memcpy(), what should the value of the size parameter be?

Question

I want to copy an int array to another int array. They use the same define for length so they\'ll always be of the same length.

What are the pros/cons of the followi

Answer 1

sizeof(X) always gives you the NUMBER OF BYTES of "X" if X is a uint16_t array of 10, then sizeof(X) will return 20

uint16_t X[10]={0};
cout<<"sizeof x: "<<sizeof(X);

$> sizeof x: 20

if you want the number of elements you have to do a bit of byte arithmetic:
8bit = 1byte
16bit = 2bytes
32bit = 4 bytes
64bit = 8 bytes

so to get the number of elements you could do:

 numb_of_elements = ( sizeof(X)/sizeof(X[0]) );

resulting in:

uint32_t source[100]={0};
memcpy((void*) dest, (void*) source, ( sizeof(source)/sizeof(source[0]) ));

of course you would probably want to make ( sizeof(X)/sizeof(X[0]) ) a constant/variable so that you don't compute each time.. ( I don't know if compilers will always optimize this)

Answer 2

How about?

memcpy(dst, src, &src[ARRAY_LENGTH] - &src[0]);

This should work even if the size of individual elements was smaller than the size taken by each item in the actual array.

Answer 3

Assuming dst is of type int*, sizeof(dst) will return the size of the pointer itself (i.e. 4 on a 32 bit system, 8 on a 64 bit system), so your second example will only every copy this many bytes, while the first one will correctly use the actual size of the content.

Answer 4

As long as dst is declared as an array with a size, sizeof will return the size of that array in bytes:

int dst[ARRAY_LENGTH];

memcpy( dst, src, sizeof(dst) ); // Good, sizeof(dst) returns sizeof(int) * ARRAY_LENGTH

If dst just happens to be a pointer to the first element of such an array (which is the same type as the array itself), it wont work:

int buffer[ARRAY_LENGTH];
int* dst = &buffer[0];

memcpy( dst, src, sizeof(dst) ); // Bad, sizeof(dst) returns sizeof(int*)

Answer 5

sizeof(dst) is correct only if dst is an array which size is known at compile time: like int arr[ARRAY_LENGTH] or a C99 variable length array; otherwise it returns the size of a pointer, not the length of the destination array.

To avoid future bug, be consistent and prefer the first form: size of type * length.

Answer 6

Will the second option always work? Regardless of the content?

It will work only if both conditions are satisfied:

• dst is regular array, not pointer
• src and dst are the same size