mq_receive: message too long

Question

I am implementing a communication between 2 processes using a queue. The problem is that when I call the function mq_receive, I get this error: Message too long.

I h

Answer 1

It seems like you need to read the docs more carefully. When you call mq_receive you should pass size of the destination buffer. This size must be greater than the mq_msgsize attribute of the queue. In addition, it seems like you have an error in queue attributes initialisation that makes proper mq_receive call impossible. Here is standard message queue session:

1. Fill mq_attr struct (doc):

   struct mq_attr attr;  
   attr.mq_flags = 0;  
   attr.mq_maxmsg = 10;  
   attr.mq_msgsize = 33;  
   attr.mq_curmsgs = 0;  
   

2. Create queue with mq_open in master process (doc):

   mqd_t queue = mq_open(qname, O_CREAT|O_RDWR, 0644, &attr);
   

3. In writer process open queue for writing:

   mqd_t queue = mq_open(qname, O_WRONLY);
   

   And send some text. Length of the text must be lesser than mq_msgsize attribute of the queue (doc):

   mq_send(queue, "some message", strlen("some message")+1, 1);
   

4. In reader process open queue for reading:

   mqd_t queue = mq_open(qname, O_RDONLY);
   

   And then allocate buffer and receive message. Size of buffer *must be greater than the mq_msgsize attribute of the queue. Here we create 50-byte buffer while mq_msgsize == 33 (doc):

   char rcvmsg[50];
   int iret = mq_receive(queue, rcvmsg, 50, NULL);
   

Also remember that you should use %ld for print long instead of %d.