Re: [systemd-devel] Returning a struct from an sd-bus method
Hi, thanks! You are correct about the signature, I was trying various
things and pasted in some half complete code, sorry. After changing to pass
the fields of the struct as individual parameters it does work correctly.
I guess I would prefer to pass the struct so that you can share a single
definition of the struct with both the client and service so as to avoid
possible errors where you pass the fields in the wrong order. Also if you
have a large struct it's a bit inconvenient to spell out every field on
both ends.
I thought it would be possible for sd-bus to marshall my C struct into a
D-bus struct. Perhaps that is not possible?
Niall
On Thu, 23 Aug 2018 at 18:35, Simon McVittie wrote:
> On Thu, 23 Aug 2018 at 16:52:38 +0200, Niall Murphy wrote:
> > struct pack {
> > int x;
> > int y;
> > };
> ...
> > return sd_bus_reply_method_return(m, "(xx)", s);
>
> Is there a reason why you're returning a struct/tuple? D-Bus methods
> can return as many things as you want[1], unlike C functions, so a more
> conventional return type would be "xx" instead of "(xx)".
>
> You also have a mismatch between your integer types: "x" is a 64-bit signed
> integer (mnemonic: the 64-bit signed and unsigned types are "x" and "t",
> which are the first letters of "sixty" that weren't already used for
> something more important) so they'll expect an int64_t. The type
> signatures for 32-bit integers are "i" and "u", depending on signedness.
> There is no correct type signature for types like int/long/long long
> whose size can vary between platform: integers in D-Bus messages are
> always fixed-size.
>
> Finally, I think the message-building API expects struct members as
> individual arguments, like
>
> sd_bus_reply_method_return(m, "xx", (int64_t) s->x, (int64_t) s->y);
>
> although I could be wrong about that.
>
> smcv
>
> [1] up to arbitrary message size limits measured in megabytes
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Re: [systemd-devel] Returning a struct from an sd-bus method
On Thu, 23 Aug 2018 at 17:34:14 +0100, Simon McVittie wrote: > Finally, I think the message-building API expects struct members as > individual arguments, like > > sd_bus_reply_method_return(m, "xx", (int64_t) s->x, (int64_t) s->y); Sorry, obviously that's correct when not using a struct. What I meant is that I think this: sd_bus_reply_method_return(m, "(xx)", (int64_t) s->x, (int64_t) s->y); might be the correct way to return a message containing a single struct. smcv ___ systemd-devel mailing list systemd-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/systemd-devel
Re: [systemd-devel] Returning a struct from an sd-bus method
On Thu, 23 Aug 2018 at 16:52:38 +0200, Niall Murphy wrote:
> struct pack {
> int x;
> int y;
> };
...
> return sd_bus_reply_method_return(m, "(xx)", s);
Is there a reason why you're returning a struct/tuple? D-Bus methods
can return as many things as you want[1], unlike C functions, so a more
conventional return type would be "xx" instead of "(xx)".
You also have a mismatch between your integer types: "x" is a 64-bit signed
integer (mnemonic: the 64-bit signed and unsigned types are "x" and "t",
which are the first letters of "sixty" that weren't already used for
something more important) so they'll expect an int64_t. The type
signatures for 32-bit integers are "i" and "u", depending on signedness.
There is no correct type signature for types like int/long/long long
whose size can vary between platform: integers in D-Bus messages are
always fixed-size.
Finally, I think the message-building API expects struct members as
individual arguments, like
sd_bus_reply_method_return(m, "xx", (int64_t) s->x, (int64_t) s->y);
although I could be wrong about that.
smcv
[1] up to arbitrary message size limits measured in megabytes
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[systemd-devel] Returning a struct from an sd-bus method
Hi,
I'm trying to return a struct from an sdbus method. I followed the simple
example of a service at
http://0pointer.net/blog/the-new-sd-bus-api-of-systemd.html. I then created
a simple method as follows:
struct pack {
int x;
int y;
};
static int foo(sd_bus_message *m, void *userdata, sd_bus_error *ret_error) {
struct pack s;
s.x = 1;
s.y = 2;
return sd_bus_reply_method_return(m, "(xx)", s);
}
static const sd_bus_vtable vtable[] = {
SD_BUS_VTABLE_START(0),
SD_BUS_METHOD("foo", "", "(xx)", foo, SD_BUS_VTABLE_UNPRIVILEGED),
SD_BUS_VTABLE_END
};
The rest of the service is as in the example. When I run the service and
call foo with busctl I get garbage:
(xx) 8589934593 140731098591648
Can someone point me to what I am doing wrong?
Thanks,
Niall
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