Re: [Vala] Using a C library

[Justin Brown <justin brown fandingo org>]

Hi Al,

Thanks for the continued help. I've been making some progress on the vapi,
but I've been putting off the compilation issue for a while now. Your reply
had me go back to compiling the vapi properly instead of as a vala program.
I think I'm still doing something slightly wrong. Structure:

src/
├── fandingo.c
├── fandingo.vala
└── vapi/
    └── libspotify.vapi

The libspotify-devel RPM from rpmfusion comes with a spotify.pc. It's in a
strange location, but that's fixed by exporting PKG_CONFIG_PATH. It's also
named libspotify, so I renamed my vapi. Trying to compile:

valac -v --pkg libspotify --vapidir vapi/ fandingo.vala


The test program just tries to create an error enum, print the message, and
print the spotify api version. I get a few errors:

/home/justin/code/fandingo/src/fandingo.vala.c:48:2: error: unknown type
name ‘sp_error’

  sp_error err = 0;

  ^

/home/justin/code/fandingo/src/fandingo.vala.c:55:46: error:
‘SPOTIFY_API_VERSION’ undeclared (first use in this function)


Seems like libspotify/api.h is not being linked correctly, which is
strange.

vapi/libspotify.vapi begins with:

[CCode (cheader = "libspotify/api.h")]

namespace Spotify {

That seems fine. However, if I generated C code with --ccode, the #include
is missing:

/* fandingo.c generated by valac 0.26.1, the Vala compiler
 * generated from fandingo.vala, do not modify */

#include <glib.h>
#include <glib-object.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

If I add #include <libspotify/api.h>, and compile the generated C with the
compilation command that valac provided earlier (cc -o
'/home/justin/code/fandingo/src/fandingo'
'/home/justin/code/fandingo/src/fandingo.c' -I/usr//include
-I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -L/usr//lib
-lgobject-2.0 -lglib-2.0 -lspotify).

Any idea why valac isn't adding "#include <libspotify/api.h>?"

After the manual compilation, I think everything is resolved with the enum
values. Errors 39-41 still produce "invalid error code" when calling
sp_error_message, but I think that might mean that libspotify doesn't have
error strings for those errors. The "jumps" (like 3 to 5 in my last email)
are working properly. Hooray.

Thanks,
Justin

On Tue, Apr 28, 2015 at 5:32 AM, Al Thomas <astavale yahoo co uk> wrote:

> > From: Justin Brown <justin brown fandingo org>
> > Cc: vala-list gnome org
> > Sent: Wednesday, 22 April 2015, 7:05
> > Subject: Re: [Vala] Using a C library
>
> > I did have a question about the C sp_error enum. There are 36 values
> [0-35] in the C library, but there are >some gaps such that the last value
> (SYSTEM_FAILURE) is 41, not 35. Every once in a while in the C enum a
> > manual value is specified that jumps the integer a few places -- for
> example:
> > SP_ERROR_TRACK_NOT_PLAYABLE        = 3,
> > SP_ERROR_BAD_APPLICATION_KEY       = 5,
> >
> > I've tried a variety of solutions. If I don't specify any integer values
> in Vala, then the codes don't >respect the "jumps," meaning that
> SP_ERROR_BAD_APPLICATION_KEY == 4. If I specify the "jumps" or all the
> > integers, then libspotify sp_error_message() complains about an invalid
> type in a string (not a segfault or >compilation error). Why isn't Vala
> using the C values properly when not specified? Also, what's the possible
> > invalid type problem when specifying the integers? I don't feel like I'm
> understanding whether it's a >int-length (unsigned/long/etc.) issue or
> something else. Any idea why the Vala enum isn't pulling the values
> > specified in libspotfiy/api.h?
>
>
> This is because you are not using a VAPI for your binding, but a Vala
> program.
>
> As I understand it now, after some experimentation, a VAPI can be seen as a
> translation file. It is used by the Vala compiler when it parses a Vala
> program
> to validate the identifier exists, in this case the Vala enum you have
> created.
>
> The second thing the Vala compiler uses the VAPI file for is to generate
> the C code
> from the Vala program using the binding. So the cname CCode attribute will
> translate the Vala program name for the enum to the C code name. The
> cprefix CCode
> attribute will prepend SP_ERROR_ to all the enum values in the C code.
> What the
> Vala compiler doesn't do is generate a C code translation of the enum in
> the VAPI
> itself. This is because the C code is already there in the C header file
> of your
> library. By placing the Vala enum in a VAPI you stop two versions of the
> enum
> being generated in C code.
>
> Hope that helps,
>
> Al
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