What is user-defined signal handler?

User-defined signal handlers are custom functions that programmers write to override the default behavior when specific signals occur in a program. Signals are software interrupts sent to indicate important events, and they can be handled in two ways: using default signal handlers or user-defined signal handlers.

Types of Signal Handlers

• Default signal handler − Built-in system behavior for each signal type
• User-defined signal handler − Custom function written by the programmer to handle specific signals

User-defined signal handlers allow programs to customize their response to signals rather than relying on default system actions. Different signals are handled in various ways: some (like window resizing) may be ignored, while others (like illegal memory access) typically terminate the program.

Signal Handler Function Structure

A signal handler function must follow specific requirements:

• Must have void return type
• Must accept exactly one int parameter (the signal number)
• Can have any function name chosen by the programmer

Example Declaration

For handling the SIGCHLD signal (child process termination), a signal handler might be declared as:

void sigchld_handler(int sig);

The integer parameter sig contains the signal number that triggered the handler. This allows a single handler function to manage multiple different signals by checking the signal number.

Common Use Cases

User-defined signal handlers are commonly used for:

• Graceful shutdown − Cleaning up resources when receiving termination signals
• Child process management − Handling SIGCHLD to prevent zombie processes
• User interaction − Responding to signals like SIGUSR1 and SIGUSR2 for custom communication
• Error recovery − Implementing custom responses to error conditions

Signal Handler Registration

To install a user-defined signal handler, programs typically use functions like signal() or sigaction():

signal(SIGCHLD, sigchld_handler);

This registers sigchld_handler as the custom handler for the SIGCHLD signal, overriding the default system behavior.

Conclusion

User-defined signal handlers provide programmers with the ability to customize program behavior in response to system signals. They must follow specific function signatures and can be designed to handle single or multiple signal types, enabling more robust and responsive applications.