[PATCH 1/5] api: don't use intermediate variables for deprecation warnings

[Ben Chan]

On Sun, Aug 20, 2017 at 1:50 PM, Aleksander Morgado <
aleksander at aleksander.es> wrote:

> On Sun, Aug 20, 2017 at 7:09 PM, Ben Chan <benchan at chromium.org> wrote:
> >
> >
> > On Sun, Aug 20, 2017 at 9:37 AM, Aleksander Morgado
> > <aleksander at aleksander.es> wrote:
> >>
> >> Using an intermediate constant variable breaks compilation with C
> >> compilers, as these variables cannot be used as initializers.
> >>
> >
> > Aleksander, do you have the compiler version and the code snippet that
> > causes the compiler error?
> >
>
> Here's an example:
>
> /* test.c */
> #include <stdio.h>
> #include <ModemManager.h>
>
> typedef struct {
>   MMModemBand  band;
>   char        *str;
> } BandName;
>
> static const BandName names[] = {
>   { .band = MM_MODEM_BAND_EUTRAN_XLII, "e-utran 43" },
> };
>
> int main(int argc, char **argv)
> {
>   guint i;
>
>   for (i = 0; i < G_N_ELEMENTS (names); i++)
>     printf ("band '%s' has id %u\n", names[i].str, names[i].band);
>   return 0;
> }
>
> Compilation fails with "error: initializer element is not constant":
> $ gcc -o test `pkg-config --cflags --libs ModemManager glib-2.0` test.c
> test.c:12:2: warning: ‘MM_DEPRECATED_MODEM_BAND_EUTRAN_XLII’ is
> deprecated: Use 'MM_MODEM_BAND_EUTRAN_42' instead
> [-Wdeprecated-declarations]
>   { .band = MM_MODEM_BAND_EUTRAN_XLII, "e-utran 43" },
>   ^
> In file included from /usr/include/ModemManager/ModemManager.h:41:0,
>                  from test.c:4:
> /usr/include/ModemManager/ModemManager-compat.h:441:18: note: declared
> here
>  static const int MM_DEPRECATED_MODEM_BAND_EUTRAN_XLII =
> MM_MODEM_BAND_EUTRAN_42;
>                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> /usr/include/ModemManager/ModemManager-compat.h:442:35: error:
> initializer element is not constant
>  #define MM_MODEM_BAND_EUTRAN_XLII MM_DEPRECATED_MODEM_BAND_EUTRAN_XLII
>                                    ^
> /usr/include/ModemManager/ModemManager-compat.h:442:35: note: in
> definition of macro ‘MM_MODEM_BAND_EUTRAN_XLII’
>  #define MM_MODEM_BAND_EUTRAN_XLII MM_DEPRECATED_MODEM_BAND_EUTRAN_XLII
>                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> /usr/include/ModemManager/ModemManager-compat.h:442:35: note: (near
> initialization for ‘names[0].band’)
>  #define MM_MODEM_BAND_EUTRAN_XLII MM_DEPRECATED_MODEM_BAND_EUTRAN_XLII
>                                    ^
> /usr/include/ModemManager/ModemManager-compat.h:442:35: note: in
> definition of macro ‘MM_MODEM_BAND_EUTRAN_XLII’
>  #define MM_MODEM_BAND_EUTRAN_XLII MM_DEPRECATED_MODEM_BAND_EUTRAN_XLII
>                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> $ gcc --version
> gcc (GCC) 7.1.1 20170630
>
> For what I could gather browsing; using constant variables as
> initializers is valid C++, but not C. The approach I took for the
> patch is what the GTK+ did when deprecating GtkStock macros (e.g.
> https://github.com/GNOME/gtk/blob/7173df1dd1a8f45c47b75d30bfbfaa
> 67f33d8e9b/gtk/deprecated/gtkstock.h#L102)
>

lgtm.

Yeah, unfortunately, it seems like C has a different rule for constant
expressions (than C++). A simple expression like this fails to compile with
gcc (even in c99, c11), but succeeds with clang:

   static const int a = 1;
   static const int b = a;   /* error: initializer element is not constant
*/

Seems a bit subtle that `static const int a = 1` isn't regarded as a
constant expression. It may be related to whether there is any guarantee on
the initialization order.


>
> --
> Aleksander
> https://aleksander.es
>
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