C++ Concurrent Programming: How to Identify and Solve Deadlock Problems?

In C concurrent programming, the deadlock problem occurs when one or more threads wait indefinitely for other threads to release resources, causing the program to hang. We can use std::lock_guard and std::unique_lock to implement deadlock detection. If a deadlock occurs, a std::system_error exception will be thrown. Methods to resolve deadlocks include acquiring locks in order, using timed locks, and deadlock recovery algorithms.

C Concurrent Programming: How to Identify and Solve Deadlock Problems

Understanding Deadlock

Deadlock is a common type of concurrent programming This error occurs when one or more threads wait indefinitely for a resource to be released by another thread. This situation causes the program to hang forever.

To understand deadlock, consider the following scenario:

• Thread A holds resource R1 and attempts to acquire resource R2.
• Thread B holds resource R2 and tries to obtain resource R1.

If both threads enter the waiting state at this time, waiting for the other party to release resources, a deadlock will occur.

Detecting Deadlock

In C, we can use locks like std::lock_guard and std::unique_lock to protect resources. These locks implement a deadlock detection mechanism. If a deadlock is detected, a std::system_error exception will be thrown.

We can detect deadlock by catching this exception:

std::mutex m1;
std::mutex m2;

void foo() {
  // 获取锁
  std::lock_guard<std::mutex> lock1(m1);
  std::lock_guard<std::mutex> lock2(m2);

  // 其他操作...
}

int main() {
  try {
    foo();
  } catch (const std::system_error& e) {
    std::cerr << "死锁检测到：异常代码 " << e.code() << std::endl;
  }
}

If a deadlock occurs while running this program, we print an error message.

Resolving Deadlock

Once a deadlock is detected, it needs to be resolved. Here are some common solutions:

• Acquiring locks in order: This can be prevented by forcing locks to be acquired in a specific order (for example, always acquire R1 first, then R2) deadlock.
• Use timing lock: The timing lock will time out after a period of time, forcing the thread to release resources.
• Deadlock recovery algorithm: Using specialized algorithms, such as the banker's algorithm, deadlocks can be detected and recovered.

Practical case

Consider the following code, which shares a bank account object between two threads:

class BankAccount {
public:
  int balance;
  std::mutex m;
};

void withdraw(BankAccount& account, int amount) {
  std::lock_guard<std::mutex> lock(account.m);
  if (account.balance >= amount)
    account.balance -= amount;
}

void deposit(BankAccount& account, int amount) {
  std::lock_guard<std::mutex> lock(account.m);
  account.balance += amount;
}

If two threads call simultaneously Withdraw and deposit functions, deadlock may occur. We can solve this problem by acquiring locks in order:

void withdraw(BankAccount& account, int amount) {
  std::lock_guard<std::mutex> lock(account.m);
  if (account.balance >= amount)
    account.balance -= amount;
}

void deposit(BankAccount& account, int amount) {
  std::lock_guard<std::mutex> lock(account.m);
  account.balance += amount;
}

The above is the detailed content of C++ Concurrent Programming: How to Identify and Solve Deadlock Problems?. For more information, please follow other related articles on the PHP Chinese website!