Question

I am trying to read two integers, stored consecutively, from a memory block (i have a pointer void *block pointing to the contents of the block) using memcpy. The first one is read just fine using:

memcpy(&test, block, sizeof(int));

I try to read the second using:

memcpy(&test, block + sizeof(int), sizeof(int));

(Of course i am having those stataments in different execution instances of the program, so the problem is not that test is being overriden)

but i fail to get the correct result! What am i doing wrong here?

Answer 1

This is nonstandard:

void *block = ...; 
block + sizeof(int); // where does this point to?

If you want to do pointer arithmetic, cast to a type of known size first (the pointer addition is implicitly multiplied by the size of the underlying type, and sizeof(unsigned char) is always 1):

void *block = ...;
(unsigned char *) block + sizeof(int);

Or use the even easier version,

void *block = ...;
(int *) block + 1;

So the final code is:

int test;
void *block = ...;
memcpy(&test, block, sizeof(test));
// do something...?
memcpy(&test, (int *) block + 1, sizeof(test));

Or a simpler version,

int test[2];
void *block = ...;
memcpy(&test, block, sizeof(test));

Don’t do the following:

test = *(int *) block;

It will crash on some systems (typically SIGBUS) if block is unaligned.