CODEWITHSUNDEEP
carraysstringstructchar
Daniel Shin
Daniel Shin

Reputation: 185

Strange characters when returning char array from struct in C

I am having some problems when returning with printf of a char array from a struct in C.

struct q_entry
{
    long mtype;
    char mtext[MAXLENGTH + 1];
};

The long mtype from the struct is returning fine, but the string is just returning some weird characters. 

int proc_obj(struct q_entry *msg)
{
    printf("\npriority: %ld name: %s\n", msg->mtype, msg->mtext);
}

It just returns some strange characters like "priority: 1 name: ▒▒(" and not "priority: 1 name: hello"

I am populating the struct using the following code

int enter(char *objname, int priority)
{
    ...

    strncpy(s_entry.mtext, objname, sizeof(s_entry.mtext) - 1);
    s_entry.mtype = priority;

    // Send the message
    if (msgsnd(s_qid, &s_entry, len, 0) == -1)
    {
        printf("error: msgsnd failed\n");
        return(-1);
    }
    else
    {
        return(0);
    }
}

I don't have much experience with C, so I don't know too much about using structs. Please let me know if more context or parts of the code is needed. Any kind of help would be very helpful.

I have added a little more code in enter above, and here is more code of the when enter and proc_obj are called

main(int argc, char **argv)
{
    int priority;

    if (argc != 3)
    {
        printf("error: incorrect number of arguments\n");
        exit(1);
    }
    else
    {
        priority = atoi(argv[2]);
    }

    if (enter(argv[1], priority) < 0)
    {
        printf("error: message entering failed\n");
        exit(1);
    }

    exit(0);
}

This is in a different file from enter and above code

int server(void)
{
    int mlen, r_qid;
    struct q_entry r_entry;

    // Initialize queue
    if ((r_qid = init_queue()) == -1)
        return(-1);

    for (;;)
    {
        if ((mlen = msgrcv(r_qid, &r_entry, MAXLENGTH, (-1 * MAXPRIOR), MSG_NOERROR)) == -1)
        {
            printf("error: msgrcv failed\n");
            exit(1);
        }
        else
        {
            proc_obj(&r_entry);
        }
    }
}

Upvotes: 0

Views: 1475

Answers (3)

phoxis
phoxis

Reputation: 61950

Answer before adding more code

It looks like the s_entry structure object is local, and enter works on the local variable. How are you calling enter and returning the structure after you have done initializing it ? note that you have int as the return type of the enter function. If you are doing return s_entry; then the output you are getting is possible, as only the first word of the structure, ie the lower sizeof (int) part of mtype is considered.

If you are using enter function like this as i described above then make the return type of enter to struct s_entry

You should check the size of len when sending the message.

Upvotes: 1

Kevin
Kevin

Reputation: 25269

You don't show the message queue call but my guess is you are somehow miscalling the API and putting garbage into the queue (and then printing garbage in the server).

Upvotes: 0

hmakholm left over Monica
hmakholm left over Monica

Reputation: 23342

The only obvious error in your code is that you should explicitly fill in a zero at s_entry.mtext[MAXLENGTH] such that the string will still be zero terminated if strncpy() hits the limit. But if that were the problem, you would see "hello" followed by strange characters. Are you sure that objname points to the text you're expecting it to point to?

Also, it looks a bit strange that proc_obj() is declared to return an int but actually does not return anything. Your compiler ought to complain about that.

Upvotes: 2

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