Cannot set POSIX message queue attribute
My environment:
- CentOS 6.5 (64-bit kernel)
- gcc 4.4.7 20120313
I'm trying to set an attribute for a POSIX message queue, but the code doesn't change the attribute. I am getting the default attribute values.
Could you please point out what the problem is with my code?
I am executing a.out as a user (not root).
#include <stdio.h>
#include <mqueue.h> // for message queue
#include <sys/stat.h>
#include <stdlib.h> // for EXIT_FAILURE
#include <string.h>
/*
gcc [file] -lrt
*/
static void showAttr(mqd_t mqd)
{
struct mq_attr attr;
mq_getattr(mqd, &attr);
printf("maxmsg = %d\n", attr.mq_maxmsg);
printf("msgsize = %d\n", attr.mq_msgsize);
printf("curmsgs = %d\n", attr.mq_curmsgs);
}
int main()
{
mqd_t mqd;
int flags;
int ret;
struct mq_attr attr;
flags = O_RDWR | O_CREAT;
attr.mq_flags = 0; // or O_NONBLOCK
attr.mq_maxmsg = 60;
attr.mq_msgsize = 120;
attr.mq_curmsgs = 0;
// POSIX IPC name should start with "/"
mqd = mq_open("/mq", flags,
// (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH),
0644,
&attr );
if (mqd < 0) {
printf("open failed\n");
exit(EXIT_FAILURE);
}
printf("open ok\n");
sleep(1);
showAttr(mqd);
ret = mq_close(mqd);
if (ret != 0) {
printf("open failed\n");
exit(EXIT_FAILURE);
}
printf("close ok\n");
return 0;
}
I found the following code works. However, when I try to install attr.mq_maxmsg (= 60), mq_open fails.
#include <stdio.h>
#include <mqueue.h> // for message queue
#include <sys/stat.h>
#include <stdlib.h> // for EXIT_FAILURE
#include <string.h>
#include <errno.h>
/*
gcc [file] -lrt
*/
static void showAttr(mqd_t mqd)
{
struct mq_attr attr;
mq_getattr(mqd, &attr);
printf("maxmsg = %d\n", attr.mq_maxmsg);
printf("msgsize = %d\n", attr.mq_msgsize);
printf("curmsgs = %d\n", attr.mq_curmsgs);
}
int main()
{
mqd_t mqd;
int flags;
int ret;
struct mq_attr attr;
flags = O_RDWR | O_CREAT;
// POSIX IPC name should start with "/"
// 1. once open without attribute setting
mqd = mq_open("/mq", flags,
(S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH) );
mq_getattr(mqd, &attr);
mq_unlink("/mq");
mq_close(mqd);
// 2. set values of attribute
// attr.mq_maxmsg = 10;
attr.mq_msgsize = 120;
// 3. allocate attribute
mqd = mq_open("/mq", flags,
(S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH),
// 0644,
&attr );
if (mqd < 0) {
printf("open failed %d\n", mqd);
exit(EXIT_FAILURE);
}
printf("open ok\n");
sleep(1);
showAttr(mqd);
ret = mq_close(mqd);
if (ret != 0) {
printf("open failed\n");
exit(EXIT_FAILURE);
}
printf("close ok\n");
return 0;
}
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2 answers
The following code works.
I used mq_unlink () before mq_open () as suggested by picker.
And another source of the problem: I set attr.mq_maxmsg to be greater than 10 (which is defined in / proc / sys / fs / mqueue / msg_max). If I set attr.mq_maxmsg less and equal to 10, it is not a problem to set the attribute.
#include <stdio.h>
#include <mqueue.h> // for message queue
#include <sys/stat.h>
#include <stdlib.h> // for EXIT_FAILURE
#include <string.h>
#include <errno.h>
/*
gcc [file] -lrt
*/
static void showAttr(mqd_t mqd)
{
struct mq_attr attr;
mq_getattr(mqd, &attr);
printf("maxmsg = %d\n", attr.mq_maxmsg);
printf("msgsize = %d\n", attr.mq_msgsize);
printf("curmsgs = %d\n", attr.mq_curmsgs);
}
int main()
{
mqd_t mqd;
int flags;
int ret;
struct mq_attr attr;
flags = O_RDWR | O_CREAT;
mq_unlink("/mq");
attr.mq_flags = 0;
attr.mq_maxmsg = 3; // ***
attr.mq_msgsize = 141;
attr.mq_curmsgs = 0;
mqd = mq_open("/mq", flags,
(S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH),
&attr );
if (mqd < 0) {
printf("open failed %d\n", mqd);
exit(EXIT_FAILURE);
}
printf("open ok\n");
sleep(1);
showAttr(mqd);
ret = mq_close(mqd);
if (ret != 0) {
printf("open failed\n");
exit(EXIT_FAILURE);
}
printf("close ok\n");
return 0;
}
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