I'm just learning how to use message queues, and I am having a little difficulty with them. I am using two completely separate applications to do the testing -- one is the "sender", and the other is the "receiver".
When I run the sender, it sends 15 strings to the pipe, but then afterward fails with a "Resource temporarily unavailable" error. I just need to consume the messages on the receiver side, but why only 15 messages? I might be sending lots of messages, so I'd like to increase this to a much larger number, like 1000 or so.
I tried to set the message queue size to 32767, so I was expecting 31 at least, but apparently msg_qbytes is not related to the number of messages that can be buffered.
The sender code looks like this:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <unistd.h>
#include <string.h>
#define MESSAGE_SIZE 1024
typedef struct msgbuf
{
long mtype;
char mtext[MESSAGE_SIZE];
};
int main(int argc, char *argv[]) {
int msgid;
int ret;
struct msqid_ds msg_settings;
long key;
struct msgbuf msg;
key = strtol(argv[1], NULL, 10);
// print the message queue ID for reading via msgrcv
printf( "Getting message queue with key = %ld\n", key);
usleep( 1000000);
msgid = msgget( (key_t)key, 0666 | IPC_CREAT);
if (msgid == -1) {
perror("msgget failed with error");
exit(EXIT_FAILURE);
}
// read in current queue settings and then set the new
// queue size.
ret = msgctl(msgid, IPC_STAT, &msg_settings);
msg_settings.msg_qbytes = 32767;
msgctl( msgid, IPC_SET, &msg_settings);
while( 1) {
msg.mtype = 1; // we'll always leave this as 1
memset( &(msg.mtext), 0, MESSAGE_SIZE);
sprintf( msg.mtext, "hi");
printf( "Sending data: %s\n", msg.mtext);
ret = msgsnd( 1, &msg, MESSAGE_SIZE, IPC_NOWAIT);
usleep( 500000);
if( ret == -1) {
perror( "msgsnd failed\n");
}
printf( "leaving...\n");
return EXIT_SUCCESS;
}
The receiver code looks like this:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#define MESSAGE_SIZE 1024
typedef struct msgbuf
{
long mtype;
char mtext[MESSAGE_SIZE];
};
int main(int argc, char *argv[]) {
long int msgtyp = 1;
int ret;
size_t msgsz;
struct msgbuf mymsg;
int msgid;
msgid = strtol(argv[1], NULL, 10);
printf( "Reading message queue with ID = %d\n", msgid);
usleep( 1000000);
while( 1) {
msgsz = (size_t)MESSAGE_SIZE;
ret = msgrcv( msgid, &mymsg, msgsz, msgtyp, IPC_NOWAIT);
if( ret == ENOMSG) {
usleep( 100000);
continue;
}
if( ret == -1) {
perror( "msgrcv failed");
} else {
printf( "Read data: %s", mymsg.mtext);
}
usleep( 100000);
}
return EXIT_SUCCESS;
}
