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[Page 1257 (continued)]

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 <csignal>. The next example demonstrates functions signal and raise.


[Page 1258]

Figure E.5. Signals defined in header <csignal>.

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 <iostream>
 4  using std::cout;
 5  using std::cin;
 6  using std::endl;
 7
 8  #include <iomanip>
 9  using std::setw;
10
11  #include <csignal>
12  using std::raise;
13  using std::signal;
14
15  #include <cstdlib>
16  using std::exit;
17  using std::rand;
18  using std::srand;
19
20  #include <ctime>
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 << "\nInterrupt signal (" << signalValue
51          << ") received.\n"
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


     1   2   3   4
  Interrupt signal (2) received.
  Do you wish to continue (1 = yes or 2 = no)? 2



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