Why do I get an exception when trying to memcpy a section of Topic is solved

[davelacey]

I've got a structure like this:


struct st {
int a;
int b;
int c; } x;


If I do the following:

memcpy(dst, (char *) &x.b, 8); 

I get an execption at runtime:

Unhandled exception: ECALL, data: 0x00000000

Why is that?

[davelacey]

The exception is due to the safe pointer bounds checks in xC.

In xC if you take a safe pointer to a structure member then the bounds on that pointer allow you to only access that member.

If you want to manipulate the memory of a structure you have to take a pointer to the whole structure - not just a member e.g.

struct st x;

char * p = &x;

In the example above you want to do something like:

memcpy(dst, ((char *) &x) + 4, 8);

Of course, this code means you know how the struct is laid out in memory. A useful macro is this:

#define struct_offset(s, x) ((char *) &s.x - (char *) &s)
 
This means you could write:
 
memcpy(dst, ((char *) &x) + struct_offset(x, b), 8);
 
This code is more explicit that it wants to treat the struct as a block of characters in memory.