Appendix C. Fundamental Types

Appendix D. Number Systems

Appendix E. C Legacy Code Topics

Appendix F. Preprocessor

Appendix G. ATM Case Study Code

Appendix H. UML 2: Additional Diagram Types

Appendix I. C++ Internet and Web Resources

Appendix J. Introduction to XHTML

Appendix K. XHTML Special Characters

Appendix L. Using the Visual Studio .NET Debugger

Appendix M. Using the GNU C++ Debugger

Bibliography

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C++ How to Program
C++ How to Program (5th Edition)
ISBN: 0131857576
EAN: 2147483647
Year: 2004
Pages: 627
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