Linux Kernel Message Queue Implementation

Linux Kernel Message Queue Implementation

[gskiran]

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[1] msgsnd (int, struct msgbuf *, int, int) INTERFACE ERROR !!

[2] msgsnd() has "struct msgbuf*" as its 2nd parameter. When I tried to write an application which uses message queues for IPC i declared my message queue buffer as -

struct my_msgq_buf
{
    long mtype;
    char *name;
    double amount;
};

I allocated the memory of name dynamically using "malloc" and wrote into the message Queue using msgsnd. And the msgsnd returns successfully. When I do an msgrcv with the above structure type, I always recieve NULL values in my structure buffer !! 

But, When I use the structure as declared below -

struct my_msgq_buf
{
    long mtype;
    char name[30];
    double amount;
};

Every thing interstingly seems to be working fine.! The interface for msgsnd and msgrcv should have "void *" pointers rather than having a "struct msgbuf *" as parameters.

[3] Kernel
[4] 2.4.7-10 
[5] 
[6]
[7] i586. Intel Pentium 166 MHz MMX.
[8]

Re: Linux Kernel Message Queue Implementation

[Erik Mouw]

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On Tue, Feb 18, 2003 at 02:22:13PM +0000, gskiran@bgl.vsnl.net.in wrote:
> [1] msgsnd (int, struct msgbuf *, int, int) INTERFACE ERROR !!

No, user error.

> [2] msgsnd() has "struct msgbuf*" as its 2nd parameter. When I tried
> to write an application which uses message queues for IPC i declared
> my message queue buffer as -
> 
> struct my_msgq_buf
> {
>     long mtype;
>     char *name;
>     double amount;
> };

Nothing wrong over here, except that your pointer is ONLY valid in the
sending process, and NOT in the receiving process.

> I allocated the memory of name dynamically using "malloc" and wrote
> into the message Queue using msgsnd. And the msgsnd returns
> successfully. When I do an msgrcv with the above structure type, I
> always recieve NULL values in my structure buffer !!
> 
> But, When I use the structure as declared below -
> 
> struct my_msgq_buf
> {
>     long mtype;
>     char name[30];
>     double amount;
> };
> 
> Every thing interstingly seems to be working fine.!

Yes, which was to be expected cause "name" is now in the the message
structure.

>  The interface for
> msgsnd and msgrcv should have "void *" pointers rather than having a
> "struct msgbuf *" as parameters.

No, it's right the way it is right now. You are making the mistake that
you think pointers are valid memory references among all processes,
which clearly is not the case.

Get the book "Advanced programming in the Unix environment" by W.
Richard Stevens. It's more than worth its price and explains SysV IPC
in a very thorough way.


Erik

-- 
J.A.K. (Erik) Mouw
Email: J.A.K.Mouw@its.tudelft.nl  mouw@nl.linux.org

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