signal

/*
Copyright (C) 2018-2024 Geoffrey Daniels. https://gpdaniels.com/

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, version 3 of the License only.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <https://www.gnu.org/licenses/>.
*/

#pragma once
#ifndef GTL_DEBUG_SIGNAL_HPP
#define GTL_DEBUG_SIGNAL_HPP

// Summary: Class to wrap signal handlers allowing the use of lambdas with scope.

#if defined(_MSC_VER)
#   pragma warning(push, 0)
#endif

#include <csignal>
#include <functional>

#if defined(_MSC_VER)
#   pragma warning(pop)
#endif

namespace gtl {
    /// @brief  The signal class implements a wrapper around signal handling allowing the use of lambdas with scope.
    /// @tparam signal_number The signal number, e.g. SIGABRT, SIGFPE, SIGILL, SIGINT, SIGSEGV, SIGTERM, etc...
    template <int signal_number>
    class signal final {
    private:
        /// @brief  The manager class is a singleton as there can only be one handler for each signal in a program.
        class manager final {
        private:
            /// @brief  Each signal manager singleton holds a lambda function to call.
            std::function<void(int)> function = nullptr;

        public:
            /// @brief  Defaulted destructor.
            ~manager() = default;

        private:
            /// @brief  Defaulted constructor, made private as this is a singleton.
            manager() = default;

        public:
            /// @brief  Singleton instance accessor function.
            /// @return Returns the static instance of this class.
            static manager& get_instance() {
                static manager instance;
                return instance;
            }

            /// @brief  Set the function to call when the signal is raised.
            /// @param  new_function The signal handler lambda function that raised signals are forwarded to.
            /// @return Returns the previously attached signal handling lambda function.
            std::function<void(int)> set_function(const std::function<void(int)> new_function) {
                std::function<void(int)> old_function = this->function;
                this->function = new_function;
                return old_function;
            }

            /// @brief  Clear the function to call when the signal is raised.
            /// @return Returns the previously attached signal handling function.
            std::function<void(int)> clear_function() {
                std::function<void(int)> old_function = this->function;
                this->function = nullptr;
                return old_function;
            }

        public:
            /// @brief  Static signal handler function that gets the singleton instance and attempts to call the contained handler.
            static void handler(int signal_id) {
                if (manager::get_instance().function) {
                    manager::get_instance().function(signal_id);
                }
            }
        };

    private:
        /// @brief  The function specification of a signal handler.
        using function_type = void (*)(int);

        /// @brief  Variable that holds any previously attached signal handler function to be restored upon destruction.
        function_type old_signal_function;

        /// @brief  Variable that holds any previously attached single handler lambda function to be restored upon destruction.
        std::function<void(int)> old_signal_lambda;

    public:
        /// @brief  The destructor resets the signal handlers to their previous values.
        ~signal() {
            std::signal(signal_number, this->old_signal_function);
            manager::get_instance().set_function(this->old_signal_lambda);
        }

        /// @brief  The constructor sets the signal handlers to new functions and stores their old values.
        /// @param  new_function The signal handler lambda function that raised signals are forwarded to.
        signal(const std::function<void(int)>& new_function) {
            this->old_signal_lambda = manager::get_instance().set_function(new_function);
            this->old_signal_function = std::signal(signal_number, &manager::handler);
        }
    };
}

#endif // GTL_DEBUG_SIGNAL_HPP