Re: recent gnulib has problems with read, write and close within structs

[Bruno Haible]

Hi Joël,

> Because recent gnulib has problems with read, write and close as
> struct fields. I am still using:
> 
> gnulib$ git log
> commit 2d431ac35c4943a3655c07ba91870d2323321b43 (HEAD -> master,
> origin/master, origin/HEAD)
> 
> Here is my code:
> 
> https://git.savannah.nongnu.org/cgit/gsequencer.git/tree/ags/object/ags_application_context.h#n112
> 
> Actually it fails here:
> 
> https://git.savannah.nongnu.org/cgit/gsequencer.git/tree/ags/X/ags_xorg_application_context.c#n431
> 
> Here is a snipped of the terminal output:
> 
> ags/X/ags_xorg_application_context.c:89: warning: "_GNU_SOURCE" redefined
>    89 | #define _GNU_SOURCE
>       |
> In file included from <command-line>:
> ././ags/config.h:605: note: this is the location of the previous definition
>   605 | # define _GNU_SOURCE 1
>       |
> In file included from ./ags/audio/midi/ags_midi_builder.h:29,
>                  from ./ags/libags-audio.h:144,
>                  from ./ags/X/ags_xorg_application_context.h:29,
>                  from ags/X/ags_xorg_application_context.c:20:
> ags/X/ags_xorg_application_context.c: In function
> 'ags_xorg_application_context_class_init':
> ags/X/ags_xorg_application_context.c:431:24: error:
> 'AgsApplicationContextClass' {aka 'struct
> _AgsApplicationContextClass'} has no member named '_write'; did you
> mean 'write'?
>   431 |   application_context->write = ags_xorg_application_context_write;
>       |                        ^~~~~
> ags/X/ags_xorg_application_context.c:432:24: error:
> 'AgsApplicationContextClass' {aka 'struct
> _AgsApplicationContextClass'} has no member named '_read'; did you
> mean 'read'?
>   432 |   application_context->read = ags_xorg_application_context_read;
>       |

Gnulib defines functions like 'read', 'write', 'close', etc. as C preprocessor
macros, in order to avoid conflicts at the linker level. This is essential for
the operation of Gnulib. So, 'write' may be defined to 'rpl_write' or to
'_write', depending on the platform. (For some compilers, it would be possible
to do this mapping at the C compiler level as opposed to the C preprocessor
level. But this does not generalize to all compilers, therefore we are avoiding
this approach.)

Yes, unfortunately this can produce an error if you define a struct with a
member 'write', then #include <unistd.h>, then use code that looks up the
member 'write' - because at this point, 'write' expands to 'rpl_write' or
'_write'.

The workaround is simple: Make sure that you include the corresponding file -
here <unistd.h> - before the struct definition. That is, in this case,
at the beginning of ags/object/ags_application_context.h, add
  #include <unistd.h>

Bruno