cygwin上带有FD_SET（）和消息队列的核心转储

Question

I have the following C code for setting up select() to listen for incoming data on both a TCP socket and a message queue:

fd_set readSet;
u32 maxFD = 0;
mqd_t parentQueue;
int serverSocket;

/* not shown: parentQueue and serverSocket were successfully opened */

FD_ZERO(&readSet);
FD_SET(serverSocket,&readSet);
FD_SET(parentQueue,&readSet);

/* next, not shown: block with select() and I/O processing */ 

My problem is that the code crashes (with core dump) when I add the message queue to readSet with FD_SET(), FD_SET(parentQueue,&readSet).

The man page says that on Linux, mqd_t can be used with select(). And parentQueue corresponds to a valid mqd_t, obtained with mq_open(). So why would FD_SET() crash ? Is it because on Cygwin, message queues are not files ? But I can see the queue in /dev/mqueue.

Answer 1

After digging a little bit, I identified the problem. On cygwin, the queue file descriptor (mqd_t) is 64 bits long (I am using a 64 bit system) and because of that the queue file descriptor falls outside the range for select, FD_SETSIZE, causing FD_SET to crash