Bad address with mq_open

问题

I am trying to open a simple queue using mq_open but I keep getting error:

"Error while opening ... 
Bad address: Bad address"

And I have no idea why.

int main(int argc, char **argv) {
    struct mq_attr attr;
    //max size of a message
    attr.mq_msgsize = MSG_SIZE; 
    attr.mq_flags = 0;
    //maximum of messages on queue
    attr.mq_maxmsg = 1024 ;
    dRegister = mq_open("/serverQRegister",O_RDONLY | O_CREAT, S_IRUSR | S_IWUSR,0664, &attr);
    if(dRegister == -1)
    {
        perror("mq_open() failed");
        exit(1);
 }
}

I updated code as suggested but still getting error ("invalid argument"):

#include <stdio.h>
#include <stdlib.h>
#include <mqueue.h>
#include <signal.h>
#include <string.h>
#include <unistd.h>
#include "serverDefinitions.h"
mqd_t dRegister;
int main(int argc, char **argv) {
    struct mq_attr attr;
    //setting all attributes to 0
    memset(&attr, 0, sizeof attr);
    //max size of a message
    attr.mq_msgsize = MSG_SIZE;  //MSG_SIZE = 4096
    attr.mq_flags = 0;
    //maximum of messages on queue
    attr.mq_maxmsg = 1024;
    dRegister = mq_open("/serverQRegister", O_RDONLY | O_CREAT, 0664, &attr);

    if (dRegister == -1) {
        perror("mq_open() failed");
        exit(1);
    }
    return 0;
}

回答1:

This call

... = mq_open("/serverQRegister",O_RDONLY | O_CREAT, S_IRUSR | S_IWUSR,0664, &attr);

specifies too many parameters. The mode seems to have been specified twice.

It should be

... = mq_open("/serverQRegister",O_RDONLY | O_CREAT, S_IRUSR | S_IWUSR, &attr);

... = mq_open("/serverQRegister",O_RDONLY | O_CREAT, 0664, &attr);

Regarding EINVAL, the man mq_open states:

EINVAL

O_CREAT was specified in oflag, and attr was not NULL, but attr->mq_maxmsg or attr->mq_msqsize was invalid. Both of these fields must be greater than zero. In a process that is unprivileged (does not have the CAP_SYS_RESOURCE capability), attr->mq_maxmsg must be less than or equal to the msg_max limit, and attr->mq_msgsize must be less than or equal to the msgsize_max limit. In addition, even in a privileged process, attr->mq_maxmsg cannot exceed the HARD_MAX limit. (See mq_overview(7) for details of these limits.)

The initialisation of attr hits the limits for either one or both of mq_maxmsg or/and mq_msgsize. Read man 7 mq_overview on how to find out the limits.

回答2:

mq_open() is a varadic function and it can take either 2 or 4 arguments, but you give it 5, which is wrong.

Make it just

dRegister = mq_open("/serverQRegister",O_RDONLY | O_CREAT, 0664, &attr);

Or use the symbolic names, S_IRUSR | S_IWUSR instead of the octal representation.

You should also initialize all the members of attr, and if you supply the mq_attr, you must set both mq_msgsize and mq_maxmsg, so make it

struct mq_attr attr;
memset(&attr, 0, sizeof attr);
//max size of a message
attr.mq_msgsize = MSG_SIZE;
attr.mq_maxmsg = 10;

(note, mq_maxmsg must be less than what the command sysctl fs.mqueue.msg_max is set to)

来源：https://stackoverflow.com/questions/30011979/bad-address-with-mq-open

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