Message queue - No proper response - c

Question

*I'm using message queues for sending and receiving messages but some parameters in the the structure msqid_ds no giving proper values why it happens?
*Time of last message send=1525240214- why this is showing junk values?
struct mesg_q
{
char msg_txt[100];
long msg_typ;
};
int main()
{
int msgid;
key_t key;
char buffer[100];
struct mesg_q msgq;
struc answerst msqid_ds info;
// key = ftok("/home/yash72/krishna/thread_test/msgq.text", 65);
msgid=msgget((key_t)1234, 0666 | IPC_CREAT);
if(msgid== -1)
{
printf("msgget failed\n");
return -1;
}
while(1)
{
printf("Text Message\n");
fgets(msgq.msg_txt,100,stdin);
if(msgsnd(msgid,&msgq,100,0)==-1)
{
printf("Send failed\n");
return -1;
}
else
{
printf("Message send\n");
}
msgctl(msgid, IPC_STAT, &info);
printf("Time of last message send=%d\n",info.msg_stime);
printf("uid=%d\n",info.msg_perm.uid);
}
}
OUTPUT:
Text Message
qwerty
Message send
Time of last message send=1525240214 // Why this is showing junk?
uid=0
Text Message
Receiver code:
Reason for junk values from this code?
struct mesg_q
{
char msg_txt[100];
long msg_typ;
};
int main()
{
int msgid;
char buffer[100];
long int rec_buff=0;
key_t key;
struct mesg_q msgq;
struct msqid_ds info;
// key = ftok("/home/yash72/krishna/thread_test/msgq.text", 65);
msgid=msgget((key_t)1234, 0666 | IPC_CREAT);
if(msgid == -1)
{
printf("Msgget failed\n");
return -1;
}
while(1)
{
if(msgrcv(msgid,&msgq,100,rec_buff,0)==-1)
{
printf("Mesg recv failed\n");
return -1;
}
else
{
printf("Mesg Recvd\n");
}
printf("Recvd mesg=%s\n",msgq.msg_txt);
msgctl(msgid, IPC_STAT, &info);
printf("Num:of bytes on queue= %d\n", info.msg_cbytes);
printf("num:of messages on queue=%d\n", info.msg_qnum);
printf("Max bytes on queue=%d\n", info.msg_qbytes);
printf("Time of last message recvd=%d\n",info.msg_rtime);
}
}
OUTPUT;
Number of bytes on queue= 0
number of messages on queue=0
Maximum bytes on queue=16384
Time of last message received=1525240214
what is the reason foe this incorrect values from struct msqid_ds ?

Answer 1

First of all you should know why to use message queue IPC, I assume before using message queue, you have gone through FIFO. In FIFO what is condition for the processes to communicate each other ? that both process should be alive while communicating.
To avoid above problem of FIFO you are using message queue, as in MQ you can put data at one time and can read at anytime, for that you have to specify the mtype i.e with what mtype process_1 is sending the data and with what mtype process_2 is receiving the data.
From the manual page of msgsnd
struct msgbuf {
long mtype; /* message type, must be > 0, I'm talking baout this */
char mtext[1]; /* message data */
};
so in both the process(reader & sender) specify the mtype
msgq.msg_typ = 1; /*specify this in both process */
Secondly, Time of last message send=1525240214- why this is showing junk values ? => Its not junk data, its the time in seconds from EPOCH time, use ctime() to print in human readable format.
sender.c
#include<stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include<stdint.h>
struct mesg_q {
char msg_txt[100];
long msg_typ;
};
int main(void) {
int msgid;
struct mesg_q msgq;
struct msqid_ds info;
msgid=msgget((key_t)1234, 0666 | IPC_CREAT);
if(msgid== -1) {
printf("msgget failed\n");
return -1;
}
while(1) {
printf("Text Message\n");
fgets(msgq.msg_txt,sizeof(msgq.msg_txt),stdin);
/* with what msg_type you are sending, you didn't mention ? mention it */
msgq.msg_typ = 1; /* I am sending with msg_type 1 */
if(msgsnd(msgid,&msgq,sizeof(msgq),msgq.msg_typ)==-1) {
printf("Send failed\n");
return -1;
}
else {
printf("Message send\n");
}
msgctl(msgid, MSG_STAT, &info);
printf("Time of last message send = %jd\n",(intmax_t)info.msg_stime);/* use the correct format specifier */
/* what ever time info.msg_stime printing, its not junk, its seconds from EPOCH, use ctime() to print in readable format */
printf("uid = %d\n",info.msg_perm.uid);
}
return 0;
}
receiver.c
struct mesg_q {
char msg_txt[100];
long msg_typ;
};
int main(void ){
int msgid;
long int rec_buff=0;
struct mesg_q msgq;
struct msqid_ds info;
msgid=msgget((key_t)1234, 0666 | IPC_CREAT);
if(msgid == -1) {
printf("Msgget failed\n");
return -1;
}
while(1) {
/* with what msg_typ you are reciving, you have to mention ? u didn't */
msgq.msg_typ = 1;
if(msgrcv(msgid,&msgq,sizeof(msgq),rec_buff, msgq.msg_typ)==-1) {
printf("Mesg recv failed\n");
return -1;
}
else{
printf("Mesg Recvd\n");
}
printf("Recvd mesg=%s\n",msgq.msg_txt);
msgctl(msgid, MSG_STAT, &info);
/* enable compiler warning, and use correct format specifier */
printf("Num:of bytes on queue= %u\n", (int)info.msg_cbytes);
printf("num:of messages on queue=%d\n", (int)info.msg_qnum);
printf("Max bytes on queue=%d\n", (int)info.msg_qbytes);
printf("Time of last message recvd=%jd\n",(intmax_t)info.msg_rtime);
}
return 0;
}