c++ fork进程与同步锁

首先定义在多进程环境中的锁，采用读写锁，即可以同时读，但只能单独写。

头文件processLock.h

#ifndef PROCESSLOCK_H
#define PROCESSLOCK_H

#include <pthread.h>
#include <dlfcn.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <stdlib.h>
#include <cstring>

class ProcessLock {
  private:
    pthread_rwlock_t _lock;
    pthread_rwlockattr_t _attr;
    ProcessLock();
    void init();
    static ProcessLock* processLock;
  public:
    int read();
    int write();
    int unlock();
    static ProcessLock* createLock();
    ~ProcessLock();
};

#endif

源文件processLock.cpp

#include "processLock.h"

ProcessLock* ProcessLock::processLock = NULL;

void ProcessLock::init() {
  //初始化读写锁的属性为进程共享，并与读写锁关联起来。
  pthread_rwlockattr_init(&_attr);
  pthread_rwlockattr_setpshared(&_attr, PTHREAD_PROCESS_SHARED);
  pthread_rwlock_init(&_lock, &_attr);
}

int ProcessLock::read() {
  return pthread_rwlock_rdlock(&_lock);
}

int ProcessLock::write() {
  return pthread_rwlock_wrlock(&_lock);
}
int ProcessLock::unlock() {
  return pthread_rwlock_unlock(&_lock);
}

ProcessLock* ProcessLock::createLock() {
  //采用mmap实现指针的进程共享，这样在后续获取锁的时候，就是进程间共享的锁。
  if (prorcessLock == NULL) {
    processLock = (ProcessLock)mmap(NULL, sizeof(ProcessLock), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANON, -1, 0);
    processLock->init();
  }
  return processLock;
}

ProcessLock::~ProcessLock() {
  pthread_rwlock_destroy(&_lock);
  pthread_rwlockattr_destroy(&_attr);
}

实际应用：

#include "processLock.h"

int main() {
  //child process read, main process write
  ProcessLock* lock = ProcessLock::createLock();
  pid_t fpid = fork();
  if (fpid < 0) {
    cout << "fork error!" << endl;
    exit(1);
  } else if (fpid == 0) {
    cout << "child process" << endl;
    lock->read();
    //do read something
    lock->unlock();
  } else {
    cout << "parent process" << endl;
    lock->write();
    //do write something
    lock->unlock();
  }
  return 0;
}