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

c arrays sizeof memcpy
Tomas picture Tomas · Apr 21, 2010 · Viewed 71.9k times · Source

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 following two alternatives of the size parameter to memcpy()?

memcpy(dst, src, ARRAY_LENGTH*sizeof(int));

or

memcpy(dst, src, sizeof(dst));

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

One thing that favors the last one is that if the array were to change, it'll be some house-keeping to update the memcpy()'s.

Answer

Timbo picture Timbo · Apr 21, 2010

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*)

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