C++Talk.NET

Guest

I was tinkering with the idea that it'd be nice if you could define a
struct that specified the layout of a file, and then be able to 'load'
that file by simply pointing a struct pointer to the beginning of the
file. You can't usually do this because of padding bytes inserted by
the compiler for efficient access. So I thought that maybe if I did
this:

struct example {
volatile char a;
volatile int x;
volatile double z;
};

That 'volatile' would prevent the compiler from adding those padding
bytes, because I've just told it I'm expecting the struct to be layed
out in a specific way by using that keyword. But in my tests, no matter
how I arrange it, I end up with a struct with the same memory layout as
one with all non-volatile members -- sizeof(example) is always 16
regardless of whether the struct members are volatile, even though the
sum of the members is only 13. I find this a little surprising, since
I'm compiling on an x86 machine so AFAIK unaligned accesses are allowed
although slow. Compiling with g++ 4.1.2 (Ubuntu). Is the compiler
ignoring the volatile keyword, or am I misinterpreting what its effect
should be?

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Seungbeom Kim · Guest

k04jg02 (AT) gmail (DOT) com wrote:

James Kanze · Guest

Seungbeom Kim wrote: