-1

I am trying to open a queue on linux in C++ and it returns a -1 error. how would I debug this issue.

 int32_t MaxMsgs;
    int32_t MaxMsgLength;
    eoMQHndlT msgQptr = -1; 
    char temp[32];
    
    mq_unlink(temp);
    
    struct mq_arrt msQAtt; msQAtt.mq_flags = 0; msQAtt.mq_maxmsg = MaxMsgs; msQAtt.mq_magsize = MaxMsgLength; msQAtt.mq_curmsgs = 0;
    
    msgQptr  = mq_open(temp,O_RDWR|O_CREAT,S_IRUSR | S_IWUSR, &msQAtt);

The variables MaxMsgs and MaxMsgLength are sent it to this call and those values are ex1: MaxMsgs = 10, MaxMsgLength=1232 and ex2:MaxMsgs = 2400, MaxMsgLength=4136 Appreciate some feedback

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From the documentation for mq_open (emphasis mine):

RETURN VALUE

On success, mq_open() returns a message queue descriptor for use by other message queue functions. On error, mq_open() returns (mqd_t) -1, with errno set to indicate the error.

So, to debug the issue you have, you should check the value of errno.

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6 Comments

I am getting a value of -1 for msgQptr . I am assuming that is the errno?
how do I print the errno?
No, errno is a separate variable that is set by various functions to indicate the specific error. -1 just indicates that there was an error. From the return value section here, there is a link to the errno manual page.
And from that page, it tells you that you can use perror and strerror to print an error message to stderr.
when I did perror("mq_open") - for 2 different instances I get - Too many open files and invalid argument. How do I debug this?
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