From mboxrd@z Thu Jan  1 00:00:00 1970
From: "H. Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>
Subject: Re: [RFC][v5][PATCH 8/8]: Define clone_with_pids() syscall
Date: Wed, 09 Sep 2009 08:51:59 -0700
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To: Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>
Cc: Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Nathan Lynch <ntl-e+AXbWqSrlAAvxtiuMwx3w@public.gmane.org>, "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>, mingo-X9Un+BFzKDI@public.gmane.org, Sukadev Bhattiprolu <sukadev-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>, torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, Alexey Dobriyan <adobriyan-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Pavel Emelyanov <xemul-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>
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On 09/09/2009 05:19 AM, Arnd Bergmann wrote:
> 
> This is a complex problem. The structure above would need a conversion
> for the pointer size that you can avoid by using a u64, but that introduces
> another problem:
> 
> 2. use a single pointer, with variable length data structures:
> 
> struct pid_set {
> 	int num_pids;
> 	pid_t pids[0];
> };
> 
> Since pid_t is always an int, you have no problem with padding or
> incompatible types, but rely on a data structure definition that
> is not in C89 (not sure about C99).
> 

C90 has these data structures, but you have to give the array a nonzero
length:

struct pid_set {
	int num_pids;
	pid_t pids[1];
};

In C99, this is spelt:

struct pid_set {
	int num_pids;
	pid_t pids[];
};

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.