Using memcpy to copy a structure into the heap

I am 开发者_开发百科trying to put a structure named "Holder" into the heap by using the following code, but I get a segmentation fault when I try to use memcpy:

Holder hold;
Holder *heapHold = memcpy(heapHold, &hold, sizeof(Holder));

Why do I segfault?

Holder hold;

This declares hold as a variable of type Holder. hold is uninitialized.

Holder *heapHold = memcpy(heapHold, &hold, sizeof(Holder));

memcpy(dest, source, size) copies size bytes from source to dest. It assumes that dest is properly allocated. Since you don't allocate memory for it, you get a segfault.

You need to allocate memory:

Holder *heapHold = malloc(sizeof *heapHold);
if (heapHold == NULL) {
    /* failed to allocate, handle error here */
} else {
    /* OK to copy.  Make sure you initialize 'hold' to something valid */
    memcpy(heapHold, &hold, sizeof *heapHold);
}

Also, memcpy() returns the first parameter. Assigning the return value to heapHold is just like saying a = a;, i.e., it doesn't do anything. For most practical cases, the return value of memcpy() is ignored.

You need to allocate memory for heapHold first. Currently memcpy is writing to some random address (heapHold is uninitialized), causing the crash.

Holder hold;
Holder *heapHold = malloc(sizeof(Holder));
memcpy(heapHold, &hold, sizeof(Holder));

Holder hold;
Holder *heapHold = malloc(sizeof(Holder));
memcpy(heapHold,&hold,sizeof(Holder));

The heapHold variable in memcpy(heapHold, &hold, sizeof(Holder)); is still not initialized (and no buffer was allocated what-so-ever) so it's pointing to invalid memory.

BTW (I always forget to mention that) - when you allocate memory using malloc, don't forget to free it later..