• Example Slot Signal C++ Booster
• Example Slot Signal C++ Jammers
• Qt Signal Slot Example C++

• The C Standard Library

Each signal stores a list of slots. A slot defines what should be done when the signal is emitted. Typically an object emits the signal when it changes state. When a signal is emitted, the slots from the signal's slot list are executed, and sometimes a result is returned. Note that libsigc signals have absolutely nothing to do with POSIX signals. For our signal and slot example, we will handle two types of widgets such as LineEdit and Label. Under the input widgets, choose “QLineEdit” and drag and drop to the Dialog form. In the same way, click a Qlabel from the Display widgets drag and drop to the Dialog form. For example, if you have an object that downloads data from the network, it's a perfectly sensible design to use a signal to indicate that the requested data arrived. But if you need to send out every single byte one by one to a consumer, use a listener interface rather than signals and slots. Slots and signals must have same parameters. Otherwise, the connection will not occur. Not only for connection, slot function must have same parameters with signal. For example, this sample doesn’t work: QObject::connect(ui.comboBox, SIGNAL (activated(int)), this, SLOT (onComboboxActivated)); But it works. Corresponding signal and slot arguments must have the same types, so for example, we could not connect a QDial's valueChanged(int) signal to a QLineEdit's setText(QString) slot. In our dial and spinbox example we used the instance.methodName syntax as we did with the example applications shown earlier in the chapter.

• C Standard Library Resources

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Description

The C library function void (*signal(int sig, void (*func)(int)))(int) sets a function to handle signal i.e. a signal handler with signal number sig.

Declaration

Following is the declaration for signal() function.

Example Slot Signal C++ Booster

Parameters

• sig − This is the signal number to which a handling function is set. The following are few important standard signal numbers −

Sr.No.	Macro & Signal
1	SIGABRT (Signal Abort) Abnormal termination, such as is initiated by the function.
2	SIGFPE (Signal Floating-Point Exception) Erroneous arithmetic operation, such as zero divide or an operation resulting in overflow (not necessarily with a floating-point operation).
3	SIGILL (Signal Illegal Instruction) Invalid function image, such as an illegal instruction. This is generally due to a corruption in the code or to an attempt to execute data.
4	SIGINT (Signal Interrupt) Interactive attention signal. Generally generated by the application user.
5	SIGSEGV (Signal Segmentation Violation) Invalid access to storage − When a program tries to read or write outside the memory it is allocated for it.
6	SIGTERM (Signal Terminate) Termination request sent to program.

• func − This is a pointer to a function. This can be a function defined by the programmer or one of the following predefined functions −

Sr.No.	Function & Description
1	SIG_DFL Default handling − The signal is handled by the default action for that particular signal.
2	SIG_IGN Ignore Signal − The signal is ignored.

Return Value

This function returns the previous value of the signal handler, or SIG_ERR on error.

Example

The following example shows the usage of signal() function.

Let us compile and run the above program that will produce the following result and program will go in infinite loop. To come out of the program we used CTRL + C keys.

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:

Example Slot Signal C++ Jammers

/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

Sending a process a signal:

A process can be sent a signal using the 'kill' command: kill -s signal-numberpid

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

Basic C signal callback function example:

File: signalExample.cpp

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

Qt Signal Slot Example C++

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

File: signalHandler.hppFile: signalHandler.cppFile: test.cppCompile: g++ signalHandle.cpp test.cppC 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.

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'Advanced Programming in the UNIX Environment' First Edition by W. Richard Stevens ISBN # 0201563177, Addison-Wesley Professional Computing Series It is the C programmers guide to programming on the UNIX platform. This book is a must for any serious UNIX/Linux programmer. It covers all of the essential UNIX/Linux API's and techniques. This book starts where the basic C programming book leaves off. Great example code. This book travels with me to every job I go to.
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