How to copy dynamically allocated array of struct?

Question

I have been struggling with this. What I have found here on stackoverflow, and other places was to just do:

memcpy(&a, &b, sizeof(b));

But for me, that did not work. So I thought, I share what worked for me without any unexpected behavior.

sizeof(b) will tell you the size of the pointer, not the size of the memory block that b points to. So, your line of code will only copy 4 or 8 bytes depending on the system architecture (32 or 64 bit). You need to pass the 3rd parameter to memcpy to really say how much memory to copy. — bruceg
– bruceg, Commented Dec 4, 2015 at 22:51
yes, I have realized/understood/read that by now :), but for some reason this is what I find as accepted solution for questions :/. — Snowman
– Snowman, Commented Dec 4, 2015 at 22:54

ouah · Accepted Answer · 2015-12-04 23:12:43Z

2

Assume you have:

#define N 42
struct struct_name *a = malloc(sizeof *a * N);  // allocate N objects
struct struct_name *b = malloc(sizeof *b * N);  // allocate N objects

then the correct memcpy call to copy the array of structure objects is:

memcpy(a, b, sizeof *a * N);

answered Dec 4, 2015 at 23:12

ouah

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Snowman · Accepted Answer · 2016-04-13 17:48:23Z

In my case previous solutions did not work properly, e.g. the one in the question! (it copied about half of only the first element).
So in case, somebody needs a solution, that will give you correct results, here it is:

memcpy(a, b, n * sizeof(*b));

More detail:

int i, n = 50; 
struct YourStruct *a, *b;

a = calloc(n, sizeof(*a));
b = malloc(n * sizeof(*b));
for (i = 0; i < n; ++i) { 
    // filling a
}

memcpy(b, a, n * sizeof(*a)); // <----- memcpy 'n' elements from 'a' to 'b'

if (a != NULL) free(a); // free 'a'
a = calloc(2*n, sizeof(*a)); // 'a' is size 2n now

memcpy(a, b, n * sizeof(*b)); // <------ memcpy back the 'n' elements from 'b' to 'a'

// do other stuff with 'a' (filling too)...

Some notes:

I used calloc for a, because in the '// filling a' part I was doing operations that required initialized data.
I could have used realloc as well.
I was kind of doing a dynamically growing array (with doubling size -> log(n)), but here I simplified it.
You should check, if memory allocation is successful or not (a,b NULL or not). But here, I removed those checks for simplification.

Thanks for pointing it out, it was malloc before, and I just copied it. That caused the typo.
the posted code can result in a seg fault event if either the calloc() or the malloc() fails. Always check (!=NULL) the returned value from any memory allocation function (malloc, calloc, realloc) before using that value
Thanks for pointing it out, but this is just a simplified version, which tries to show usage of memcpy.

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