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Re: [Paparazzi-devel] I need an expert in C

From: Michele Santucci
Subject: Re: [Paparazzi-devel] I need an expert in C
Date: Fri, 4 Sep 2009 09:31:04 +0200

Hallo Cédric,
if I understood your problem the workaround could be a compiler directive to override the
default  compiler memory aligment policy, of course this directive is strongly bound to the
compiler itself but it usually sounds like a 'pragma' directive i.e. #pragma option push -a1,
#pragma pack(1). Anyway look into compiler manual and you will find this information easily.

Sent: Friday, September 04, 2009 8:47 AM
Subject: [Paparazzi-devel] I need an expert in C



My OSD makes good progress but I am stuck with some C trick.

The messages are stored in an array each char, short or long following each other.

I want to map this message into different structures (for the gps message, the batterie message, etc.).


The crude way to do that (it works), is the following :



But that means that I will write a dozen of this for each message…


The (not so smart) way is to do that :




but there is a trap…

When you make a structure, the compiler makes sure that the short and long are aligned in the memory

and add gaps after let's say a char to make sure that the next long is well aligned in the memory.

That means that the data in my array and in the structure are not aligned. The process to put the gaps is called "padding".


Some compiler and processor allow one to disable "padding". I doubt that this is the case with arm.


So does anyone of you have a smart idea for a clean work around ? Otherwise I will just go the crude way…





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