Unix Programming - Unknown Storage Size Error for union semun

Author

Unknown Storage Size Error for union semun

kvnsmnsn@hotmail.com

2007-09-22, 1:17 pm

I'm taking a class on Internet Programming, and the lab I'm currently
working on requires us to use threads and semaphores. I'm looking at
a slide from that class titled "Unix Semaphore Code" that says:

[] Creation
o union semun arguments;
o key_t key = 1;
o int flags = 0777 | IPC_CREAT;
o int semid = semget(key, 1, flags);
o argument.val = initialvalue;
o semctl(semid, 0, SETVAL, argument);
[] Destruction
o int ignored_int;
o union semun ignored;
o semctl(semid, ignored_int, IPC_RMID, ignored);

I wanted to create an array of two semaphores, so I wrote the follow-
ing <main> function:

#include <pthread.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

...

int main ( int argCount
, char** arguments)
{
pthread_t Write[ 2];
union semun argument; <=================
key_t key = 1;
int flags = 0700 | IPC_CREAT;
int before[ 2];
int pairs[ 2][ 2];
int result;
int parity;
int end;
printf( "Before creation of semaphores.\n");
for (parity = EVEN; parity <= ODD; parity++)
{ before[ parity] = semget( key, 1, flags);
argument.val = parity;
semctl( before[ parity], 0, SETVAL, argument);
}
...

But when I try to compile it I get the error message
"CountSharer.c:44: error: storage size of 'argument' isn't known".
Line 44 is the line my arrow is pointing to up above. The slide up
above is the only reference to <union semun> that I know of. Does
anyone know what I need to do to my <argument> variable to keep this
error from occurring?

---Kevin Simonson

"You'll never get to heaven, or even to LA,
if you don't believe there's a way."
from _Why Not_

Ben Bacarisse

2007-09-22, 7:16 pm

kvnsmnsn@hotmail.com writes:

> I'm taking a class on Internet Programming, and the lab I'm currently
> working on requires us to use threads and semaphores. I'm looking at
> a slide from that class titled "Unix Semaphore Code" that says:
>
> [] Creation
> o union semun arguments;
> o key_t key = 1;
> o int flags = 0777 | IPC_CREAT;
> o int semid = semget(key, 1, flags);
> o argument.val = initialvalue;
> o semctl(semid, 0, SETVAL, argument);
> [] Destruction
> o int ignored_int;
> o union semun ignored;
> o semctl(semid, ignored_int, IPC_RMID, ignored);
>
> I wanted to create an array of two semaphores, so I wrote the follow-
> ing <main> function:
>
> #include <pthread.h>
> #include <stdio.h>
> #include <sys/types.h>
> #include <sys/ipc.h>
> #include <sys/sem.h>
>
> ...
>
> int main ( int argCount
> , char** arguments)
> {
> pthread_t Write[ 2];
> union semun argument; <=================
> key_t key = 1;
> int flags = 0700 | IPC_CREAT;
> int before[ 2];
> int pairs[ 2][ 2];
> int result;
> int parity;
> int end;
> printf( "Before creation of semaphores.\n");
> for (parity = EVEN; parity <= ODD; parity++)
> { before[ parity] = semget( key, 1, flags);
> argument.val = parity;
> semctl( before[ parity], 0, SETVAL, argument);
> }
> ...
>
> But when I try to compile it I get the error message
> "CountSharer.c:44: error: storage size of 'argument' isn't known".

You will need either a good book or access to manual pages. On my
system "man semctl" tells me:

"The calling program must define this union as follows:

union semun {
int val; /* Value for SETVAL */
struct semid_ds *buf; /* Buffer for IPC_STAT, IPC_SET */
unsigned short *array; /* Array for GETALL, SETALL */
struct seminfo *__buf; /* Buffer for IPC_INFO
(Linux specific) */
};"

so it is up to you to define this union when you want to use those
features of semctl (those that require a fourth argument). Your local
system may require something else, but the manual pages should tell you.

--
Ben.

Joachim Schmitz

2007-09-23, 7:29 am

"Ben Bacarisse" <ben.usenet@bsb.me.uk> schrieb im Newsbeitrag
news:87d4wa4703.fsf@bsb.me.uk...
> kvnsmnsn@hotmail.com writes:
>
> You will need either a good book or access to manual pages. On my
> system "man semctl" tells me:
>
> "The calling program must define this union as follows:
>
> union semun {
> int val; /* Value for SETVAL */
> struct semid_ds *buf; /* Buffer for IPC_STAT, IPC_SET */
> unsigned short *array; /* Array for GETALL, SETALL */
> struct seminfo *__buf; /* Buffer for IPC_INFO
> (Linux specific) */
> };"
>
> so it is up to you to define this union when you want to use those
> features of semctl (those that require a fourth argument). Your local
> system may require something else, but the manual pages should tell you.
Any idea/reason why that union semun is not part of sys/sem.h?

Bye Jojo

Peter J. Holzer

2007-09-23, 7:29 am

On 2007-09-23 08:58, Joachim Schmitz <nospam.jojo@schmitz-digital.de> wrote:
> "Ben Bacarisse" <ben.usenet@bsb.me.uk> schrieb im Newsbeitrag
> news:87d4wa4703.fsf@bsb.me.uk...
> Any idea/reason why that union semun is not part of sys/sem.h?

The prototype of the function semctl is

int semctl (int __semid, int __semnum, int __cmd, ...);

I have the nasty suspicion that the union was introduced into the
man-page as a semi-formal way of declaring that the fourth parameter
could be either an int or one of three pointer types but wasn't actually
intended to be used as a union. On most unix systems it doesn't make a
difference whether you call

semctl (sem_id, 0, SETVAL, 15);
or
union semun su;
su.val = 15;
semctl (sem_id, 0, SETVAL, su);

resp.

struct semid_ds ds;
semctl (sem_id, 0, SETVAL, &ds);
or
union semun su;
su.buf = &ds;
semctl (sem_id, 0, SETVAL, su);

either because int and pointers are the same size or because at least
the first 4 parameters are passed in registers.

As further evidence for this suspicion I cite the OSF/1 man-page for
semctl (ca. 1992), which declares the union as:

union semun {
int val;
struct semun *buf;
ushort array[];
};

which isn't even legal C, and if the DEC C compiler supported array[] as
an extension (many compilers at that time did) the semantics would not
have been the same as that of ushort *array (which was obviously
intended).

hp

--
_ | Peter J. Holzer | I know I'd be respectful of a pirate
|_|_) | Sysadmin WSR | with an emu on his shoulder.
| | | hjp@hjp.at |
__/ | http://www.hjp.at/ | -- Sam in "Freefall"