C and C++ signal handling and C++ signal classes and examples

Signals:

Description: Signals are software interrupts delivered to a process by the operating system. Signals can also be issued by the operating system based on system or error conditions. There is a default behavior for some (i.e. a process is terminated when it receives an inturrupt SIGINT signal by pressing keystrokes ctrl-C) but this tutorial shows how to handle the signal by defining callback functions to manage the signal. Where possible, this allows one to close files and perform operations and react in a manner defined by the programmer.

Note that not all signals can be handled.

Types of signals:

Signal Value Description

SIGHUP	1	Hangup (POSIX) Report that user's terminal is disconnected. Signal used to report the termination of the controlling process.
SIGINT	2	Interrupt (ANSI) Program interrupt. (ctrl-c)
SIGQUIT	3	Quit (POSIX) Terminate process and generate core dump.
SIGILL	4	Illegal Instruction (ANSI) Generally indicates that the executable file is corrupted or use of data where a pointer to a function was expected.
SIGTRAP	5	Trace trap (POSIX)
SIGABRT SIGIOT	6	Abort (ANSI) IOT trap (4.2 BSD) Process detects error and reports by calling abort
SIGBUS	7	BUS error (4.2 BSD) Indicates an access to an invalid address.
SIGFPE	8	Floating-Point arithmetic Exception (ANSI). This includes division by zero and overflow.The IEEE Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Std 754-1985) defines various floating-point exceptions.
SIGKILL	9	Kill, unblockable (POSIX) Cause immediate program termination. Can not be handled, blocked or ignored.
SIGUSR1	10	User-defined signal 1
SIGSEGV	11	Segmentation Violation (ANSI) Occurs when a program tries to read or write outside the memory that is allocated for it by the operating system, dereferencing a bad or NULL pointer. Indicates an invalid access to valid memory.
SIGUSR2	12	User-defined signal 2
SIGPIPE	13	Broken pipe (POSIX) Error condition like trying to write to a socket which is not connected.
SIGALRM	14	Alarm clock (POSIX) Indicates expiration of a timer. Used by the alarm() function.
SIGTERM	15	Termination (ANSI) This signal can be blocked, handled, and ignored. Generated by "kill" command.
SIGSTKFLT	16	Stack fault
SIGCHLD SIGCLD	17	Child status has changed (POSIX) Signal sent to parent process whenever one of its child processes terminates or stops. See the YoLinux.com Fork, exec, wait, waitpid tutorial
SIGCONT	18	Continue (POSIX) Signal sent to process to make it continue.
SIGSTOP	19	Stop, unblockable (POSIX) Stop a process. This signal cannot be handled, ignored, or blocked.
SIGTSTP	20	Keyboard stop (POSIX) Interactive stop signal. This signal can be handled and ignored. (ctrl-z)
SIGTTIN	21	Background read from tty (POSIX)
SIGTTOU	22	Background write to tty (POSIX)
SIGURG	23	Urgent condition on socket (4.2 BSD) Signal sent when "urgent" or out-of-band data arrives on a socket.
SIGXCPU	24	CPU limit exceeded (4.2 BSD)
SIGXFSZ	25	File size limit exceeded (4.2 BSD)
SIGVTALRM	26	Virtual Time Alarm (4.2 BSD) Indicates expiration of a timer.
SIGPROF	27	Profiling alarm clock (4.2 BSD) Indicates expiration of a timer. Use for code profiling facilities.
SIGWINCH	28	Window size change (4.3 BSD, Sun)
SIGIO SIGPOLL	29	I/O now possible (4.2 BSD) Pollable event occurred (System V) Signal sent when file descriptor is ready to perform I/O (generated by sockets)
SIGPWR	30	Power failure restart (System V)
SIGSYS	31	Bad system call

See: /usr/include/bits/signum.h

Signals which can be processed include: SIGINT, SIGABRT, SIGFPE, SIGILL, SIGSEGV, SIGTERM, SIGHUP

List all signals available to the system:

Use the command: kill -l

$ kill -l 1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP 6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1 11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP 21) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ 26) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR 31) SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3 38) SIGRTMIN+4 39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8 43) SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13 48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12 53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7 58) SIGRTMAX-6 59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2 63) SIGRTMAX-1 64) SIGRTMAX

Sending a process a signal:

A process can be sent a signal using the "kill" command: kill -s signal-number pid

Where the pid (process id) can be obtained using the "ps" command.

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C Signal handler and Example:

Basic C signal callback function example:

File: signalExample.cpp

01 #include <stdio.h> 02 #include <stdlib.h> 03 #include <signal.h> 04 05 // Define the function to be called when ctrl-c (SIGINT) signal is sent to process 06 void 07 signal_callback_handler(int signum) 08 { 09 printf("Caught signal %d/n",signum); 10 // Cleanup and close up stuff here 11 12 // Terminate program 13 exit(signum); 14 } 15 16 int main() 17 { 18 // Register signal and signal handler 19 signal(SIGINT, signal_callback_handler); 20 21 while(1) 22 { 23 printf("Program processing stuff here./n"); 24 sleep(1); 25 } 26 return EXIT_SUCCESS; 27 }

Example to handle ctrl-c
Compile: gcc signalExample.cpp
Run: a.out
Results:

Program processing stuff here.
Program processing stuff here.
Caught signal 2

The function prototype: void (*signal (int sig, void (*func)(int)))(int);

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C++ Signal Registration and Handling Class:

File: signalHandler.hpp

01 #ifndef __SIGNALHANDLER_H__ 02 #define __SIGNALHANDLER_H_ 03 #include <stdexcept> 04 using std::runtime_error 05 06 class SignalException : public runtime_error 07 { 08 public: 09 SignalException(const std::string& _message) 10 : std::runtime_error(_message) 11 {} 12 }; 13 14 class SignalHandler 15 { 16 protected: 17 static bool mbGotExitSignal; 18 19 public: 20 SignalHandler(); 21 ~SignalHandler(); 22 23 static bool gotExitSignal(); 24 static void setExitSignal(bool _bExitSignal); 25 26 void setupSignalHandlers(); 27 static void exitSignalHandler(int _ignored); 28 29 }; 30 #endif

File: signalHandler.cpp

01 #include <signal.h> 02 #include <errno.h> 03 04 #include "signalHandler.hpp 05 06 bool SignalHandler::mbGotExitSignal = false; 07 08 /** 09 * Default Contructor. 10 */ 11 SignalHandler::SignalHandler() 12 { 13 } 14 15 /** 16 * Destructor. 17 */ 18 SignalHandler::~SignalHandler() 19 { 20 } 21 22 /** 23 * Returns the bool flag indicating whether we received an exit signal 24 * @return Flag indicating shutdown of program 25 */ 26 bool SignalHandler::gotExitSignal() 27 { 28 return mbGotExitSignal; 29 } 30 31 /** 32 * Sets the bool flag indicating whether we received an exit signal 33 */ 34 void SignalHandler::setExitSignal(bool _bExitSignal) 35 { 36 mbGotExitSignal = _bExitSignal; 37 } 38 39 /** 40 * Sets exit signal to true. 41 * @param[in] _ignored Not used but required by function prototype 42 * to match required handler. 43 */ 44 void SignalHandler::exitSignalHandler(int _ignored) 45 { 46 mbGotExitSignal = true; 47 } 48 49 /** 50 * Set up the signal handlers for CTRL-C. 51 */ 52 void SignalHandler::setupSignalHandlers() 53 { 54 if (signal((int) SIGINT, SignalHandler::exitSignalHandler) == SIG_ERR) 55 { 56 throw SignalException("!!!!! Error setting up signal handlers !!!!!"); 57 } 58 }

File: test.cpp

01 #include <iostream> 02 #include "signalHandle.hpp" 03 using namespace std; 04 05 main() 06 { 07 int iret; 08 09 try 10 { 11 SignalHandler signalHandler; 12 13 // Register signal handler to handle kill signal 14 signalHandler.setupSignalHandlers(); 15 16 // Infinite loop until signal ctrl-c (KILL) received 17 while(!signalHandler.gotExitSignal()) 18 { 19 sleep(1); 20 } 21 22 iret = EXIT_SUCCESS; 23 } 24 catch (SignalException& e) 25 { 26 std::cerr << "SignalException: " << e.what() << std::endl; 27 iret = EXIT_FAILURE; 28 } 29 return(iret); 30 }

Compile: g++ signalHandle.cpp test.cpp

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C Signal Man Pages:

C functions:

• signal - ANSI C signal handling
• raise - send a signal to the current process
• strsignal - return string describing signal (GNU extension)
• psignal - print signal message
• sigaction - POSIX signal handling functions
• sigsetops - POSIX signal set operations
• sigvec - BSD software signal facilities
• alarm - set an alarm clock for delivery of a signal

Commands:

• kill - terminate a process
• ps - report a snapshot of the current processes.