[2/2] darwin: Don't use sbrk

  This patch gets rid of this warning on macOS:

    CXX    main.o
  /Users/simark/src/binutils-gdb/gdb/main.c:492:27: error: 'sbrk' is deprecated [-Werror,-Wdeprecated-declarations]
    lim_at_start = (char *) sbrk (0);
                            ^
  /usr/include/unistd.h:585:1: note: 'sbrk' has been explicitly marked deprecated here
  __deprecated __WATCHOS_PROHIBITED __TVOS_PROHIBITED
  ^
  /usr/include/sys/cdefs.h:176:37: note: expanded from macro '__deprecated'
  #define __deprecated    __attribute__((deprecated))
                                         ^

sbrk on macOS is not useful for our purposes, since sbrk(0) always
returns the same value.  From what I read, brk/sbrk on macOS is just an
emulation, it always returns a pointer in a 4MB section reserved for
that.

So instead of letting users use "maint set per-command space on" and
print silly results, I think we should just disable that feature for
this platform (as we do for platforms that don't have sbrk).

I defined a HAVE_USEFUL_SBRK macro and used that instead of HAVE_SBRK.

gdb/ChangeLog:

	* common/common-defs.h (HAVE_USEFUL_SBRK): Define.
	* main.c: Use HAVE_USEFUL_SBRK instead of HAVE_SBRK.
	* maint.c: Likewise.
	* top.c: Likewise.
---
 gdb/common/common-defs.h | 4 ++++
 gdb/main.c               | 2 +-
 gdb/maint.c              | 4 ++--
 gdb/top.c                | 2 +-
 4 files changed, 8 insertions(+), 4 deletions(-)
  

On 07/04/2018 05:30 AM, Simon Marchi wrote:
> This patch gets rid of this warning on macOS:
> 

> sbrk on macOS is not useful for our purposes, since sbrk(0) always
> returns the same value.  From what I read, brk/sbrk on macOS is just an
> emulation, it always returns a pointer in a 4MB section reserved for
> that.
> 

> --- a/gdb/common/common-defs.h
> +++ b/gdb/common/common-defs.h
> @@ -105,4 +105,8 @@
>  /* String containing the current directory (what getwd would return).  */
>  extern char *current_directory;
>  

Suggest adding a comment here based on what you had in the intro:

/* sbrk on macOS is not useful for our purposes, since sbrk(0) always
   returns the same value.  brk/sbrk on macOS is just an emulation
   that always returns a pointer to a 4MB section reserved for
   that.  */

> +#if defined (HAVE_SBRK) && !__APPLE__
> +#define HAVE_USEFUL_SBRK 1
> +#endif
Thanks,
Pedro Alves
  

On 2018-07-04 06:49, Pedro Alves wrote:
> On 07/04/2018 05:30 AM, Simon Marchi wrote:
>> This patch gets rid of this warning on macOS:
>> 
> 
>> sbrk on macOS is not useful for our purposes, since sbrk(0) always
>> returns the same value.  From what I read, brk/sbrk on macOS is just 
>> an
>> emulation, it always returns a pointer in a 4MB section reserved for
>> that.
>> 
> 
>> --- a/gdb/common/common-defs.h
>> +++ b/gdb/common/common-defs.h
>> @@ -105,4 +105,8 @@
>>  /* String containing the current directory (what getwd would return). 
>>  */
>>  extern char *current_directory;
>> 
> 
> Suggest adding a comment here based on what you had in the intro:
> 
> /* sbrk on macOS is not useful for our purposes, since sbrk(0) always
>    returns the same value.  brk/sbrk on macOS is just an emulation
>    that always returns a pointer to a 4MB section reserved for
>    that.  */

Of course, that's a good idea.  Otherwise somebody will curse in 10 
years, wondering why we did this, just like we did for the #if 
__APPLE_CC__ in gettext :).  I'm pushing with that added, thanks for 
providing the comment.

Simon