Example Slot Signal C++

04-02-2021

Example Slot Signal C++

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Qt Signal Slot Example C
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Qt Signal Slot Example C++

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Emitting a signal from QML and listening to it from C. To create a signal in QML, simply add the following line to the object which will emit the signal. Signal submitTextField(string text) Here I have created a signal, submitTextField, which will pass a string as an argument to any connecting slots (if they choose to receive it). 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. While being better in many regards, the new connection syntax in Qt5 has one big weakness: Connecting overloaded signals and slots. In order to let the compiler resolve the overloads we need to use staticcasts to member function pointers, or (starting in Qt 5.7) qOverload and friends. 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.

Unlike a console mode application, which is executed in a sequential manner, a GUI based application is event driven. Functions or methods are executed in response to user’s actions like clicking on a button, selecting an item from a collection or a mouse click etc., called events.

Widgets used to build the GUI interface act as the source of such events. Each PyQt widget, which is derived from QObject class, is designed to emit ‘signal’ in response to one or more events. The signal on its own does not perform any action. Instead, it is ‘connected’ to a ‘slot’. The slot can be any callable Python function.

In PyQt, connection between a signal and a slot can be achieved in different ways. Following are most commonly used techniques −

A more convenient way to call a slot_function, when a signal is emitted by a widget is as follows −

Suppose if a function is to be called when a button is clicked. Here, the clicked signal is to be connected to a callable function. It can be achieved in any of the following two techniques −

Example

In the following example, two QPushButton objects (b1 and b2) are added in QDialog window. We want to call functions b1_clicked() and b2_clicked() on clicking b1 and b2 respectively.

When b1 is clicked, the clicked() signal is connected to b1_clicked() function

When b2 is clicked, the clicked() signal is connected to b2_clicked() function

Example

The above code produces the following output −

Output

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

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.

C Signal handler and Example:

Basic C signal callback function example:

File: signalExample.cpp

Example to handle ctrl-c
Compile: gcc signalExample.cpp

Run: a.out
Results:

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

C++ Signal Registration and Handling Class:

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

C Signal Man Pages:

C functions:

Qt Signal Slot Example C

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.