Message queue
- C course
- Advanced
- Lesson 34/83
- 5 min
Learn message queues with msgget and msgsnd.
Message queue
A message queue is an IPC structure where processes send and receive typed messages in order. It is created with msgget(key, flags); the IPC_PRIVATE key produces a new queue. Each message has a long mtype field; the receiver uses it as a filter.
The message structure is user-defined, e.g. struct Msg { long mtype; char mtext[16]; };. It is sent with msgsnd(msqid, &msg, sizeof(msg.mtext), 0) and received with msgrcv(msqid, &msg, sizeof(msg.mtext), 1, 0). mtype = 1 targets; 0 accepts any type. msgctl(msqid, IPC_RMID, NULL) removes the queue.
Message struct and flow
The first field of the message struct must always be long mtype; otherwise msgsnd and msgrcv misbehave. The mtext field is optional; you pass its size explicitly to the functions.
With fork the child can be the receiver and the parent the sender. The child waits on msgrcv, which is a blocking call. When the parent calls msgsnd the child wakes up, takes the message and prints it. wait(NULL) lets the parent reap the child, then msgctl(IPC_RMID) removes the queue.
Practice Send data via message queue
Define the struct struct Msg { long mtype; char mtext[16]; };. Set up a queue with int msqid = msgget(IPC_PRIVATE, 0600 | IPC_CREAT);. Fork a child with pid_t pid = fork();. In the child: struct Msg rmsg; msgrcv(msqid, &rmsg, sizeof(rmsg.mtext), 1, 0);, printf("%s\n", rmsg.mtext);, _exit(0);. In the parent: struct Msg msg; msg.mtype = 1; strncpy(msg.mtext, "merhaba mq", sizeof(msg.mtext) - 1);, msgsnd(msqid, &msg, sizeof(msg.mtext), 0);, wait(NULL);, msgctl(msqid, IPC_RMID, NULL);.
Starter code
#include <stdio.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <sys/wait.h>
#include <unistd.h>
#include <string.h>
// Define message struct
// Set up queue and fork child receiver
// Parent sends message and cleans up
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <sys/wait.h>
#include <unistd.h>
#include <string.h>
struct Msg { long mtype; char mtext[16]; };
int main(void) {
int msqid = msgget(IPC_PRIVATE, 0600 | IPC_CREAT);
pid_t pid = fork();
if (pid == 0) {
struct Msg rmsg;
msgrcv(msqid, &rmsg, sizeof(rmsg.mtext), 1, 0);
printf("%s\n", rmsg.mtext);
_exit(0);
}
struct Msg msg;
msg.mtype = 1;
strncpy(msg.mtext, "merhaba mq", sizeof(msg.mtext) - 1);
msgsnd(msqid, &msg, sizeof(msg.mtext), 0);
wait(NULL);
msgctl(msqid, IPC_RMID, NULL);
return 0;
}Expected output
merhaba mq
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does msgget(IPC_PRIVATE, flags) produce?
- Existing queue
- New unique queue Correct answer
- Empty message
- Deleted queue
Explanation: IPC_PRIVATE always produces a new and unique queue.
2 Match message queue functions
| Term | Meaning |
|---|---|
msgget |
Creates queue |
msgsnd |
Sends message |
msgrcv |
Receives message |
msgctl |
Removes queue |
3 msgsnd call
msgsnd(msqid, &msg, sizeof(msg.mtext), ____);msgsnd(msqid, &msg, sizeof(msg.mtext), 0);Explanation: The 0 flag waits until the queue has space.