after msgget() , cout or printf are not working - c++

Question

After using msgget() system call to create a msgQ im not able to use cout or printf.
Below is my code trying to receive from a message Queue.
The "printf" at line 28 does not print anything. But it works fine if i add one more "cout" or "printf" statement(say at line 29).Please let me know for any flaws in my code.
1 #include<sys/errno.h>
2 #include<sys/wait.h>
3 #include<sys/ipc.h>
4 #include<stdio.h>
5 #include<iostream.h>
6 #include<sys/msg.h>
7 #include<sys/types.h>
8
9 struct messgQ
10 {
11 char text[1024];
12 long int mtype;
13 };
14
15
16 int main()
17 {
18 struct messgQ R;
19
20 R.mtype=1;
21
22 int qid=0;
23 qid=msgget((key_t)1234,0766|IPC_CREAT);
24 cout<<"\n 1 MsgQ created with id ="<<qid;
25
26 if( qid > 0 )
27 {
28 printf("\n MsgQ created with id =%d",qid);
29 }
30
31 int rc=0,run=1;
32 while( run )
33 {
34 memset(R.text,0x00,sizeof(R.text));
35 msgrcv(qid,&R,sizeof(R.text),1,0);
36 cout<<"\n Recvd:"<<R.text;
37 if( !strncmp(R.text,"bye",3) )
38 {
39 cout<<"\n Exiting";
40 run=0;
41 }
42 }
43 rc=msgctl(qid,IPC_RMID,NULL);
44 if (rc < 0)
45 perror(strerror(errno));
46 }

Answer 1

The msgget documentation says
RETURN VALUE
If successful, the return value will be the message queue identifier (a nonnegative integer), otherwise -1 with errno indicating the error.
So your check on line 26 is flawed, it should be:
if( qid >= 0 )
Regarding your output, stdout is normally line buffered when you print to a terminal. That means output isn't actually written out until you write a newline. So don't start your lines with a newline, but end with one instead.
printf(" MsgQ created with id =%d\n",qid);
cout<<"Recvd:"<<R.text << '\n';
cout<<"Exiting\n";
Alternatively, force the flushing by doing
cout.flush();
Or if you're using the C stdio functions such as printf, flush it with
fflush(stdout);