POSIX消息队列，错误号90，消息太长

Question

I am currently doing a minimal IPC using POSIX Message Queues. I have one pipe that only will pass uint8_t 's as commands and another pipe that will pass strings with a length up to 128 characters. The command pipe is working fine. But the stringpipe will always give me errno number 90, which means message too long . I have written a minimal example demonstrating the issue (please note: i kept it minimal, so there is no errorhandling whatsoever besides the error upon receiving).

#include <fcntl.h>
#include <sys/stat.h>
#include <mqueue.h>
#include <errno.h>
#include <time.h>
#include <string.h>
#include <stdio.h>

int msg_size = 128;

int send()
{
    struct mq_attr attr = {0, 10, msg_size + 1, 0};
    mqd_t mq = mq_open("/test", O_RDWR | O_CREAT, 00644, &attr);
    char msg[msg_size] = {0};
    strncpy(msg, "this_is_a_test", msg_size);
    mq_send(mq, msg, msg_size, 0);
}

int recv()
{
    struct mq_attr attr = {0, 10, msg_size + 1, 0};
    mqd_t mq = mq_open("/test", O_RDWR | O_CREAT, 00644, &attr);
    char msg[msg_size] = {0};
    int res = mq_receive(mq, msg, msg_size, NULL);
    if (res == -1)
    {
        printf("Errno: %d\n", errno);
    }
    else
    {
        printf("Message: %s\n", msg); 
    }
}

int main()
{
    send();
    recv();
    return 0;
}

Compile with:

g++ -o mq mq.c -lrt

Answer 1

If you read the mq_receive manual page you will will see that EMSGSIZE means

msg_len was less than the mq_msgsize attribute of the message queue

_{[Emphasis mine]}

And that's true, you set the mq_msgsize attribute to msg_size + 1 and then you receive msg_size which is one less than the mq_msgsize attribute.

You don't need that +1 when setting the mq_msgsize attribute, just remove it.

Answer 2

the following code

1. cleanly compiles
2. actually works
3. You do need to have a /dev/mqueue device already created.
4. do not use known system function names for your local function names
5. use perror() to properly output error messages. Note: the output will automatically include the system message related to errno
6. note the usage of (size_t)attr.mq_msgsize in the call to mq_receive()
7. note only need to call mq_open() once
8. note the mqd_t mqdes and struct mq_attr variables are in the file global space so all the functions can see them. An alternate possibility is to pass them as parameters.
9. note the MSG_SIZE is defined via #define so it does not consume space on the stack nor on the file global space and only needs to be defined once rather than in each function
10. header files that are not used in the code should not be included.
11. note the attr is cleared to all 0x00 before setting the individual fields
12. note that only the actual length of the message is specified in the call to mq_send(), so no trash is sent/received

and now the code

#include <fcntl.h>
#include <sys/stat.h>
#include <mqueue.h> // mq_open(), mq_send(), mq_receive()

//#include <errno.h>
//#include <time.h>
#include <string.h> // strncpy(), strlen()

#include <stdio.h>  // printf(), perror()
#include <stdlib.h> // exit(), EXIT_FAILURE

#define MSG_SIZE  (128)

static mqd_t mqdes = -1;
static struct mq_attr attr;

void sendQueue()
{
    char msg[ MSG_SIZE ] = {0};
    strncpy(msg, "this_is_a_test", MSG_SIZE);
    if( -1 == mq_send(mqdes, msg, strlen(msg), 5) )
    {
        perror( "mq_send failed" );
        exit( EXIT_FAILURE );
    }

    else
    {
        printf( "%s\n", "msg sent successfully");
    }
}

void recvQueue()
{
    char msg[ MSG_SIZE ] = {0};

    ssize_t res = mq_receive(mqdes, msg, (size_t)attr.mq_msgsize, NULL);
    if (res == -1)
    {
        perror("mq_receive failed");
    }

    else
    {
        printf("Message: %s\n", msg);
    }
}

int main( void )
{
    mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH;

    memset( &attr, 0x00, sizeof(struct mq_attr) );
    attr.mq_maxmsg = 3;
    attr.mq_msgsize = MSG_SIZE;
    attr.mq_flags = 0;
    attr.mq_curmsgs = 0;

    char *queueName = "/test";

    mqdes = mq_open( queueName, O_RDWR|O_CREAT, mode, &attr);
    if( -1 == mqdes )
    {
        perror( "mq_open failed");
        exit( EXIT_FAILURE );
    }

    // implied else, mq_open successful


    sendQueue();
    recvQueue();
    return 0;
}