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Re: open_safer on amd64

From: Eric Blake
Subject: Re: open_safer on amd64
Date: Wed, 20 May 2009 19:00:26 -0600
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According to Bruno Haible on 5/20/2009 5:08 PM:
> Sergey Poznyakoff wrote:
>> When trying to compile open-safer.c on amd64 I get:
>> open-safer.c: In function 'open_safer':
>> open-safer.c:46: warning: 'mode_t' is promoted to 'int' when passed through 
>> '...'
>> open-safer.c:46: warning: (so you should pass 'int' not 'mode_t' to 'va_arg')
>> open-safer.c:46: note: if this code is reached, the program will abort
> You can ignore this warning. The code is fine.
> We know no way of getting rid of this warning, except by passing fewer warning
> options to gcc.

Well, we COULD file a gcc bug for being too strict on this particular
warning.  In earlier versions of POSIX, the intent was that mode_t could
be narrower than int, and that all programmers had to use only symbolic
constants in that argument.  But POSIX 2008 changed its stance by adding
these words:

The <sys/stat.h> header shall define the following symbolic constants for
the file mode bits encoded in type mode_t, with the indicated numeric
values. These macros shall expand to an expression which has a type that
allows them to be used, either singly or OR'ed together, as the third
argument to open() without the need for a mode_t cast. The values shall be
suitable for use in #if preprocessing directives.

which effectively requires that mode_t and int be compatible types when
passed through va_arg.  POSIX 2008 also recognized traditional open-coding
(like 0660) instead of symbolic macros for mode_t values.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             address@hidden
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