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Date: Wed, 24 Sep 2014 08:21:44 +0200
From: Gabriel Paubert <paubert AT iram DOT es>
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Subject: Re: [geda-user] Banging my head against the guile-for-windows wall
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On Wed, Sep 24, 2014 at 06:04:32AM +0200, Peter Stuge wrote:
> Kai-Martin Knaak wrote:
> > read.c: In function 'try_read_ci_chars':
> > read.c:983:3: warning: implicit declaration of function 'alloca' [-Wimplicit-function-declaration]
> >    char *chars_read = alloca (num_chars_wanted);
> >    ^
> > read.c:983:22: warning: incompatible implicit declaration of built-in function 'alloca'
> >    char *chars_read = alloca (num_chars_wanted);
> >                       ^
> 
> This is ugly but if void * and int are the same size it is harmless.
> 
> MSDN says the correct header file is malloc.h and that the function
> is called _alloca() as opposed to alloca(). Try including the header
> and adding a conditional define, into all source files with that
> warning.
> 
> #include <malloc.h>
> #ifndef alloca
> #define alloca _alloca
> #endif
> 
> 
> > posix.c: In function 'scm_execl':
> > posix.c:1144:3: warning: passing argument 2 of 'execv' from incompatible pointer type
> >    execv (exec_file, exec_argv);
> >    ^
> > In file included from /usr/local/src/mxe/usr/i686-pc-mingw32.static/include/unistd.h:13:0,
> >                  from ../lib/unistd.h:40,
> >                  from posix.c:50:
> > /usr/local/src/mxe/usr/i686-pc-mingw32.static/include/process.h:118:42: note: expected 'const char * const*' but argument is of type 'char **'
> >  _CRTIMP intptr_t __cdecl __MINGW_NOTHROW execv (const char*, const char* const*);
> >                                           ^
> 
> These are harmless, it's a mismatch between const and non-const. When
> the function takes const and a non-const is passed, there's no problem.
> 

I consider this one a spurious warning, it may probably be silenced by inserting
an useless and ugly cast, but I really wonder on which drugs the compiler
writers were when they decided to add it. You should always be able to pass
a non-const argument to a const parameter.

> 
> > From my scarce coding experiences long long ago "incompatible pointer
> > type" and "incompatible implicit declarations" ring a serious bell.
> 

Indeed. For the first one there are unavoidable occurences in low level code
but it can fixed by a proper cast (especially if the message is caused
by the distinction between char, signed char and unsigned char, one of 
the problems of C and derived languages).

> In principle correct, but when the only difference is const vs
> non-const it's not so bad.

Huh? I consider this message extremely stupid. Passing a const to 
a non-const is an error, the other way around should be silently 
accepted.
	
	Gabriel