如何检查邮件是否为空

Question

So I've been doing this for a whole day and didn't figure it out. I need to check if an empty message is passed to the receiver or server and then unlink queue. This is what I have:

while((c=getopt(argc, argv, ":q:"))!=-1){
    switch(c){
        case 'q':
            q = 1;
            Q = optarg;
            break;
    }
}
int oprimek_vrsta = -1;
char *msg = malloc(maxmsg_len + 1);

if(q != 0){
    oprimek_vrsta = mq_open(Q, O_RDWR|O_CREAT|O_EXCL, 0660, &lastnosti_vrste);
    if(oprimek_vrsta == -1){
        perror("error creating queue");
        return -1;
    }

    if(mq_getattr(oprimek_vrsta, &lastnosti_vrste) == -1){
        perror("error reading attributes");
        return -1;
    }
    while(loop){
        memset(msg, 0, 4096);
        munmap(msg, 4096);
        msg_len = mq_receive(oprimek_vrsta, msg, maxmsg_len, &priority);
        if(msg_len == -1){
            perror("error reading message");
            loop = 0;
            free(msg);
            mq_close(oprimek_vrsta);
            mq_unlink(Q);
            return -1;
        }else{
            write(1, msg, strlen(msg));
        }
    }
}

Answer 1

From mq_receive manual:

mq_receive() removes the oldest message with the highest priority
from the message queue ... If the queue is empty, then, by default, 
mq_receive() blocks until a message becomes available, 
or the call is interrupted by a signal handler.  
If the O_NONBLOCK flag is enabled for the message
queue description, then the call instead fails immediately with the error EAGAIN.

RETURN VALUE: On success, mq_receive() and mq_timedreceive() return the 
number of bytes in the received message; on error, -1 is returned, 
with errno set to indicate the error.

the return value of mq_receive is not what you're looking for. You need to check whether your received message length is 0.

I also highly reccomend you to read this helpful tutorial about POSIX message queues.

Here's a working example:

#include <stdio.h>
#include <mqueue.h>
#include <stdlib.h>
#include <string.h> /* memset */
#include <unistd.h> /* close */
#include <sys/mman.h>

#define QUEUE_NAME "/test"
#define MAX_MSG_LEN 100

int main(int argc, char **argv)
{
    int q;
    char* Q;
    char c;
    int loop = 1;
    int msg_len;

    struct mq_attr lastnosti_vrste;

    lastnosti_vrste.mq_flags = 0;
    lastnosti_vrste.mq_maxmsg = 10;
    lastnosti_vrste.mq_msgsize = 100;
    lastnosti_vrste.mq_curmsgs = 0; 


    while((c=getopt(argc, argv, "q:")) != -1)
    {
        switch(c)
        {
            case 'q':
                mq_unlink(Q);
                q = 1;
                Q = QUEUE_NAME;
            break;
        }
    }

    int oprimek_vrsta = -1;
    char *msg = malloc(MAX_MSG_LEN + 1);

    if(q != 0)
    {
        oprimek_vrsta = mq_open(Q, O_RDWR|O_CREAT|O_EXCL, 0660, &lastnosti_vrste);
        if(oprimek_vrsta == -1){
            perror("error creating queue");
            return -1;
        }

        if(mq_getattr(oprimek_vrsta, &lastnosti_vrste) == -1){
            perror("error reading attributes");
            return -1;
        }
        while(loop)
        {

            memset(msg, 0, 4096);
            munmap(msg, 4096);
            msg_len = mq_timedreceive(oprimek_vrsta, msg, MAX_MSG_LEN, 0);
            if(strlen(msg) == 0)
            {
                perror("error reading message");
                loop = 0;
                mq_close(oprimek_vrsta);
                mq_unlink(Q);
                return -1;
            }
            else
            {
                write(1, msg, strlen(msg));
            }
        }

        free(msg);
    }

return 0;
}