如何"buffer" UNIX 信号

如何以阻塞函数在被调用时解封的方式缓冲UNIX信号？在我们的软件中，我们使用套接字。我们现在想通过信号取消阻止/取消recv()呼叫。问题是，如果在输入recv()之前发送信号，那么它就会丢失，recv()永远不会解锁。

主函数如下所示：

bool signalHandlerSetup;
bool signalSent;
int main(int argc, char* argv[])
{
pthread_t thread;
signalHandlerSetup = false;
signalSent = false;
// Block the SIGUSR1 signal in the main thread
sigset_t signalSet;
sigemptyset(&signalSet);
sigaddset(&signalSet, SIGUSR1);
pthread_sigmask(SIG_BLOCK, &signalSet, NULL);
// Setup the signal handler for all future threads
struct sigaction signalAction;
signalAction.sa_flags = 0;
signalAction.sa_handler = [](int signalNumber) {};
sigaction(SIGUSR1, &signalAction, NULL);
pthread_create(&thread, NULL, secondaryThreadFunction, NULL);
std::cout << "Started thread" << std::endl;
// Wait until the signal handler is setup
while (!signalHandlerSetup);
pthread_kill(thread, SIGUSR1);
signalSent = true;
pthread_join(thread, NULL);
std::cout << "Joined thread" << std::endl;
}

辅助线程只是创建一个套接字并尝试从中读取：

void* secondaryThreadFunction(void *arg)
{
// Setup socket
int internSocket = setupSocket();
// Setup the signal handling
sigset_t signalSet;
sigemptyset(&signalSet);
sigaddset(&signalSet, SIGUSR1);
pthread_sigmask(SIG_UNBLOCK, &signalSet, NULL);
signalHandlerSetup = true;
while (!signalSent);
char buffer;
std::cout << "recv()..." << std::endl;
ssize_t bytesRead = recv(internSocket, static_cast<void*>(&buffer), sizeof(buffer), 0);
std::cout << "recv() canceled" << std::endl;
close(internSocket);
}

如您所见，在输入recv()函数之前显式发送信号以说明问题。

除了缓冲之外，可能还有其他解决方案，但每种解决方案

• 等到辅助线程被阻塞(导致主线程也阻塞或繁忙循环(
• 不断发送信号(另一个繁忙循环(

编辑： 不应仅仅为了取消阻塞辅助线程而关闭套接字。我们可能希望在解锁后再次在同一套接字上运行。

编辑2： 我已经能够用马丁·詹姆斯和达伦·史密斯提供的解决方案来解决这个问题。我现在正在将select()函数与事件文件描述符结合使用。作为参考，这里是工作解决方案：

int eventDescriptor;
int main(int argc, char* argv[])
{
pthread_t thread;
// Create the event file descriptor
eventDescriptor = eventfd(
0,  // Initial value
0   // Flags
);
if (eventDescriptor == -1)
{
std::cout << "Failed to create event file descriptor" << std::endl;
return -1;
}
pthread_create(&thread, NULL, secondaryThreadFunction, NULL);
std::cout << "Started thread" << std::endl;
// Notify the event descriptor
uint64_t valueToWrite = 1;
if (write(eventDescriptor, &valueToWrite, sizeof(uint64_t)) == -1)
{
std::cout << "Failed to write to event file descriptor" << std::endl;
return -1;
}
pthread_join(thread, NULL);
std::cout << "Joined thread" << std::endl;
close(eventDescriptor);
return 0;
}

这是辅助线程：

void* secondaryThreadFunction(void *arg)
{
// Setup socket
int internSocket = setupSocket();
// Set up the file descriptor set
fd_set readFileDescriptorSet;
FD_ZERO(&readFileDescriptorSet);
FD_SET(internSocket, &readFileDescriptorSet);
FD_SET(eventDescriptor, &readFileDescriptorSet);
char buffer;
std::cout << "select()..." << std::endl;
int fileDescriptorsSet = select(std::max(internSocket, eventDescriptor) + 1, &readFileDescriptorSet, NULL, NULL, NULL);
if (FD_ISSET(eventDescriptor, &readFileDescriptorSet))
{
std::cout << "select() canceled via event" << std::endl;
}
else if (FD_ISSET(internSocket, &readFileDescriptorSet))
{
std::cout << "select() canceled through socket" << std::endl;
ssize_t bytesRead = recv(internSocket, static_cast<void*>(&buffer), sizeof(buffer), 0);
}
close(internSocket);
}