Message too long - buffer is larger than message size - c

Question

I'm trying to teach myself message queues, and I'm using pthreads that talk to each other.
I know that the buffer in mq_receive should be larger than attr.mq_msgsize, and it is (twice the size).
I haven't even sent a message yet.
EDIT: John Bollinger ran this on his machine and it worked. This may be an OS-dependent problem. I'm running Mint 18 XFCE
EDIT EDIT: rebooting fixed the behaviour? Not going to question this or complain.
This code is based off of an example I found online : https://github.com/arembedded/mq_example
Here is the code :
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <signal.h>
#include <sys/time.h>
#include <sys/stat.h>
#include <mqueue.h>
#include <fcntl.h>
#include <errno.h>
using namespace std;
#define PIN_MSG_NAME "/pin_msg"
#define DB_MSG_NAME "/db_msg"
#define MESSAGE_QUEUE_SIZE 15
pthread_t ATM;
pthread_t DB_server;
pthread_t DB_editor;
void* run_ATM(void* arg);
void* run_DB(void* arg);
static struct mq_attr mq_attribute;
static mqd_t PIN_MSG = -1;
void sig_handler(int signum){
//ASSERT(signum == SIGINT);
if (signum == SIGINT){
cout << "killing application" << endl;
pthread_cancel(ATM);
pthread_cancel(DB_server);
pthread_cancel(DB_editor);
}
}
int main(int argc, char const *argv[])
{
pthread_attr_t attr;
signal(SIGINT, sig_handler);
mq_attribute.mq_maxmsg = 10; //mazimum of 10 messages in the queue at the same time
mq_attribute.mq_msgsize = MESSAGE_QUEUE_SIZE;
PIN_MSG = mq_open(PIN_MSG_NAME , O_CREAT | O_RDWR, 0666, &mq_attribute);
if (PIN_MSG == -1){
perror("creating message queue failed ");
}
pthread_attr_init(&attr);
pthread_attr_setstacksize(&attr, 1024*1024);
long start_arg = 0; //the start argument is unused right now
pthread_create(&ATM, NULL, run_ATM, (void*) start_arg);
pthread_create(&DB_server, NULL, run_DB, (void*) start_arg);
pthread_join(ATM, NULL);
pthread_join(DB_server, NULL);
sig_handler(SIGINT);
}
void* run_ATM(void* arg) {
int status;
char accountNumber[15];
char PIN[15];
cout << "ATM is running" << endl;
cout << "Please input an account number > ";
cin >> accountNumber;
status = mq_send(PIN_MSG, accountNumber, sizeof(accountNumber) + 1, 1);
if (status < 0){
perror("sending message failed");
}
}
void* run_DB(void* arg){
cout << "Database server running" << endl;
int status;
char received_acct_number[30];
while(1){
status = mq_receive(PIN_MSG, received_acct_number, sizeof(received_acct_number), NULL);
if (status < 0){
perror("error:");
} else {
cout << received_acct_number << endl;
}
}
}
Am I missing something? Why is there a message coming in at all, and why is it too large?

Answer 1

You talk about your receive buffer size as if the error is reported by mq_receive(), but as you observe, your buffer is long enough to receive any message that can be enqueued on your queue, and moreover, you seem not to be expecting an incoming message at all.
Although I'm not sure how you could be confused in this way, I'm inclined to think that the problem occurs in sending a message:
char accountNumber[15];
...
status = mq_send(PIN_MSG, accountNumber, sizeof(accountNumber) + 1, 1);
Your queue's message length limit is 15 bytes, and you're trying to enqueue a 16-byte message. Moreover, your send buffer is in fact shorter than 16 bytes in the first place; if mq_send() tried to copy 16 bytes of message then that would produce undefined behavior.