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Re: Parsing struct memory?

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On Thursday, March 16, 2017 at 4:58:24 PM UTC-3, Thiago Adams wrote:
> Having some struct;
> 
> struct {
>   char c;
>   void* p;
>   double d; 
> };
> 
> Is it possible to parse it just knowing each type?
> Something similar of var_arg but for structs.
> 
> char c = arg(pointer, char);
> void* p = arg(pointer, void*);
> double d = arg(pointer, double);
> 
> I was trying https://en.wikipedia.org/wiki/Data_structure_alignment
> 
> for x86 this is working on visual studio to return the next pointer.
> 
> #define NEXT(n)   ( (sizeof(n) + sizeof(int) - 1) & ~(sizeof(int) - 1) )
> 
> For x64 not. Changing int for __int64 doesn't work as well.


This one is working on x86 and x64. 

The previous macro was copied from va_arg x86 and I think on the stack the alignment was always the same as int.

I am comparing the offset with the calculated one.

#include <stdlib.h>
#include <stdio.h>
#include <stddef.h>

#define NEW_OFFSET(offset, type)  (((offset) + sizeof(type) - 1) & ~(sizeof(type) - 1))


int main()
{
	struct X {  

		char a1;
		char a2;
		void* a3;
		char a4;
	} x;

	int i = 0;

	i = offsetof(struct X, a1);
	printf("%d %d\n", i, (int) NEW_OFFSET(i,char));

	i = offsetof(struct X, a2);
	printf("%d %d\n", i, (int)NEW_OFFSET(i, char));

	i = offsetof(struct X, a3);
	printf("%d %d\n", i, (int)NEW_OFFSET(i, void*));

	i = offsetof(struct X, a4);
	printf("%d %d\n", i, (int)NEW_OFFSET(i, char));


	printf("\n");

    return 0;
}

For my specific problem, I can build the struct. (build the memory) and doing this probably the program is very portable.

I need to copy values to heap and then later to copy from heap to stack.

I am not sure if I must used the correct alignment or if it is optional.
To cast some memory for double for instance, does it need to be aligned?

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Thread

Parsing struct memory? Thiago Adams <thiago.adams@gmail.com> - 2017-03-16 12:58 -0700
  Re: Parsing struct memory? Thiago Adams <thiago.adams@gmail.com> - 2017-03-16 13:21 -0700
    Re: Parsing struct memory? Ian Collins <ian-news@hotmail.com> - 2017-03-17 11:16 +1300
      Re: Parsing struct memory? James Kuyper <jameskuyper@verizon.net> - 2017-03-16 22:09 -0400
        Re: Parsing struct memory? Ian Collins <ian-news@hotmail.com> - 2017-03-17 15:23 +1300
        Re: Parsing struct memory? David Brown <david.brown@hesbynett.no> - 2017-03-17 10:11 +0100
          Re: Parsing struct memory? "James R. Kuyper" <jameskuyper@verizon.net> - 2017-03-17 11:35 -0400
    Re: Parsing struct memory? gordonb.7zilv@burditt.org (Gordon Burditt) - 2017-03-16 20:17 -0500
      Re: Parsing struct memory? James Kuyper <jameskuyper@verizon.net> - 2017-03-16 21:56 -0400
  Re: Parsing struct memory? supercat@casperkitty.com - 2017-03-16 13:51 -0700
    Re: Parsing struct memory? Lowell Gilbert <lgusenet@be-well.ilk.org> - 2017-03-16 19:39 -0400
      Re: Parsing struct memory? BartC <bc@freeuk.com> - 2017-03-17 12:16 +0000
      Re: Parsing struct memory? "Peter \"Shaggy\" Haywood" <phaywood@alphalink.com.au> - 2017-03-24 18:06 +1100
        Re: Parsing struct memory? supercat@casperkitty.com - 2017-03-24 14:35 -0700
  Re: Parsing struct memory? Keith Thompson <kst-u@mib.org> - 2017-03-16 19:14 -0700
    Re: Parsing struct memory? scott@slp53.sl.home (Scott Lurndal) - 2017-03-17 12:45 +0000
  Re: Parsing struct memory? scott@slp53.sl.home (Scott Lurndal) - 2017-03-17 12:41 +0000
  Re: Parsing struct memory? Thiago Adams <thiago.adams@gmail.com> - 2017-03-17 08:17 -0700
  Re: Parsing struct memory? Thiago Adams <thiago.adams@gmail.com> - 2017-03-20 05:07 -0700

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