如何读取消息队列中的动态消息

Question

I need to send a message of not a certain size, and so that the second program displays the size of the received message, allocates memory for it, receives it and prints it

my first program waits until the user enters a message, allocates memory for it and sends the size of the message to another program and then sends the message itself.

my second program receives a message with the size of the message that will arrive, but at the same time it does not receive the message itself and I do not understand how to solve this problem

prog 1. sender

#include <stdio.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <sys/types.h>
#include <stdlib.h>


struct msgbuf
{
long type;
char mtext[1];
};

struct size
{
long type;
char sz[1];
};

  void send_message(int mqid)
 {
char mes[1000];
struct msgbuf *buffer;
struct size buf;
buf.type = 1;
int length;

//memset(buffer.mtext, 0, sizeof(buffer.mtext));

for(int i = 0; i < 4;i++){

    fgets(mes, sizeof(mes), stdin);
    sprintf(buf.sz, "%ld", strlen(mes));
    length = strlen(mes)+1;
    if(msgsnd(mqid, &buf,sizeof(buf.sz), 0)<0)
        perror("msgsnd");
    printf("Size: %d\n",strlen(mes));
    buffer = malloc(sizeof(struct msgbuf)-1 + length);
    buffer->type = 2;
    strncpy(buffer->mtext, mes, strlen(mes));
    if(msgsnd(mqid, &buffer, length, 0)<0)
        perror("msgsnd");
    else
        printf("Send message: %s\n", buffer->mtext);

    //free(buffer);

}
}

 int main()
 {
int key = ftok("comm", 8);
if(key < 0)
    perror("ftok");

int id = msgget(key, 0600);
if(id < 0)
    perror("msgget");

send_message(id);

return 0;
 }

prog 2

#include <stdio.h>
   #include <string.h>
   #include <sys/ipc.h>
   #include <sys/msg.h>
   #include <sys/types.h>
   #include <stdlib.h>


   struct msgbuf
   {
    long type;
    char mtext[1];
   };

   struct size
   {
    long type;
    char sz[1];
   };



void receive_message(int mqid)
{
    struct msgbuf *buffer;
    struct size szbuf;
    int a;

    for(int i = 0; i < 4;i++){
        if(msgrcv(mqid, &szbuf, sizeof(szbuf.sz), 1,0)<0)
                        perror("msgrcv");
        else
            printf("Size: %s\n", szbuf.sz);

        printf("Hello\n");

        a = atoi(szbuf.sz);
        printf("%d\n", a);

        /*szbuf = (struct size*) malloc(sizeof(struct size) - 1 + a);
        if(msgrcv(mqid, szbuf, sizeof(szbuf->sz), 2,0)<0)
                        perror("msgrcv");
                else
                        printf("Size: %s\n", szbuf->sz);*/

        struct msgbuf* buffer = (struct msgbuf*)malloc(sizeof(struct msgbuf)-1 + a);
        if(msgrcv(mqid, buffer, sizeof(buffer->mtext), 2,0)<0)
            perror("msgrcv");
        else
            printf("Received message: %s\n", buffer->mtext);

        free(buffer);


    }
   }

   int main()
   {
    int key = ftok("comm", 8);
    if(key < 0)
        perror("ftok");

    int id = msgget(key, 0600|IPC_CREAT|IPC_EXCL);
    if(id < 0)
        perror("msgget");

    receive_message(id);

    if(msgctl(id, IPC_RMID, 0)<0)
        perror("msgctl");

    return 0;
   }

the recipient program should print the size of the incoming message and the message itself

Answer 1

Your case can use the socket ipc, can check msg peak ( also be msg size), then allocate memory and do the real receive.