Assertion failed: dummy == 0 at src/signaler.cpp:369

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Issue description

When using libzmq for C++, we encountered a program crash.It said：Assertion failed: dummy == 0 (libzmq-4.3.5\src\signaler.cpp:369) The error occurs sporadically and is not fixed, but it basically happens after receiving (RECV) a message. I Use C++ libzmq, The other side is python.

Environment

• libzmq version (commit hash if unreleased): 4.3.5
• OS: windows server 2019

Minimal test code / Steps to reproduce the issue

1. void *context = zmq_ctx_new();
2. void *responder = zmq_socket(context, ZMQ_REP);
3. while (true)
4. {
5. 复制

   zmq_msg_t recv_msg;

6.   zmq_msg_init(&recv_msg);
8.   int nbytes = zmq_msg_recv(&recv_msg, responder, 0);
9.   if (nbytes < 0)
10. {
11.       zmq_msg_close(&recv_msg);
12.       continue;
13. }          
15.   size_t nReqNum = zmq_msg_size(&recv_msg);
16.   char* chRecCmd1 = new char[nReqNum + 1];
17.   memcpy(chRecCmd1, zmq_msg_data(&recv_msg), nReqNum);
18.   chRecCmd1[nReqNum] = '\0';
20.   //some bizcode....
22.   delete[] chRecCmd1;
23.   chRecCmd1 = NULL;
24. }

What's the actual result? (include assertion message & call stack if applicable)

Assertion failed: dummy == 0 (libzmq-4.3.5\src\signaler.cpp:369) Sometimes it crashed after Line18.Sometimes it crashed at or after Line20. It is not fixed in the same position.It seems crashed after recv a message but It never get into Line11(recv bytes are alwayes greater than 0,It seems i received the message successfully). But I don't know why it crashed after success receving.Please help.

What's the expected result?

Expected result is: No crash.