pthread在分离的线程上创建错误11

Question

I have a server application, which waits on a queue, fetches incoming messages, and spawns a thread to process the received message and send a reply.

The pthread portion/options I am using are as follows:

pthread_attr_t child_attr;
pthread_attr_setdetachstate(&child_attr, PTHREAD_CREATE);

// other code here

while (true)
{

    // code here to wait on valid message (msg)
    if (valid_message(msg))
    {
        pthread_t child_thread;
        MessageProcessor * processor = new MessageProcessor();
        if (0 == pthread_create(&child_thread, &child_attr, processor->process, (void *) msg))
        {
            printf("Thread dispatch successful\n");
        }
        else
        {
            printf("Error %d: could not create thread\n", errno);
        }
    }
}

// other code here
pthread_attr_destroy(&child_attr);

Every time I run this, the error code displayed is 11 , which apparently would indicate that my process has crossed the max threads threshold, based on stuff I've read on the Internet.

However,

• This is happening right from the beginning, not after I have run the application for a while.
• The threads are created detached, so I shouldn't have to use pthread_join().
• I used top as well as ps -p <PID> -lfT to check how many threads were in use by the application, and only 3 were (one for main server, one for the message receiver, and one for a message sender for the queue system)

PS :

The "process" prototype is as follows:

class MessageProcessor
{
    MessageProcessor();
    static void * MessageProcessor::process(void * arg);
}

void * MessageProcessor::process(void * arg)
{
    // do something here with arg
}

Answer 1

Like all pthreads functions, pthread_create does not set errno to report errors, it returns an error number instead. To see why it failed you need to print the return value, not errno .

const int err = pthread_create(&child_thread, &child_attr, processor->process, (void *) msg);

if (err == 0)
  printf("Thread dispatch successful\n");
else
  printf("Error %d: could not create thread\n", err);

POSIX specifies errno like so:

The value of errno shall be defined only after a call to a function for which it is explicitly stated to be set [...] The value of errno should only be examined when it is indicated to be valid by a function's return value.

Since pthread_create is not documented to set errno it means the value is not defined after a call to pthread_create and should not be examined.

Answer 2

errno 11 is usually EAGAIN, in this case it means no more processes (linux treats threads as light weight processes - see the clone manual page)available.

The while(true) loop will run forever making processes.

Note if you have a special version of Linux like ARM the error number 11 is NOT required to be EAGAIN. So take this answer with a grain of salt.

Answer 3

Your code is using an uninitialized child_attr , you'll have to do:

  pthread_attr_init(&child_attr); 
  pthread_attr_setdetachstate(&child_attr, PTHREAD_CREATE_DETACHED);