Search code examples
cmemcpy

Using memcpy to read two integers from a memory block

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?

Solution

  • 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.