From: Andrew Lunn <andrew@lunn.ch>
To: Philip Soeberg <ecos@soeberg.net>
Cc: ecos-discuss@sources.redhat.com
Subject: Re: [ECOS] Deciphering ISO C (Chap 6.3.2.3 - Pointers)
Date: Mon, 10 Nov 2003 22:16:00 -0000 [thread overview]
Message-ID: <20031110221615.GL7282@lunn.ch> (raw)
In-Reply-To: <E1AJK5Y-0004tk-00@londo.lunn.ch>
> cyg_uint16* res_16;
> cyg_uint16* p_16;
> cyg_uint16 u_16;
>
> p_16 = 0x0u;
> u_16 = 0x0u;'
> res_16 = (p_16 + 0x555u);
>
> The above code yields res_16 == 0xaaa
> I was sort of hoping for 0x555 instead...
>
> Altering the addition line to this:
> res_16 = (cyg_uint16*)(u_16 + 0x555);
> corrects the problem...
>
> why?
Pointer arithmetic. Take the following example bits of code....(which probably has many syntax errors etc)
char foo[4]="bar";
char foo1[4];
int a[]={0,1,2,3};
int a1[3];
for (i = 0; i < 3; i++)
{
char * pfoo = &foo;
char * pa = & foo1;
*(pfoo+i) = pfoo[i];
*(pa+i) = a[i];
}
Its a contrived way of copying foo into foo1 and a into a1.
You want *(pfoo+i) to be byte by byte since you are copying characters, but
you want *(pa+i) to be word by word since its copying ints.
In
res_16 = (p_16 + 0x555u);
p_16 is of type cyg_uint16 *, so you are adding 0x555 16bit words, ie 0xaaa.
With
res_16 = (cyg_uint16*)(u_16 + 0x555);
u_16 is a plain cyg_uint16, so its not pointer arithmetic, its normal
arithmetic which you then cast to a pointer afterwards.
Andrew
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next prev parent reply other threads:[~2003-11-10 22:16 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-11-10 20:28 [ECOS] Building from OS X Rib Rdb
2003-11-10 20:34 ` Gary Thomas
2003-11-10 22:00 ` [ECOS] Deciphering ISO C (Chap 6.3.2.3 - Pointers) Philip Soeberg
[not found] ` <E1AJK5Y-0004tk-00@londo.lunn.ch>
2003-11-10 22:16 ` Andrew Lunn [this message]
2003-11-11 0:27 ` [ECOS] Building from OS X Rib Rdb
2003-11-11 0:37 ` Gary Thomas
2003-11-11 0:49 ` Rib Rdb
2003-11-11 1:18 ` Rib Rdb
2003-11-13 9:37 ` [ECOS] " John Dallaway
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