Basic example message queues reader and writer

Message queues

• In this program, you are going to learn

• How to open a message queue ?

• How to get/set message queue attributes ?

• How to receive a message from a message queue?

• How to send a message to a message queue?

• How to use message queues APIs ?

  • mq_open

  • mq_getattr

  • mq_receive

  • mq_send

  • mq_close

Topics in this section,

• program 1: message queues single reader and single writer : single data transfer

• program 2: message queues single reader and single writer : multiple data transfer

program 1 : message queues single reader and single writer : single data transfer

Topics in this section,

• message queues FAQs

• Program1 : Step 1: Sequence Diagram for reader.c

• Program1 : Step 2: Program for reader.c

• Program1 : Step 3: Compile and Execute reader.c

• Program1 : Step 4: Sequence Diagram for writer.c

• Program1 : Step 5: Program for writer.c

• Program1 : Step 6: Compile and Execute writer.c

• Summary

program 1 : message queues single reader and single writer : multiple data transfer

Topics in this section,

• message queues FAQs

• Program2 : Step 1: Sequence Diagram for reader.c

• Program2 : Step 2: Program for reader.c

• Program2 : Step 3: Compile and Execute reader.c

• Program2 : Step 4: Sequence Diagram for writer.c

• Program2 : Step 5: Program for writer.c

• Program2 : Step 6: Compile and Execute writer.c

• Summary

message queues FAQs

Let us answer few basic questions in this message queues

What does mq_open do in this context?

See Answer

mq_open is used to open or create a message queue. It creates a new message queue or opens an existing one identified by queue_name.

Why use O_CREAT | O_RDWR as flags?

See Answer

O_CREAT ensures the creation of the message queue if it does not exist, and O_RDWR grants read and write access to the message queue.

What is the significance of the 0666 permission parameter?

See Answer

It sets the permission bits for the message queue, allowing read and write access for all users.

Can mq_open open an existing queue?

See Answer

Yes, if the queue with the specified name already exists, mq_open will open that existing queue.

What happens if the queue with queue_name does not exist?

See Answer

If the queue does not exist, and O_CREAT is specified, mq_open will create a new queue with the given name.

How does mq_open handle errors during queue creation?

See Answer

mq_open returns -1 on failure. You can check errno for specific error details. Common issues include insufficient permissions or system resources.

Why pass NULL as the last parameter?

See Answer

The last parameter (struct mq_attr) is used for setting message queue attributes. Passing NULL indicates using default attributes.

Can multiple processes open the same message queue?

See Answer

Yes, multiple processes can open the same message queue by using the same queue_name.

What is the lifetime of the message queue created with mq_open?

See Answer

The message queue persists until it is explicitly closed using mq_close or until the system is shut down.

How do I unlink (delete) a message queue created with mq_open?

See Answer

You can use mq_unlink(queue_name) to unlink (delete) the message queue.
This does not close the queue but removes its reference in the file system.

Program1 : Step 1: Sequence Diagram for reader.c

Program1 : Step 5: Program for reader.c

• There are many functions used in message queues. We can classify those functions based on functionalities.

  • mq_open

  • mq_getattr

  • mq_receive

  • mq_send

  • mq_close

• mq_open is used to open message queues. For example,

mq = mq_open(queue_name, O_CREAT | O_RDWR, 0666, NULL);

• mq_getattr is used to get message queue attributes. For example,

mq_getattr(mq, &attr);

• mq_receive is used to receive a message from a message queue. For example,

ret = mq_receive(mq, buffer, attr.mq_msgsize, NULL);;

• mq_send is used to send a message to a message queue. For example,

ret = mq_send(mq, message, strlen(message) + 1, 0);

• mq_close is used to close the opened message queues. For example,

mq_close(mq);

• See the full program below,

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <mqueue.h>
#include <unistd.h>
#include <signal.h>

int main(void) 
{
  struct mq_attr attr;
  char* buffer;
  int ret;
  mqd_t mq;
  const char* queue_name = "/my_queue";
  const char* message = "Hello, Writer!";

  mq = mq_open(queue_name, 
  O_CREAT | O_RDWR, 0666, NULL);
  
  if (mq == (mqd_t)-1) {
    perror("mq_open");
    return -1;
  }

  ret = mq_getattr(mq, &attr);
 
  if (ret < 0) {
    perror("mq_getattr");
    (void)close(mq);
    return -2;
  }

  buffer = (char*)malloc(
  attr.mq_msgsize);

  if (buffer == NULL) {
    perror("malloc");
    (void)close(mq);
    return -3;
  }

  ret = mq_receive(mq, buffer, 
  attr.mq_msgsize, NULL);

  if (ret < 0) {
    perror("mq_receive");
    (void)close(mq);
    return -4;
  }

  printf("Received message: %s\n",
  buffer);
  free(buffer);

  ret = mq_send(mq, message, 
  strlen(message) + 1, 0);
 
  if (ret < 0) {
    perror("mq_send");
    (void)close(mq);
    return -5;
  } else {
    printf("messageSent = %s\n", 
    message);
  }
  (void)mq_close(mq);

  return 0;
}

Program1 : Step 6: Compile and Execute reader.c

$ gcc -o reader reader.c -lrt

$ sudo ./reader

Received message: Hello, reader!
messageSent = Hello, Writer!

Program1 : Step 4: Sequence Diagram for writer.c

Program1 : Step 5: Program for writer.c

• There are many functions used in message queues. We can classify those functions based on functionalities.

  • mq_open

  • mq_send

  • mq_getattr

  • mq_receive

  • mq_close

• mq_open is used to open message queues. For example,

mq = mq_open(queue_name, O_CREAT | O_RDWR, 0666, NULL);

• mq_send is used to send a message to a message queue. For example,

ret = mq_send(mq, message, strlen(message) + 1, 0);

• mq_getattr is used to get message queue attributes. For example,

ret = mq_getattr(mq, &attr);

• mq_receive is used to receive a message from a message queue. For example,

ret = mq_receive(mq, buffer, attr.mq_msgsize, NULL);;

• mq_close is used to close the opened message queues. For example,

mq_close(mq);

• See the full program below,

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <mqueue.h>
#include <unistd.h>
#include <signal.h>

int main(void) 
{
  mqd_t mq;
  int ret;
  char *buffer;
  const char* queue_name = "/my_queue";
  const char* message = "Hello, reader!";
  struct mq_attr attr;

  mq = mq_open(queue_name, 
  O_CREAT | O_RDWR, 0666, NULL);
  
  if (mq == (mqd_t)-1) {
    perror("mq_open");
    return -1;
  }

  ret = mq_send(mq, message, 
  strlen(message) + 1, 0);
 
  if (ret < 0) {
    perror("mq_send");
    (void)close(mq);
    return -2;
  } else {
    printf("messageSent = %s\n", 
    message);
  }

  ret = mq_getattr(mq, &attr);
 
  if(ret < 0) {
    perror("mq_getattr");
    (void)close(mq);
    return -3;
  }  

  buffer = (char*)malloc(
  attr.mq_msgsize);

  if (buffer == NULL) {
    perror("malloc");
    (void)close(mq);
    return -4;
  }

  memset(buffer, 0, 
  sizeof(buffer));
  
  ret = mq_receive(mq, buffer, 
  attr.mq_msgsize, NULL);

  if (ret < 0) {
    perror("mq_receive");
    (void)close(mq);
    return -5;
  }

  printf("Received message: %s\n", 
  buffer);
  free(buffer);     
  (void)mq_close(mq);

  return 0;
}

Program1 : Step 6: Compile and Execute writer.c

$ gcc -o writer writer.c -lrt

$ sudo ./writer

messageSent = Hello, reader!
Received message: Hello, Writer!

Program2 : Step 1: Sequence Diagram for reader.c

Program2 : Step 2: Program for reader.c

• There are many functions used in message queues. We can classify those functions based on functionalities.

  • mq_open

  • mq_getattr

  • mq_receive

  • mq_send

  • mq_close

• mq_open is used to open message queues. For example,

mq = mq_open(queue_name, O_CREAT | O_RDWR, 0666, NULL);

• mq_getattr is used to get message queue attributes. For example,

mq_getattr(mq, &attr);

• mq_receive is used to receive a message from a message queue. For example,

ret = mq_receive(mq, buffer, attr.mq_msgsize, NULL);;

• mq_send is used to send a message to a message queue. For example,

ret = mq_send(mq, message, strlen(message) + 1, 0);

• mq_close is used to close the opened message queues. For example,

mq_close(mq);

• See the full program below,

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <mqueue.h>
#include <unistd.h>
#include <signal.h>

#define NUM_MESSAGES 10

int main(void) 
{
  struct mq_attr attr;
  char* buffer;
  int ret, i;
  mqd_t mq;
  const char* queue_name = "/my_queue";
  const char* message = "Hello, Writer!";

  mq = mq_open(queue_name, 
  O_CREAT | O_RDWR, 0666, NULL);
  
  if (mq == (mqd_t)-1) {
    perror("mq_open");
    return -1;
  }

  ret = mq_getattr(mq, &attr);
 
  if (ret < 0) {
    perror("mq_getattr");
    (void)close(mq);
    return -2;
  }

  i = 0;
  while (i < NUM_MESSAGES) {
  buffer = (char*)malloc(
  attr.mq_msgsize);

  if (buffer == NULL) {
    perror("malloc");
    (void)close(mq);
    return -3;
  }

  ret = mq_receive(mq, buffer, 
  attr.mq_msgsize, NULL);

  if (ret < 0) {
    perror("mq_receive");
    (void)close(mq);
    return -4;
  }

  printf("Received message: %s\n",
  buffer);
  free(buffer);

  ret = mq_send(mq, message, 
  strlen(message) + 1, 0);
 
  if (ret < 0) {
    perror("mq_send");
    (void)close(mq);
    return -5;
  } else {
    printf("messageSent = %s\n", 
    message);
  }
  ++i;
  }

  (void)mq_close(mq);

  return 0;
}

Program2 : Step 3: Compile and Execute reader.c

$ gcc -o reader reader.c -lrt

$ sudo ./reader

Received message: Hello, reader!
messageSent = Hello, Writer!
Received message: Hello, reader!
messageSent = Hello, Writer!
Received message: Hello, reader!
messageSent = Hello, Writer!
Received message: Hello, reader!
messageSent = Hello, Writer!
Received message: Hello, reader!
messageSent = Hello, Writer!
Received message: Hello, reader!
messageSent = Hello, Writer!
Received message: Hello, reader!
messageSent = Hello, Writer!
Received message: Hello, reader!
messageSent = Hello, Writer!
Received message: Hello, reader!
messageSent = Hello, Writer!
Received message: Hello, reader!
messageSent = Hello, Writer!

Program2 : Step 1: Sequence Diagram for writer.c

Program2 : Step 5: Program for writer.c

• There are many functions used in message queues. We can classify those functions based on functionalities.

  • mq_open

  • mq_send

  • mq_getattr

  • mq_receive

  • mq_close

• mq_open is used to open message queues. For example,

mq = mq_open(queue_name, O_CREAT | O_RDWR, 0666, NULL);

• mq_send is used to send a message to a message queue. For example,

ret = mq_send(mq, message, strlen(message) + 1, 0);

• mq_getattr is used to get message queue attributes. For example,

ret = mq_getattr(mq, &attr);

• mq_receive is used to receive a message from a message queue. For example,

ret = mq_receive(mq, buffer, attr.mq_msgsize, NULL);;

• mq_close is used to close the opened message queues. For example,

mq_close(mq);

• See the full program below,

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <mqueue.h>
#include <unistd.h>
#include <signal.h>

int main(void) 
{
  mqd_t mq;
  int ret;
  char *buffer;
  const char* queue_name = "/my_queue";
  const char* message = "Hello, reader!";
  struct mq_attr attr;

  mq = mq_open(queue_name, 
  O_CREAT | O_RDWR, 0666, NULL);
  
  if (mq == (mqd_t)-1) {
    perror("mq_open");
    return -1;
  }

  ret = mq_send(mq, message, 
  strlen(message) + 1, 0);
 
  if (ret < 0) {
    perror("mq_send");
    (void)close(mq);
    return -2;
  } else {
    printf("messageSent = %s\n", 
    message);
  }

  ret = mq_getattr(mq, &attr);
 
  if(ret < 0) {
    perror("mq_getattr");
    (void)close(mq);
    return -3;
  }  

  buffer = (char*)malloc(
  attr.mq_msgsize);

  if (buffer == NULL) {
    perror("malloc");
    (void)close(mq);
    return -4;
  }

  memset(buffer, 0, 
  sizeof(buffer));
  
  ret = mq_receive(mq, buffer, 
  attr.mq_msgsize, NULL);

  if (ret < 0) {
    perror("mq_receive");
    (void)close(mq);
    return -5;
  }

  printf("Received message: %s\n", 
  buffer);
  free(buffer);     
  (void)mq_close(mq);

  return 0;
}

Program2 : Step 6: Compile and Execute writer.c

$ gcc -o writer writer.c -lrt

$ sudo ./writer

messageSent = Hello, reader!
Received message: Hello, Writer!
messageSent = Hello, reader!
Received message: Hello, Writer!
messageSent = Hello, reader!
Received message: Hello, Writer!
messageSent = Hello, reader!
Received message: Hello, Writer!
messageSent = Hello, reader!
Received message: Hello, reader!
messageSent = Hello, reader!
Received message: Hello, reader!
messageSent = Hello, reader!
Received message: Hello, reader!
messageSent = Hello, reader!
Received message: Hello, reader!
messageSent = Hello, reader!
Received message: Hello, reader!
messageSent = Hello, reader!
Received message: Hello, reader!

Summary

Message queues API	Learning
mq_open	creates a new message queue or opens an existing one, depending on the specified flags and parameters.
mq_getattr	is employed to obtain the attributes of an open message queue, including its current state and configuration.
mq_receive	receiving (reading) messages from a message queue.
mq_send	sending (writing) messages to a message queue.
mq_close	used to close a message queue descriptor.

See Also

• Previous topic

  • raw_af_packet_tcp_htons_ETH_P_ALL

• Current topic

  • Message_queues

• Other IPCs

  • NamedPipes

  • Netlink

  • Shared_Memory

  • Shared_Memory_2_FDS

  • SocketPair

  • Timerfd