Very Computer

Hiya-

I'm working through Chap. 12 of Matthew & Stones' "Beginning Linux
Programming", and in a sample program (msg2.c) on message queues, in the
"sender" program the following lines makes the program fail:

...
        if (msgsnd(msgid, (void *)&some_data, BUFSIZ, 0) == -1) {
            fprintf(stderr, "msgsnd failed\n");
            exit(EXIT_FAILURE);
        }
...

After a lot of back and forth, it seems to fail because of BUFSIZ.  If I
arbitrarily make the first line:

        if (msgsnd(msgid, (void *)&some_data, 1024, 0) == -1) {

it works.

Any hints?  This is on Linux 2.0.32...

I don't know if I'm giving the right amount of info or not enough, thus
I append the whole program below.

TIA.  Bob L.
--

http://www.veryComputer.com/~rm*/
---

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>

struct my_msg_st {
    long int my_msg_type;
    char some_text[BUFSIZ];

Quote:};

    msgid = msgget((key_t)1234, 0666 | IPC_CREAT);

    if (msgid == -1) {
        fprintf(stderr, "msgget failed\n");
        exit(EXIT_FAILURE);
    }

    while(running) {
        printf("Enter some text: ");
        fgets(buffer, BUFSIZ, stdin);
        some_data.my_msg_type = 1;
        strcpy(some_data.some_text, buffer);

                if (msgsnd(msgid, (void *)&some_data, 1024, 0) == -1) {
/*        if (msgsnd(msgid, (void *)&some_data, BUFSIZ, 0) == -1) { */
            fprintf(stderr, "msgsnd failed\n");
            exit(EXIT_FAILURE);
        }

        if (strncmp(buffer, "end", 3) == 0) {
            running = 0;
        }
    }

    exit(EXIT_SUCCESS);

Quote:}

Take a look a the kernel variable msgsize witch give the maximum size of
a message in a message queue.
I don't where you can found this variable in linux, but :

   * with SCO, type sysadmsh,
   * with Solaris the file is /etc/system.

There is other variables that may have interest. You got the total size
of all the message in one queue, the size max of one message.

            Thomas

--
Thomas Rink
Lehrstuhl f. Biophysik
Ruhr-Universitaet Bochum
D-44780 Bochum, Germany

I've gotten three replies so far, that all correctly point to the
problem being that BUFSIZ  probably exceeds MSGMAX (maximum size of a
message queue msg.).  After investigation I found with my system that
these are set to 8092 and 4080 bytes respectively.  Oddly, replacing
BUFSIZ with 4080 - 1 bytes doesn't work; I didn't try all values, but
when I went down to 4000 bytes it did finally work.

(This was a very useful exercise in that I found out how to get the
value of a given define, besides reading through the header files, which
tend to define one define in terms of another.  My first trick was to
define, say, BUFSIZ myself, then the compiler warned me it was also
defined in a certain header file and even gave the line number, then I
could look there easily!  The second way was to run the source code
through the preprocessor and look at the output...)

OK, the question that remains in my mind is why such experienced
programmers of such an excellent book would have made a "mistake" like
this?  Presumably they tested their code before releasing it?  Perhaps
in some library upgrade BUFSIZE was increased to 8092 bytes, thus
breaking the code with an "old" msgsnd, MAXMSG = 4080 bytes?  Is this
possible, or are defines like BUFSIZ, etc. more "frozen" than this view
would imply and it's more probably a programmer's error?

Sorry if this seems like I am creating a tempest out of a teapot.  Just
trying to plumb the depths of this mysterious grab-bag of lore called
"Unix programming".  ;-)

And my grateful thanks to those who took the time to reply.

Bob L.
--

http://www.veryComputer.com/~rm*/

John Kroll

You are sending the some_data to the message queue. What I have read the
value BUFSIZ or 1024 is the size of the passed structure some_data. If your
structure is smaller then this can cause a problem. Thus replace BUFSIZ or
1024 with sizeof( some_data ). This should resolve the problem. Also,
determine that some_text can contain the data in the buffer. This can be
another reason why your program has a problem.

Claudio