Detailed explanation of C++ function debugging: How to debug problems in multi-threaded functions?

C Multi-thread debugging can use GDB: 1. Enable debugging information compilation; 2. Set breakpoints; 3. Use info threads to view threads; 4. Use thread <n> to switch threads; 5. Use next, stepi, locals debugging. Actual case debugging deadlock: 1. Use thread apply all bt to print the stack; 2. Check the thread status; 3. Single-step the main thread; 4. Use condition variables to coordinate access to solve the deadlock.

# Detailed explanation of C function debugging: How to debug problems in multi-threaded functions?

Introduction
Multi-threaded programming can significantly improve the performance of applications, but it also brings a more complex debugging process. This article will delve into how to debug multi-threaded functions in C and provide a practical case to demonstrate debugging techniques.

Debugging Multithreading with GDB
GDB (GNU Debugger) is a powerful tool for debugging C multithreaded code. To use GDB to debug a multi-threaded function, follow these steps:

1. Enable debugging information when compiling the code (for example: g -gmulti ...).
2. Set a breakpoint in GDB (for example: break main).
3. Run the program and stop it at the desired location (for example: run args).
4. Use the info threads command to view the thread list.
5. Use the thread <n> command to switch to a specific thread.
6. Use other GDB commands for debugging, such as next, stepi, and locals for single-stepping, line-by-line execution, and inspection respectively local variables.

Practical case: debugging a deadlock multi-threaded function
The following is a practical case of debugging a deadlock multi-threaded function:

#include <iostream>
#include <thread>
#include <mutex>

std::mutex mutex;

void thread_func() {
  while (true) {
    std::lock_guard<std::mutex> guard(mutex);
    std::cout << "Thread is holding the lock" << std::endl;
    std::this_thread::sleep_for(std::chrono::seconds(1));
  }
}

int main() {
  std::thread t(thread_func);  // Start the thread
  std::lock_guard<std::mutex> guard(mutex);  // Attempt to acquire the lock in main
  std::cout << "Main thread is waiting for the lock" << std::endl;
  t.join();  // Wait for the thread to finish
}

Debugging process
While debugging this function in GDB, we found that it was deadlocked because the main thread tried to acquire a lock held by another thread. To solve this problem, we can perform the following steps:

1. Use the thread apply all bt command to print the call stack in all threads.
2. Observe that both the main thread and another thread are waiting for the same lock.
3. Use the thread info <n> command to check the status of another thread and find that it is sleeping.
4. Use the next command to step into the main thread and find that it cannot acquire the lock, thus deadlocking.

Solution
To resolve this deadlock, we can use condition variables to coordinate access between threads. Here is a modified code snippet:

#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>

std::mutex mutex;
std::condition_variable cv;

void thread_func() {
  while (true) {
    std::unique_lock<std::mutex> guard(mutex);
    cv.wait(guard);  // Wait for the condition variable to be notified
    std::cout << "Thread is holding the lock" << std::endl;
    std::this_thread::sleep_for(std::chrono::seconds(1));
  }
}

int main() {
  std::thread t(thread_func);  // Start the thread
  std::unique_lock<std::mutex> guard(mutex);
  cv.notify_all();  // Notify the other thread to acquire the lock
  guard.unlock();  // Release the lock in main
  t.join();  // Wait for the thread to finish
}

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