1. RE: Semaphore help
jbrown105 at speedymail.org wrote:
> I have tried to implement the code for semaphores, but it seems
> not to be working. The main limitation is that fact that one of
> the paramters of a function (semctl) has a union as its type.
> How does one create/pass a union as a paramter to a C func?
> (It would be nice to also have examples of using semaphores, but
> I have the lpg (linux programmers guide) in addition to the
> man pages, so if none can be provided, I can manage.)
All a union is is a storage location that can contain different
size values. To use it all you need to do is know the largest
value that will fit in the union and this will be the size that
you will use when allocating the union size.
Then you can store any value that is listed in the union values.
in the same storage. The only thing you have to be careful
to use the correct size for the value when poking it into the storage.
I hope this makes sense.
Bernie
2. RE: Semaphore help
- Posted by a.tammer at hetnet.nl
Jun 05, 2002
Hi JBrown/Bern,
Say it Euphoria-way would be a union is an object
In fact in C it is always(?) a pointer to something
like an Euphoria-object.
EUrs
antoine tammer
3. RE: Semaphore help
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> -----Original Message-----
> From: jbrown105 at speedymail.org [mailto:jbrown105 at speedymail.org]
> Sent: Thursday, 6 June 2002 9:56
> To: EUforum
> Subject: Re: Semaphore help
>
>
>
> On 0, Bernie Ryan <xotron at localnet.com> wrote:
> >
> >
> > jbrown105 at speedymail.org wrote:
> > > I have tried to implement the code for semaphores, but it seems
> > > not to be working. The main limitation is that fact that one of
> > > the paramters of a function (semctl) has a union as its type.
> > > How does one create/pass a union as a paramter to a C func?
> > > (It would be nice to also have examples of using semaphores, but
> > > I have the lpg (linux programmers guide) in addition to the
> > > man pages, so if none can be provided, I can manage.)
> >
> > All a union is is a storage location that can contain different
> > size values. To use it all you need to do is know the largest
> > value that will fit in the union and this will be the size that
> > you will use when allocating the union size.
> > Then you can store any value that is listed in the union values.
> > in the same storage. The only thing you have to be careful
> > to use the correct size for the value when poking it into
> the storage.
> > I hope this makes sense.
> >
> > Bernie
> >
>
> It does. However I am unclear as how to define a C func which
> uses a union.
>
> union semun {
> int val; /* value for SETVAL */
> struct semid_ds *buf; /* buffer for IPC_STAT,
> IPC_SET */
> unsigned short int *array; /* array for GETALL, SETALL
> */
> struct seminfo *__buf; /* buffer for IPC_INFO */
> };
>
> int semctl (int semid, int semnum, int cmd, union semun arg);
>
> Thats the C prototype. How do I use define_c_func() to have a union
> as a parameter? I got this far:
>
> semctl_ = define_c_func(so, "semctl", {C_INT, C_INT, C_INT, }, C_INT)
>
> How do I finish it?
>
semctl_ = define_c_func(so, "semctl", {C_INT, C_INT, C_INT, C_POINTER},
C_INT)
Because the largest item in the union is a pointer.
When calling this routine, you just need to decide what you are sending in
the last parameter. It is expecting one of ...
val
address of buf
address of array
address of __buf
depending on what the cmd value is.
-----------
Derek.
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4. RE: Semaphore help
jbrown105 at speedymail.org wrote:
> On 0, Bernie Ryan <xotron at localnet.com> wrote:
> >
> >
> > jbrown105 at speedymail.org wrote:
> > > I have tried to implement the code for semaphores, but it seems
> > > not to be working. The main limitation is that fact that one of
> > > the paramters of a function (semctl) has a union as its type.
> > > How does one create/pass a union as a paramter to a C func?
> > > (It would be nice to also have examples of using semaphores, but
> > > I have the lpg (linux programmers guide) in addition to the
> > > man pages, so if none can be provided, I can manage.)
> >
> > All a union is is a storage location that can contain different
> > size values. To use it all you need to do is know the largest
> > value that will fit in the union and this will be the size that
> > you will use when allocating the union size.
> > Then you can store any value that is listed in the union values.
> > in the same storage. The only thing you have to be careful
> > to use the correct size for the value when poking it into the storage.
> > I hope this makes sense.
> >
> > Bernie
> >
>
> It does. However I am unclear as how to define a C func which
> uses a union.
>
> union semun {
> int val; /* value for SETVAL */
> struct semid_ds *buf; /* buffer for IPC_STAT,
> IPC_SET */
> unsigned short int *array; /* array for GETALL, SETALL
> */
> struct seminfo *__buf; /* buffer for IPC_INFO */
> };
>
> int semctl (int semid, int semnum, int cmd, union semun rg);
>
> Thats the C prototype. How do I use define_c_func() to have a union
> as a parameter? I got this far:
>
> semctl_ = define_c_func(so, "semctl", {C_INT, C_INT, C_INT, }, C_INT)
>
> How do I finish it?
>
> TIA,
> jbrown
semctl_ = define_c_func(so, "semctl", {C_INT, C_INT, C_UINT, },C_INT)
The largest value in the union is a pointer which needs a size of
C_UINT ( 4 bytes unsigned integer ) in the last parameter.
the C_INT tells Euphoria to only allow a signed value
if you use C_UINT then let Euphoria think that any
negative number is just a large positive number.
If that doesn't work then just create two different functions
with diferent parameter types but the same name.
semctl_int = define_c_func(so, "semctl", {C_INT, C_INT, C_INT, },C_INT)
semctl_pointer = define_c_func(so, "semctl", {C_INT, C_INT, C_UINT,
},C_INT)
Euphoria is using the the definition to determine what the
sign of a parameter, the size in bytes, and the number of
arguments. Then it looks up the name of the function in the SO/DLL
and sees if it can finds its name and grabs a pointer to it.
"C" will use the parameters depending how euphoria puts them
on the stack and Euphoria will use the definition to type check
parameters that you are using.
Bernie
