Creating Posix message queue with attribute get error (invalid argument)

[Hill]

Why not Following code always get "Invalid argument" error?

$ ./a.out /tmp.333

Wrong!
: Invalid argument

#include <mqueue.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/stat.h>
#include <iostream>
using namespace std;

#define FILE_MODE S_IRUSR|S_IWUSR

struct mq_attr attr;

int main(int argc, char** argv){
int c, flags;
mqd_t mqd;
flags = O_RDWR|O_CREAT;

if( argc != 2){

printf("usage: mqcreate <name>\n");
return -1;
}

attr.mq_maxmsg = 512;
attr.mq_msgsize = 512;

mqd = mq_open( argv[argc -1 ], flags, FILE_MODE, &attr);

if(mqd < 0){
perror("Wrong!\n");
return -1;
}

mq_close(mqd);
exit(0);
}

[red floyd]

Hill wrote:
> Why not Following code always get "Invalid argument" error? Who can
> help me out?

Did you have a C++ language question? Unix syscalls are not part of
the C++ language. Please try comp.unix.programmer.

Replied to there, and followup set to there.

[Dmitry V. Krivenok]

Hill wrote:
> Why not Following code always get "Invalid argument" error?
>
> $ ./a.out /tmp.333
>
> Wrong!
> : Invalid argument
>

Hi Hill!

It's common problem when working with POSIX MQ.
Your system disallows you to create MQ containing 512 messages, since
system-wide limit is 10 by default.

You can solve your problem by changing hard limit as follows:

krivenok@olimpico_work 10:44:27 /tmp $ g++ -lrt -Wall mq.cpp
mq.cpp: In function 'int main(int, char**)':
mq.cpp:16: warning: unused variable 'c'
krivenok@olimpico_work 10:44:37 /tmp $ ./a.out /tmp.333
Wrong!
: Invalid argument
krivenok@olimpico_work 10:44:40 /tmp $ sudo sh -c 'echo 512 >
/proc/sys/fs/mqueue/msg_max'
krivenok@olimpico_work 10:44:46 /tmp $ ./a.out /tmp.333
krivenok@olimpico_work 10:44:47 /tmp $

POSIX message queues are well-documented and I suggest you to read
man 2 mq_open and man 7 mq_overview carefully.

[Hill]

On Sep 4, 2:50 pm, "Dmitry V. Krivenok" <krive...@orangesystem.ru>
wrote:

Thanks Dmitry very much for the kindly help. Now this issue is fixed.
Following code can work well.

#include <mqueue.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/stat.h>
#include <iostream>
using namespace std;

#define FILE_MODE S_IRUSR|S_IWUSR

struct mq_attr attr;

int main(int argc, char** argv){
int c, flags;
mqd_t mqd;
flags = O_RDWR|O_CREAT;

if( argc != 2){

printf("usage: mqcreate <name>\n");
return -1;
}

attr.mq_flags = O_NONBLOCK;
attr.mq_maxmsg = 5;