Simple IPC via linux messages - c++

Question

I'm trying to write some example of client-server application which uses linux messages. Here is my code:
#include <mqueue.h>
#include <ctype.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#define MSG_SIZE 4096
#define MSG_MAX_COUNT 1024
#define MSG_TYPE_TO_UPPER 0
#define MSG_TYPE_EXIT 1
#define MQ_NAME "msg_queue"
namespace {
int str_toupper(char *str)
{
int len = 0;
for(; str[len]; ++len) {
str[len] = toupper(str[len]);
}
return len;
}
}
int main(int argc, char** argv)
{
if(argc != 2) {
fprintf(stderr, "Usage: msg_queue (client|server)\n");
exit(EXIT_FAILURE);
}
struct mq_attr attr; // MQueue attributes
mqd_t mqd; // MQueue descriptor
char buf[MSG_SIZE]; // Msg buffer
unsigned int type; // Msg type(priority)
// Set up MQueue attributes
attr.mq_maxmsg = MSG_MAX_COUNT;
attr.mq_msgsize = MSG_SIZE;
attr.mq_flags = 0;
attr.mq_curmsgs = 0;
mqd = mq_open(MQ_NAME, O_RDWR | O_CREAT, 0664, &attr);
if(mqd == -1) {
fprintf(stderr, "mq_open() failed for \""MQ_NAME"\": %s\n", strerror(errno));
exit(EXIT_FAILURE);
}
if(strcmp(argv[1], "server") == 0) {
while(mq_receive (mqd, buf, MSG_SIZE, &type) != -1) {
if(type == MSG_TYPE_EXIT) {
mq_unlink(MQ_NAME);
mq_close(mqd);
break;
} else if(type == MSG_TYPE_TO_UPPER) {
int len = str_toupper(buf);
if(mq_send (mqd, buf, len, MSG_TYPE_TO_UPPER) == -1) {
fprintf(stderr, "Server: mq_send() failed: %s", strerror(errno));
}
}
}
} else if(strcmp(argv[1], "client") == 0) {
while(1) {
printf("Input a message: <type>(0 - TO_UPPER, 1 - EXIT) <message>\n");
scanf("%u %s", &type, buf);
if(mq_send (mqd, buf, strlen(buf), type) == -1) {
fprintf(stderr, "Client: mq_send() failed: %s", strerror(errno));
}
if(type == MSG_TYPE_TO_UPPER) {
if(mq_receive (mqd, buf, MSG_SIZE, &type) == -1) {
fprintf(stderr, "Client: mq_receive() failed: %s", strerror(errno));
}
printf("\"%s\" received\n", buf);
} else if(type == MSG_TYPE_EXIT) {
mq_unlink(MQ_NAME);
mq_close(mqd);
break;
}
}
} else {
fprintf(stderr, "Usage: msg_queue (client|server)\n");
exit(EXIT_FAILURE);
}
return 0;
}
What is my mistake? It always prints error from the line 47 - fprintf(stderr, "mq_open() failed for \""MQ_NAME"\": %s\n", strerror(errno)); with errno = EINVAL.

Answer 1

I see two issues:
Your message queue name must begin with a / on Linux. See mq_overview(7):
Each message queue is identified by a name of the form
/somename; that is, a null-terminated string of up to NAME_MAX (i.e., 255)
characters consisting of an initial slash, followed by one or more characters,
none of which are slashes.
MSG_MAX_COUNT is most likely above your system limits. It must be less than (or equal to) the /proc/sys/fs/mqueue/max_size. See mq_open(3):
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 other limit is probably ok, but you should verify it too.